The interaction of ethylene adsorbed on Ni(111) with gas-phase H atoms has been investigated. The major adsorbed reaction product is identified by high-resolution electron energy loss spectroscopy to be ethylidyne (C−CH₃). This study is the first direct spectroscopic observation of a C−CH₃ species adsorbed on Ni in an ultrahigh-vacuum environment. Spectra of four isotopomers, C−CH₃, ¹³C−¹³CH₃, C−CD₃, and ¹³C−¹³CD₃, are reported, and a complete and consistent vibrational assignment of their fundamental modes is presented. Based on this assignment, a force field is derived from the measured vibrational frequencies using a normal-modes analysis and is found to be in good agreement with that deduced from IR spectra of an ethylidyne species in an organometallic complex. Inspection of the eigenvectors of the normal-mode displacements reveals that substantial mixing of harmonic bond motions is the origin of the unusual upshift in frequency of the C−C stretching mode upon deuteration. A quantitative determination of the relative dynamic bond dipole moments demonstrates that the changes in intensity and dipole activity of the C−C stretching and symmetric CH₃ deformation modes upon deuteration, phenomena common to all C−CD₃ spectra, also arise from extensive mixing of bond motions. A detailed analysis of the spectra strongly suggests a C_3v or C₃ local environment for ethylidyne and a 3-fold hollow adsorption site.

We report that both surface-bound H atoms and bulk H atoms, upon moving out from the bulk of a Ni single crystal to its surface of a (111) orientation, are reactive with adsorbed C₂H₂, but the two kinds of H atoms have unique product distributions. Both bulk H and surface-bound H react with C₂H₂ to produce adsorbed ethylidyne, CCH₃, while only bulk H hydrogenates C₂H₂ to gas-phase ethylene and ethane, the products of interest in acetylene hydrogenation catalysis for the purification of ethylene streams. Their distinct reactivities arise from both their different directions of approach to the π orbitals of the unsaturated hydrocarbon and their substantially different energetics. These observations demonstrate that H embedded in the metal catalyst is a reactant in alkyne hydrogenation and is not solely a source of surface-bound H which then reacts with acetylene, as proposed from correlations between the hydrogenation activity of Raney Ni and Pd catalysts and the amount of H absorbed in these catalysts. The reactivities of these two kinds of H atoms are clearly distinguished in this experiment because of the capability to synthesize either bulk H or surface-bound H cleanly in an ultrahigh vacuum environment.

The conformation of cinchonidine in solution has been investigated by NMR techniques as well as theoretically. Three conformers of cinchonidine are shown to be substantially populated at room temperature, Closed(1), Closed(2), and Open(3), with the latter being the most stable in apolar solvents. The stability of the closed conformers relative to that of Open(3), however, increases with solvent polarity. In polar solvents the three conformers have similar energy. The relationship between relative energies and the dielectric constant of the solvent is not linear but resembles the form of an Onsager function. Ab initio and density functional reaction field calculations using cavity shapes determined by an isodensity surface are in good agreement with experiment for solvents which do not show strong specific interaction with cinchonidine. The role of the conformational behavior of cinchonidine is illustrated using the example of the platinum-catalyzed enantioselective hydrogenation of ketopantolactone in different solvents.

The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+.

Multicomponent thin films with spectral hole burning capacity at room temperature were synthesized by using molecular beam and pulsed laser deposition techniques All materials were activated by Sm²⁺ in low-symmetry alkaline earth sites, the synthesis involved the control of ionic diffiision rate during multilayer growth and special reduction of Samarium. Enhancement of hole burning rate by 1-2 orders is obtained in nanocrystalline films as compared to bulk and microcrystalline materials New hypothetic mechanism involving the creation of Sm-defect (photochromic) centers is proposed for reversible photoburning.

The isomerization reaction of cubic N8 to the planar bicyclic structure analogous to pentalene has been investigated using multiconfigurational self-consistent field and second-order perturbation theory (CASPT2). Comparative calculations using density functional theory have also been performed. Five local minima on the energy surface have been found, and the transition states between each two consecutive minima have been determined. The results show that all steps in the isomerization process, except one, can proceed via a set of transition states with moderately high energy barriers (10–20kcal/mol).

The HF/3s2pld and MP2/3s2pld structures, energies and vibrational frequencies were calculated for ten N8 isomers, corresponding to ten analogous CH structures. Comparative calculations using density functional theory (DFT), with a cc-pVTZ basis set, were also performed. All ten structures were found to be local minima on the energy hypersurface at the Hartree-Fock (HF) level, whereas at the second-order Möller-Plesset (MP2) level nine structures were stable. At the DFT level, eight local minima were found. The total energies were recomputed using 4s3p2dlf basis sets at the HF and MP2 levels of theory.

An implementation of the Douglas–Kroll (DK) transformation is described within a new relativistic quantum chemistry code, MAGIC, which performs calculations on systems containing heavy atoms. This method reduces the computational cost in terms of memory requirements that are associated with completeness identities in the DK implementation by factorizing the one-electron matrices into smaller ones that depend only on two atoms at a time. Examples are presented.

A study on the UF6 monomer and dimer was carried out within the density functional method. The U−F distance in the UF6 monomer was optimized at different levels of theory, pointwise, assuming octahedral geometry, (1) by using an all-electron basis for both U and F in a nonrelativistic calculation; (2) by using a relativistic effective core potential (RECP) on U and nonrelativistic effective core potential (ECP) on the fluorines; and (3) by using RECP on the U atom and an all-electron basis on the F atoms. Atomization energies of 23.11, 33.92, and 35.66 eV were obtained at the three levels, respectively. Relativistic effects lead to about a 50% increase in the atomization energy. For the UF6 dimer, the potential energy curve, as a function of the intermolecular U−U distance, was computed at level 2, and the rotational barrier between the two monomers was determined. Similar calculations were performed on the corresponding PuF6 species. Comparisons are made with experiment and other theoretical studies, where available.

An efficient approach for evaluating effective core potential integrals which involve projection operators has been implemented in the MAGIC quantum chemistry program. The methodology is presented and its performance is examined through illustrative calculations on transition metal and actinide compounds.

Remote site deprotonation of a coordinated imidazole ligand switches the reduction potential of coordinated iron over a narrow pH range from +0.920 to –0.460 V.

We present a local density functional study of Cd4S and Cd4S4 clusters inside sodalite cages of different compositions (Al:Si ratios). The composition of the framework determines the cluster → cage charge transfer and strongly affects the atomic structure of the inclusion. The energy gap and the character of the highest occupied (HOMO) and lowest unoccupied (LUMO) electronic states depend on the size and stoichiometry of the included clusters, as well as on the overall stoichiometry of the composite. The calculated gap for Cd4S inclusions in aluminosilicate and aluminate sodalite (at half and full packing respectively) is 2.5 eV (i.e., about twice the calculated gap of bulk CdS), while for Cd4S4 in aluminosilicate and pure silica sodalite (at half packing) it is 1.7-1.9 eV (i.e., about 1.5 times the gap of bulk CdS). Our results indicate that simple confinement arguments are usually insufficient to predict the behavior of semiconductor−zeolite composites.

The mechanism of the electrophilic substitution reaction of ferrocene has been investigated using density functional theory. In particular, reactions with two hard electrophiles (protonation and acetylation) and one soft electrophile (mercuration) have been studied at the LDA and B-PW91 levels of theory using a triple-ζ STO basis set. A general description of the reactions has been obtained, leading to results in agreement with experiment. Acetylation is found to occur via exo attack, whereas mercuration follows an endo mechanism. In the case of protonation, evidence for a rapid equilibrium between metal-protonated and agostic ring-protonated ferrocene is obtained, and no clear conclusion concerning the exo or endo mechanism can be deduced. The calculated proton affinities corresponding to both metal-protonated and agostic ring-protonated structures are in excellent agreement with experiment.

A series of compounds containing the [Mo3-μ3S-(μS2)3-(dtc)3]+ complex (dtc = diethyldithiocarbamate) with the anions I- (1), I- and Br- (2), S2- (3), ClO4- (4), NO3- (5), and SO42- (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH2Cl2 in the crystal having the composition [Mo3S7(dtc)3]I·0.5CH2Cl2 (1a), which was in contradiction to the previous protocol. The corresponding ClO4- compound (4a) is isotypic. Crystal data: C15.5H31Cl2Mo3N3O4S13, orthorhombic space group Aba2, a = 25.816(5) Å, b = 17.761(4) Å, c = 16.250(3) Å, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms Sax of the disulfido bridges. The Raman data showed a significant decrease of the Seq−Sax stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the Seq−Sax bond length and with a significant shortening of the X···Sax distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO3- and ClO4- the Sax···X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I- and S2- significant covalency is observed. In addition, the general reaction of [Mo3S7]4+ complexes with a nucleophile was modeled. With regard to the side-on bonding of the μ-S2 groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to μ3-S, are less pronounced.

The Section for Chemical Research (SCR) of the NSCS intends to provide a forum for chemists active in research, so as to promote exchange of ideas and collaboration. The SCR organizes or supports symposia, seminars, and scientific meetings, with a special emphasis on the Fall Meeting of the NSCS, which is de facto the largest annual forum of Swiss chemistry. Particular attention is paid by the SCR to the promotion of research results obtained by young chemists so as to help them starting their career. At present, the SCR membership stands at 465 members.

In a previous work we have presented a numerical procedure for the calculation of the internal chemical hardness tensor at the molecular orbital resolution level from standard density functional calculations. In this article we describe an improvement of our method using the thermal extensions of density functional theory. Furthermore, new concepts are introduced in the orbitally resolved theory of chemical reactivity. Traditional molecular orbital theories of chemical reactivity are based only on considerations concerning the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of molecules, supposed to describe the behavior towards electrophiles, respectively, nucleophiles. By applying our methodology to two test molecular systems, namely water and ferrocene, we show how chemical reactivity can be differentiated against hard and soft electrophiles (acids) and hard and soft nucleophiles (bases). As a by-product of the numerical algorithms being used, a self-consistent method for calculating the molecular chemical potential is also described.

Experiments in which the Raman linewidth was measured as a function of temperature (7-1183 K) and pressure (0-400 bar) were performed on the (111) and (100) planes of single crystals of the cubic anti-fluorite Li₂S. The temperature dependence of the lattice constant was determined by x-ray diffraction (11-295 K). From these results and published Brillouin scattering data for this host, the volume thermal expansion coefficient as a function of temperature was obtained as well as the isothermal compressibility and the isothermal Raman mode Grüneisen parameter. Using the thermodynamic approach within the quasi-harmonic approximation, we show that below 400 K the volume effects describe well the temperature dependence of the Raman linewidth whereas above this temperature there are direct anharmonic effects appearing. Above approximately 850 K new Raman lines appear that are A_⊥ and E polarized.

A new synthetic strategy has been developed to introduce bent and rigid tridentate 2,6-bis(benzimidazol-2‘-yl)pyridine cores into rodlike ligands L11-17. The crystal structure of the nonmesogenic ligand L13 (C39H37N5O4, triclinic, P, Z = 2) shows the expected trans−trans conformation of the tridentate binding unit, which provides a linear arrangement of the semirigid aromatic sidearms. The crystal structure of the related mesogenic ligand L16 (C61H81N5O4, triclinic, P, Z = 2) demonstrates the fully extended conformation adopted by the lipophilic side chains, leading to a slightly helically twisted I-shaped molecule. A rich and varied mesomorphism results which can be combined with the simultaneous tuning of electronic and photophysical properties via a judicious choice of the spacers between the rigid central core and the semirigid lipophilic sidearms. Ligands L13,14 react with Ln(NO3)3·xH2O to give quantitatively and selectively the neutral 1:1 complexes [Ln(Li)(NO3)3] (Ln = La to Lu), which are stable in the solid state at room temperature but partially dissociate in acetonitrile to give the cationic species [Ln(Li)(NO3)2]+. The crystal structure of [Lu(L13)(NO3)3]·3CH3CN (30, LuC45H46N11O13, monoclinic, C2/c, Z = 8) reveals a U-shaped arrangement of the ligand strand arising from the cis−cis conformation of the coordinated tridentate binding unit. This drastic geometric change strongly affects the thermal behavior and the photophysical and electronic properties of the lipophilic complexes [Ln(L14)(NO3)3]. Particular attention has been focused on structure−properties relationships, which can be modulated by the size of the lanthanide metal ions.

Theoretical studies on structure and stretching frequency of the CO molecule physisorbed on the MgO(100) or ZnO(1010) surfaces are reported. The properties of the adsorbed molecule were investigated by means of the recently developed formalism of Kohn-Sham equations with constrained electron density (KSCED). The KSCED method makes it possible to divide a large system into two subsystems and to study one of them using Kohn-Sham-like equations with an effective potential which takes into account the interactions between subsystems. This method (KSCED) was shown to be adequate to study the properties of the CO molecule adsorbed on the MgO(100) surface as reported in a previous paper (Wesolowski et. al.: J. Mol. Struct., THEOCHEM, in press). The effect of the interactions with the surface on the CO stretching frequency and geometry was analyzed for vertically bound (C-down) CO at the Zn-site of the ZnO(1010) surface. The ZnO(1010) surface was represented using several cluster models: Zn²⁺, (ZnO₃)^4-, or Zn₉O₉ embedded in a matrix of point charges. The KSCED frequency shift of the CO stretching vibration is blue-shifted and in good agreement with experiment.

The migrative insertion of CO into the Ni–CH=CH2 bond has been investigated by both static and dynamic density functional methods. The stationary points of the potential surface for the migrative insertion of CO into the Ni–CH=CH2 bond have been characterized using Cl(CO)2Ni–CH=CH2 as a model compound. Such a reaction has been found exothermic by 16 kJ mol−1, with an energy barrier of 9 kJ mol−1. Dynamic simulations have also been performed on Cl(CO)2Ni–CH=CH2 and show that the migrative insertion begins from the cis isomer and occurs via a simultaneous detachment of the vinyl group from the metal and formation of the vinyl–carbonyl bond.

The ground- and excited-state properties of both gas phase and crystalline ruthenocene, Ru(cp)₂, are investigated using density functional theory. A symmetry-based technique is employed to calculate the energies of the multiplet splittings of the singly excited triplet states. For the crystalline system, a Buckingham potential is introduced to describe the intermolecular interactions between a given Ru(cp)₂ molecule and its first shell of neighbors. The overall agreement between experimental and calculated ground- and excited-state properties is very good as far as absolute transition energies, the Stokes shift and the geometry of the excited states are concerned. An additional energy lowering in the ³B₂ component of the 5a1′→4e1″ excited state is obtained when the pseudolinear geometry of Ru(cp)₂ is relaxed along the low-frequency bending vibration.

The condensed Fukui functions fk of maleimide (1H-pyrrole-2,5-dione) have been calculated using a numerical integration scheme implemented in the deMon program package. The condensed functions show that soft nucleophiles interact with the α carbon atoms, whereas hard nucleophiles interact with the carbonyl carbon atoms, in accordance with the experimental evidence. The present method yields extremely few dispersed values of fk, whatever the basis sets, the numerical grids, and the exchange-correlation functionals used. Finally, the validity of the method has been successfully tested on a set of organic and organometallic molecules.

n view of further application to the study of molecular and atomic sticking on dust particles, we investigated the capability of the “freeze-and-thaw” cycle of the Kohn–Sham equations with constrained electron density (KSCED) to describe potential energy surfaces of weak van der Waals complexes. We report the results obtained for C₆H₆⋯X(X=O₂, N₂, and CO) as test cases. In the KSCED formalism, the exchange-correlation functional is defined as in the Kohn–Sham approach whereas the kinetic energy of the molecular complex is expressed differently, using both the analytic expressions for the kinetic energy of individual fragments and the explicit functional of electron density to approximate nonadditive contributions. As the analytical form of the kinetic energy functional is not known, the approach relies on approximations. Therefore, the applied implementation of KSCED requires the use of an approximate kinetic energy functional in addition to the approximate exchange-correlation functional in calculations following the Kohn–Sham formalism. Several approximate kinetic energy functionals derived using a general form by Lee, Lee, and Parr [Lee et al., Phys. Rev. A. 44, 768 (1991)] were considered. The functionals of this type are related to the approximate exchange energy functionals and it is possible to derive a kinetic energy functional from an exchange energy functional without the use of any additional parameters. The KSCED interaction energies obtained using the PW91 [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] exchange-correlation functional and the kinetic energy functional derived from the PW91 exchange functional agree very well with the available experimental results. Other considered functionals lead to worse results. Compared to the supermolecule Kohn–Sham interaction energies, the ones derived from the KSCED calculations depend less on the choice of the approximate functionals used. The presented KSCED results together with the previous Kohn–Sham ones [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997)] support the use of the PW91 functional for studies of weakly bound systems of our interest.

The coordination properties of ortho- and meta-substituted [(2-diphenylphosphanylethyl)phenyl]methanol 4a and 4b toward ruthenium(II) have been investigated. To ensure coordination of both the arene and the tethered phosphine, the labile ruthenium arene dimer [RuCl₂(EtO₂CC₆H₅)]₂ (7) was synthesized and structurally characterized. Both the ortho and meta isomers [Ru(4a)Cl₂] (9a) and [Ru(4b)Cl₂] (9b) were characterized by X-ray crystallography. The lack of reactivity of the benzylic alcohol functionality in complexes 9a and 9b toward various P and C electrophiles is rationalized with extended Hückel calculations.

Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M_1−xFe_x(bpy)₃](PF₆)₂ (M=Cd, Mn and Zn,x≈0.01, bpy=2,2′-bipyridine) under external pressure are presented. Below 50 K the high-spin→low-spin relaxation is an almost temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn_1−xFe_x(bpy)₃](PF₆)₂ and [Zn_1−xFe_x(bpy)₃](PF₆)₂show a phase transition at ≈1.1 kbar, which increases the tunnelling rate by a factor of about 6.

The thermal spin transition in the diluted mixed crystal [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ (x = 0.00025, (6-mepy)₃tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔH_HL⁰ and ΔS_HL⁰ of 1551(50) cm⁻¹ and 7.5(5) cm⁻¹/K and a molecular volume of reaction, ΔV_HL⁰, of 22(2) ųresult. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)₃tren](PF₆)₂from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm⁻¹ and an interaction constant, Γ, of 109(15) cm⁻¹. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ and the LS system [Zn_1−xFe_x(py)₃tren](PF₆)₂ in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in[Zn_1−xFe_x(py)₃tren](PF₆)₂ is more than three orders of magnitude larger than the one in[Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.

The spin-crossover compound [Fe(pic)₃]Cl₂EtOH (pic = 2-picolylamine) shows an unusual two-step spin transition. This is thought to be caused by specific nearest-neighbour interactions and short-range correlations and requires a theoretical treatment of the elastic interactions between the spin-changing molecules beyond the mean-field approximation. Such short-range correlations also influence the high-spin → low-spin relaxation following the light-induced population of the high-spin state at cryogenic temperatures, leading to characteristic deviations from the predictions of a mean-field treatment. These deviations are directly observable by comparison of the full and unperturbed relaxation curves with curves for which the short-range correlations were destroyed using an appropriate irradiation technique. Monte Carlo simulations including both nearest-neighbour and long-range interactions give a description of the observed relaxation curves which is consistent with the thermal spin equilibrium.

The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo[2,2,2]octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14±0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm—Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed.

The dynamics of the intermediate generated upon diffusional electron transfer (ET) quenching of 9,10-dicyanoanthracene by electron donors of varying oxidation potential in acetonitrile has been investigated using several transient grating techniques. With most of the donor/acceptor pairs studied, the transient grating spectrum cannot be differentiated from those of the free ions. Exciplex fluorescence, with the same lifetime as that of the ion pair, is observed with all donors. To extract from the measured kinetics the rate constant of exciplex dissociation, k^EX_dia , and of back ET, k^EX_BET , within these exciplexes, three different schemes have been considered. The best agreement is obtained by assuming that charge recombination predominantly takes place within the exciplex. The obtained k^EX_BET values are substantially different from the BET rate constants deduced indirectly from the free-ion yields and with a donor-independent rate constant of separation. For each class of donors, k^EX_BET exhibits a logarithmic free energy dependence with a slope of about −2 eV^-1. Moreover, k^EX_dia is not constant but increases continuously with diminishing donor's oxidation potential.

The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the triplet state by molecules with both a sufficiently small oxidation potential and low triplet state was investigated in the picosecond to microsecond time scales. In the longer time scale, the product distribution depends strongly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the other product. Picosecond transient grating measurements reveal that if TT is more exergonic than ET, the TT product is predominantly formed by two sequential ET reactions, i.e., by spin-allowed back ET within the triplet geminate ion pair formed upon ET quenching. However, if ET is more exergonic than TT, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes are exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism.

Bicyclonucleosides bearing a 5-deoxy-5-N-hydroxyamino-3,N5-(1,1-ethano)-β-o-furanosyl sugar moiety (15-18) have been prepared by glycosidation of the corresponding bicyclosugars obtained via an intramolecular reverse Cope elimination. The configuration of the asymmetric carbon of the 1,1-ethano bridge is the most important factor directing the conformation of the N-hydroxypyrrolidine ring and its invertomers ratio as shown by variable temperature H NMR experiments.

A new phosphine, the diphenyldibenzobarrelenephosphine 2, was designed to study the barrier to rotation of the P−H group around the C−^•P bond. After homolytic scission of a P−H bond by radiolysis, the EPR spectrum of the resulting phosphinyl radical, trapped in a single crystal of 2, was studied at 77 K and at room temperature. The directions of the ³¹P hyperfine eigenvectors were compared with the bond orientations of the undamaged compound as determined from its crystal structure. The temperature dependence of the EPR spectrum was analyzed by using the density matrix formalism; this showed that interaction between the phosphinyl hydrogen and the phenyl ring bound to the ethylenic bond is determinant for explaining the potential energy profile. DFT investigations are consistent with these experimental results.

Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-• and for the monophosphaallylic radical (HP•−CHCH2) ↔ (HPCH−•CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical.

Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)–, of ab initio calculations performed on the model radical anion (HPCPH)– at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)–. The structure of the radical anion is compared with that of the neutral molecule.

ENDOR measurements on the ¹⁹F^- nuclei in the first four shells of KZnF₃ containing Dy³⁺ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F^- ions has been carried out. For the Dy³⁺ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanism of rare-earth ion-ligand coupling. 

A recent investigation of the (BaF₂–MgF₂) phase diagram produced several new compounds which are suitable hosts for Rare Earth impurities. We present results on single crystals of Ba₂Mg₃F₁₀ doped with Eu²⁺. The local structure and optical properties of this system were investigated by luminescence emission and by EPR. We observed two different Eu²⁺ sites. Both show C_s point symmetry and an important ground state splitting. Correlating our EPR and optical results with the new Ba₂Mg₃F₁₀ structure data allowed the assignment of each of them to a specific barium lattice site. The luminescence emission of both the 4f⁷–4f⁶5d and the 4f⁷–4f⁷ transitions is observed. The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell ⁶P_7/2→⁸S_7/2transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f⁶5d configuration are approximately degenerate with the ⁶P_7/2 manifold.

Eu²⁺ was introduced into pure and oxygen codoped BaMgF₄ single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is C_s. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba_0.8Eu_0.2MgF₄ single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu²⁺EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm⁻¹. The results due to the Eu²⁺ impurity are given and discussed qualitatively within the 4f⁷ ↔ 4f⁶5d¹ scheme.

Accurate dissociation energies were determined for gas-phase complexes between 1-naphthol and benzene, d₆-benzene and cyclohexane, using the stimulated emission pumping resonant two-photon ionization spectroscopy technique in supersonic jets. The dissociation energies obtained for the electronic ground state are surprisingly large being D₀ = 5.07±0.07 kcal/mol for 1-naphthol · benzene, 5.08±0.06 kcal/mol for 1-naphthol · d₆-benzene, and 6.92±0.03 kcal/mol for 1-naphthol · cyclohexane, respectively. The dissociation energies scale well with the parallel molecular polarizabilities.

Molecular modelling and site-directed mutagenesis were used to identify eleven amino acid residues which may be involved in antagonist binding of the human tachykinin NK1 receptor. Recombinant receptors were expressed in mammalian cells using the Semliki Forest virus system. Wild type and mutant receptors showed similar expression levels in BHK and CHO cells, verified by metabolic labelling. Binding affinities were determined for a variety of tachykinin NK1 receptor antagonists in SFV-infected CHO cells. The binding affinity for GR203040, CP 99,994 and CP 96,345 was significantly reduced by mutant Q165A. The mutant F268A significantly reduced the affinity for GR203040 and CP 99,994 and the mutant H197A had reduced affinity for CP 96,345. All antagonists seemed to bind in a similar region of the receptor, but do not all rely on the same binding site interactions. Functional coupling to G-proteins was assayed by intracellular Ca2+ release in SFV-infected CHO cells. The wild type receptor and all mutants except A162L and F268A responded to substance P stimulation.

The tetranuclear complexes {(μ₄-TCNX)[Ru(NH₃)₅]₄}(A)₈ and (μ₄-TCNX)[Mn(CO)₂(C₅Me₅)]₄ [A = PF₆ or CF₃SO₃; TCNX = TCNE (tetracyanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2,4,5-tetracyanobenzene)] were studied by variable-temperature (2−300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH₃)₅](PF₆)₂ (PhCN = benzonitrile) and {(μ-L)[Ru(NH₃)₅]₂}(PF₆)₄ (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin−spin exchange coupling patterns. The manganese systems are characterized by exchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = ¹/₂ sites, identified as Ru^III/Ru^II mixed-valence pairs.

A full configuration interaction study on the BH molecule is presented. The potential energy curves of 20 different electronic states have been calculated correlating the four valence electrons. On the two most important states, i.e. the X1Sigma+ and A1Pi states, a complete study has been performed. This includes the effect of core electron correlation, estimated via truncated configuration interaction techniques. The dissociation energy of the molecule in the two states and the height of the predissociative barrier in the A1Pi state have been determined with basis sets of increasing quality.

We present a method for the direct generation of the lists of strings, suited for integral-driven full-CI (FCI) algorithms. This method generates the string lists each time they are used, and hence sensibly reduces the memory requirements, compared to our previous method that precalculates the lists. It was also extended to permit a truncation of the string space, according to the level of excitation.

Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

We present results of a first-principles molecular-dynamics simulation of the vibrational spectrum of a model semiconductor surface where, close to the bulk melting temperature TM, the top bilayer becomes diffusive. The specific example chosen is Ge(111). In place of the ordinary surface vibrations, we obtain a spectrum more akin to that of a confined liquid. In particular, we predict a dramatic decrease in the intensity of high-frequency (bond-stretching) surface mode intensity, associated with a local melting-induced breaking of hard covalent bonds in the top bilayer.

It is shown that whereas the spherical and spheroidal jellium models are inadequate to describe lithium clusters, only the ellipsoidal jellium model is adequate. The corresponding result, obtained by Yannouleas and Landman, was unpublished at the time of submission of our paper.

IR spectra of anthracene and pyrene derivatives, serving as models for isolated, linear and isolated, compact PAHs, respectively, have been calculated using ab-initio quantum mechanical methods. The separate and combined effects of ionization and multiple dehydrogenation have been studied. This study confirms and refines the trends of our preliminary paper on the smallest possible PAH, naphthalene. If small PAHs are responsible for any UIR bands, they should be ionized and partially dehydrogenated, with a few triple bonds at the periphery of the carbon skeleton. In the appendix are given the complete IR spectra of all the isomers of the derivatives of anthracene and pyrene calculated for the purpose of this study. Tables I are for anthracene and Tables II for pyrene. Positions of the the missing hydrogens in the dehydrogenated species are referred as in Figures 1 and 2 of the original publication.

In orthorhombic Ba6Mg7F26, the Ba2+ ion can be partially replaced by Sr2+ to form a mixed, disordered compound. This new compound has a refined composition of Ba5.24 (4)Sr0.76 (4)Mg7F26. The volume decreases after the partial substitution from 1074.64 (10) to 1064.31 (11) Å3. The structure has two barium sites, Ba1 on Wyckoff site 8(l) with Cs symmetry and a coordination number of 12 + 1, and Ba2 on site 4(j) with C2v symmetry and a coordination number of 12; only the Ba2+ on site 4(j) is partially replaced by Sr2+. The distorted octahedral fluoride environment around magnesium shows a tendency to become more irregular in the Sr-substituted compound.

The crystal structures of cubic and tetragonal NiCr2O4 and of tetragonal CuCr2O4 have been refined and their cell parameters have been measured, using single-crystal and powder X-ray diffraction. It has been observed that the differences in the Jahn–Teller distortions in both compounds are reflected in their M2+ –O tetrahedral environments and in their cell parameters. A calculation of the atomic shifts during the tetragonal–cubic phase transition showed that the average shifts for the oxygen atoms are 0.10(1) and 0.04(1) Å for the nickel and chromium atoms, respectively. Crystals of both compounds jump when they go through the phase transition. This behaviour is especially spectacular for NiCr2O4 since the phase transition takes place at 320 K, this temperature being reached when the crystals are illuminated. A comparison with organic jumping crystals is presented, and characteristics of the chromite crystals are discussed.

In order to provide a possible explanation for the lack of detection of both HSiN and HNSi in the interstellar medium, an ab initio study of the Si+ + NH3 reaction is presented: it includes accurate energetic considerations and sketches dynamics discussions as well. It is unambiguously concluded that the X1A1 ground state of the SiNH2+ cation is the only exit channel of this reaction assuming interstellar conditions. The rotational and vibrational constants of this species are reported to stimulate its experimental and astrophysical searches. Upon dissociative recombination, it is likely that SiNH2+ can evolve toward HNSi: unfortunately, the dramatic weakness of the dipole moment of the latter species (0.05 D) makes it an unlikely candidate for today's radiotelescopes. At variance with HNSi, the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysical searches, but under interstellar conditions, we show that it can derive neither from the unimolecular HNSi ↔ HSiN equilibration nor from the Si+ + NH3, N + SiH3+ or N+ + SiH3 reactions as sometimes incorrectly stated in the astrophysical models that deduce interstellar silicon chemistry from that of carbon. Throughout this study, the very hazardous character of conclusions deduced from isoelectronic considerations should be considered as the leading feature: the finishing stroke to such isoelectronic analogies is given by our study of the H+ + HNSi ↔ HSiN + H+ reactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes.

A helium gas pressure cell for pressures up to 1 kbar (0.1 GPa) has been developed in conjunction with a closed-cycle He refrigerator allowing variable temperatures between 15 and 300 K. Both cell and refrigerator are equipped with optical windows suitable for photophysical measurements, such as temperature- and pressure-dependent absorption spectroscopy or laser flash photolysis. Examples of measurements on iron(II) spin-crossover systems are given. In these compounds, comparatively small external pressures induce significant changes in the thermodynamic equilibrium as well as in the relaxation dynamics.

The interaction of H2 with the Si(111)-(7 × 7) surface is investigated by means of density functional slab calculations on a (4 × 2) reconstructed model surface. A viable mechanism for β1 desorption is identified, which involves thermally activated SiH2 units at adatom sites. This mechanism leads to adsorption and desorption barriers of 1.0 and 2.4 eV, respectively, in agreement with experiment. An explanation for the two components observed in the β1 peak of temperature-programmed desorption spectra is proposed. The lattice deformation energy at the transition state for desorption is large (not, vert, similar0.6 eV). If we assume that this remains in the surface after H2 desorption, the low translational energy of desorbing molecules which has been observed experimentally can in part be explained.

A local density functional study of Si41O( Al31, H1)-substituted offretites is presented. Proton siting and dynamical properties are investigated within the First-Principles Molecular Dynamics method, using a periodically repeated unit cell. Results for monoaluminated offretites, e.g., with one Al per unit cell, show that the proton is located inside the channel of the zeolite, where it is accessible to incoming molecules for reaction. Calculated vibrational spectra of the framework, extracted from a dynamical simulation, reproduce experimental data well. The determined OH stretching frequencies show a rather weak dependence on the H1 position. A comparison of these frequencies with those of offretites containing three Al per unit cell does not indicate a significative chan

The crystal structure of the title compound, [Pd2Br2(C25H34P)2], a new binuclear phospha-alkene compound containing a trivalent P atom shows a centrosymmetric dimeric arrangement. The Pd2Br2 core is planar and adopts an irregular diamond shape. The coordination of the Pd atom is square planar. No stacking interactions were observed in the molecular packing.

An hybrid functional which includes a larger amount of pure exchange has been specially designed for the description of hydrogen-only systems. Both the H abstraction by H2 and the H2n + 1+ clusters have been investigated. Comparison with experimental values shows that the proposed functional gives dissociation energies and vibrational frequencies better than previous ab initio calculations. The results compare favorably with those obtained by a coupled clusters method [CCSD(T)], also performed in this work for sake of reference data.

Temperature-dependent Raman measurements between 190 and 358 K yield conformational enthalpy differences between 550 and 690 cal mol⁻¹ for racemic liquid 2-butanol. The conformational properties of 2-butanol were also studied using MM2 and MM3 calculations. The conformer 1, where C1 and C4 are in anti position, was found to be the most stable comformer with the MM3 calculations. Conformer 2, which has C1 and O5 in anti arrangement, has ca. 500 cal mol⁻¹ higher energy than 1. Comparisons of the calculated MM3 vibrational frequencies with the Raman spectra suggest that the most stable conformer in the liquid phase also adopts a Cl,C4 anti conformation.

The present investigation is concerned with the possible effects of material-related properties (molecular mass, glass transition and melting temperatures, crystallinity, tacticity) and particle-related properties (shape, size, specific surface area) on the compression characteristics of the chosen model polymer powder: poly(vinyl chloride) (PVC). Four grades were selected known in literature for providing compacts of varied mechanical strength. The compression characteristics were determined using an instrumented single-punch tableting machine. The differences in tableting characteristics could not be ascribed to any of the material-related properties, but a direct relationship was observed between the compact strength and the specific surface area of the particles, as measured by nitrogen adsorption. The compact hardness was thus only dependent on the inter- and the intraparticulate contact area, which in turn is dictated by the very peculiar morphology of the grains of the PVC powders, whether prepared by emulsion or suspension polymerization.

Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

A mechanism for SiCl2 formation and desorption in the etching of Si(100)-(2×1) at low chlorine coverages is analyzed using first-principles calculations. We find that the two monochlorinated Si atoms of a surface dimer can rearrange into a metastable SiCl2(a) adsorbed species plus a Cl-free Si atom. Desorption of SiCl2 occurs via a two-step mechanism, in which the adsorbed species is preliminarily stabilized by the diffusion away of the free Si atom. The energy barrier to form SiCl2(a) is lower on a dimer next to a dimer vacancy than in an undamaged region of the surface, consistent with recent STM observation of preferential linear growth of etch pits along dimer rows.

The growth of thin films made from Samarium-doped alkaline earth fluoro halides (AEFH) of composition SrxCa1−xFCl:Sm2+ (0 ⩽ x ⩽ 1) is presented and the possibilities are studied to increase significantly the inhomogeneous width of the Sm2+ optical zero phonon transitions. The best films were obtained when grown with a molecular beam deposition (MBD) method involving two separate molecular beams: one for the alkaline earth fluoride, the other one for the alkaline earth halide (Cl or Br). The results demonstrate that the double beam MBD technique employed is able to produce pure and mixed Matlockite films with targeted composition. The results of mainly optical studies of the samarium f–f transitions and of other complementary techniques are used to assess the composition and homogeneity of the films. With the aid of a model the composition dependence of the positions of specific optical f–f emission lines is established. Their inhomogeneous linewidth is compared with that of corresponding emission lines obtained from bulk samples of the same chemical composition. The linewidths of the films are only slightly larger ( ∼ 1.5–2 times). Thus, the film morphology cannot be exploited to increase substantially the inhomogeneous broadening of the luminescence lines. A novel approach to increase this broadening was devised, theoretically modeled and successfully tested by using multilayered sandwich-type thin films in conjunction with interdiffusion. Films with cation disorder of composition SrxCa1−xFCl (x = 0.5 /0/ 0.5/ 0/..) were grown. The 5D1→7F0 Sm2+ emission linewidth is thereby increased to 70 cm−1 full width half maximum. A width of 100cm−1 may be obtained within the composition range x = 0, x = 1. This represents an enhancement by a factor of 3–5 in comparison with the largest values obtained in appropriate mixed bulk AEFH of constant composition. A factor >50 is gained in comparison with pure bulk AEFH hosts. The room temperature (RT) homogeneous linewidths, on the other hand, are similar to those found in bulk mixed crystals of constant composition. The intrafilm host cation diffusion during film growth of the sandwich structures was further studied. A diffusion constant of 2⋅10−19m2s−1 for the Sr and Ca ions was deduced from this observation. These films are among the most promising materials for optical mass data storage through RT hole-burning.

Single crystal X-ray diffraction analysis was performed on crystals with composition Ba_{1 − δ}Sr_δMgF₄ (δ ≤ 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of δ = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(II) doped samples show significant inhomogeneous line broadening and confirm the disorder On the Ba Site.

Laser flash photolysis experiments were performed on the mixed crystal [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ (x=0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude.

Applicability of the approximate kinetic energy functionals to study hydrogen-bonded systems by means of the formalism of Kohn–Sham equations with constrained electron density (KSCED) [Cortona, Phys. Rev. B 44, 8454 (1991); Wesołowski and Warshel, J. Phys. Chem. 97, 8050 (1993); Wesołowski and Weber, Chem. Phys. Lett. 248, 71 (1996)] is analyzed. In the KSCED formalism, the ground-state energy of a molecular complex is obtained using a “divide-and-conquer” strategy, which is applied to the Kohn–Sham-like equations to obtain the electron density of a fragment embedded in a larger system. The approximate kinetic energy functional enters into the KSCED formalism in two ways. First, the effective potential in which the electrons of each fragment move contains a component which is expressed by means of a functional derivative of an approximate kinetic energy functional (functional derivative of the non-additive kinetic energy). Second, the KSCED energy functional contains a component (non-additive kinetic energy) which is expressed using the approximate kinetic energy functional. In this work, the KSCED energies and densities of (H₂O)₂, (HF)₂, (HCl)₂, and HFNCH are compared to the ones obtained using the standard supermolecule Kohn–Sham approach. The following factors determining the agreement between the KSCED and supermolecule Kohn–Sham results are analyzed: the analytical form of the gradient-dependent terms in the approximate kinetic energy functional and the number of atom-centered orbitals used to expand electron density of fragments. The best agreement between the supermolecule Kohn–Sham and the KSCED results is obtained with the kinetic energy functional derived following the route of Lee, Lee, and Parr [Lee et al., Phys. Rev. A 44, 768 (1991)] from the exchange functional of Perdew and Wang [Perdew and Wang, in Electronic Structure of Solids ’91, edited by P. E. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11]. The difference between the KSCED and the supermolecule Kohn–Sham energies of studied complexes amounts to less than 0.35 kcal/mol at the equilibrium geometry.

The edge-bridged tetrahedral geometry of five-coordinate d0 complexes [MD2L3] with strong π-donors D is analyzed with extended Hückel methodology as well as density functional theory. It is shown that this geometry, also encountered in bent metallocene systems [MCp2L3], can be considered as a distortion of a regular trigonal bipyramid arising from a second-order Jahn−Teller distortion of e‘ symmetry (in D3h) and corresponds to a deformation along a reversed-Berry pathway. This model was tested with a structure-correlation analysis of all experimentally determined [MD2L3] structures, thus allowing a mapping of the reversed-Berry pathway. The catalytic potential of these complexes and their isolobal relationship to [MCp2L3] are emphasized.

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H2O-ax)(H2O-eq)4L]2+ general formula. We have studied a series of complexes, where L = H2O, MeCN, Me2SO, H2CCH2, CO, and F2CCH2. We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E°‘(+2/+3), increases with the π-accepting capabilities of the ligands. For L = N2, the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)−N2 bond. The water exchange reactions have been followed by 17O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H2O)5(H2CCH2)]2+ (ΔVax and ΔVeq = +6.5 ± 0.5 and +6.1 ± 0.2 cm3 mol-1) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F2CCH2 CO < Me2SO < N2 < H2CCH2 < MeCN < H2O. The increase of the lability is accompanied by a decrease of the E°‘ values, but no change was found in the calculated Ru−H2Oeq bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N2 MeCN < H2O < CO < Me2SO < H2CCH2 < F2CCH2. A variation of the Ru−H2Oax bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru−H2Oax bond energies. It appears, also, that a decrease of the electronic density along the Ru−Oax bond and the increase of the lability can be related to an increase of the π-accepting capability of the ligand. For L = N2, the calculations have shown that the Ru(II)−N2 bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N2 ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H2O)5N2]2+ complex.

As recently reported by Klopper and Lüthi, there is a discrepancy between experiment and high-level quantum chemical calculations as to the value of the heterolytic metal–ligand bond disruption enthalpy of ferrocene. Indeed their ab initio calculations lead to a best estimate of 655 kcal/mol, whereas the experimental value is 635 kcal/mol. We report here results obtained using density functional theory. In addition to ferrocene, other metallocenes such as vanadocene, manganocene, nickelocene and ruthenocene, have also been investigated. Gradient-based corrections are crucial for a quantitative description of bond dissociation, our best estimate for ferrocene being 663 kcal/mol.

We have used density functional theory, both within the local density (LDA) and generalized gradient (GGA) approximations, to study the structure, energetics, and vibrational properties of zeolite offretite in the presence of different monovalent cations (H+, Na+, K+, and Cu+). We find that the spatial locations of the most favorable cation-binding sites are similar for the different cations, being related to the minima of the electrostatic potential. However, the relative stability of the sites does depend on the nature of the counterion, as well as on the Al/Si ratio and on the mutual interactions between cations. At low Al/Si ratios, the preferred site for H+ is in the channel, where it is accessible for reaction with incoming molecules. For both Na+ and Cu+, the most stable site is within the 6-fold ring of the gmelinite cage, but for Na+, two other sites are present within a few tenths of a kilocalorie/mole from the lowest site (small site selectivity). For K+, two sites, one inside the cancrinite cage and the other near the 8-fold ring of the gmelinite cage, are very close in energy, consistent with the X-ray experiments on natural hydrated and dehydrated offretites. Dynamical simulations have been carried out for H− and Na−offretite. The vibrational spectrum of the framework agrees well with the available experiment. OH stretching frequencies calculated for a number of different H+ locations show that more "open" positions, e.g., in the channel, have higher frequencies, in agreement with experiment.

Although density functional theory (DFT) is more and more commonly used as a very efficient tool for the study of molecules and bulk materials, its applications to weakly bonded systems remain rather sparse in the literature, except studies that consider hydrogen bonding. It is, however, of essential interest to be able to correctly describe weaker van der Waals complexes. This prompted us to investigate more precisely the reliability of several widely-used functionals. The equilibrium geometries and the binding energies of C₆H₆···X (X = O₂, N₂, or CO) complexes are determined within the standard Kohn−Sham approach of DFT using different exchange−correlation functionals and at the MP2 level of theory for comparison. It is comprehensively concluded that extreme care must be taken in the choice of the functional since only those that behave properly at large and intermediate values of the reduced density gradient s give relevant results. The PW91 exchange functional, the enhancement factor of which does not diverge at increasing s, appears as the most reliable for the studied systems. It is furthermore demonstrated that the quality of the DFT results is determined by the exchange energy component of the total energy functional.

Quantum chemical calculations have been performed using density functional theory to model the mechanism of selective catalytic reduction of NO by NH3 on vanadium oxide. The reaction is initiated by NH3 adsorption on a Brønsted site modeled as a dimer cluster model representative of vanadium oxide, containing a terminal VO adjacent to a V−OH group. The calculations indicate that the adsorbed NH3 behaves as NH4+, which is supported by calculated IR spectra. Subsequently NO reacts with this activated NH3 to yield NH2NO and finally the reaction products N2 and H2O. The present results give support to a dual-site Eley−Rideal-type mechanism involving a Brønsted site and agree with isotopic labeling studies.

The enantioselective hydrogenation of ketopantolactone toR-(−)-pantolactone was investigated on 5 wt% Pt/Al2O3chirally modified with cinchonidine. The influence of catalyst pretreatment conditions, hydrogen pressure, temperature, solvent polarity, and catalyst, reactant, and modifier concentrations was studied in a slurry reactor. An enantiomeric excess (ee) of 79% at full conversion was achieved in toluene after optimization of pressure, temperature, and amount of modifier. Good ee could be obtained only after rigorous removal of traces of oxygen and water during catalyst pretreatment and from the hydrogenation reaction mixture. Molecular modeling studies (performed using molecular mechanics, semiempirical, andab initiomethods) provided a feasible structure for the diastereomeric transition complex formed between cinchonidine and ketopantolactone and an explanation for the observed enantiodifferentiation in apolar medium. The calculations indicate that formation of the complex affordingR-(−)-pantolactone is energetically favored with cinchonidine, whereas the near enantiomer cinchonine favorsS-pantolactone, in agreement with experimental observations. Interestingly, in apolar solvents, where the alkaloid modifier is not protonated, the modeling suggests similar structures for the diastereomeric transition complexes for the hydrogenation of ketopantolactone and methyl pyruvate.

QSAR concerning the anti-HIV and cytotoxic activities of a series of HEPT analogues has been established using a Hansch-type approach (TSAR™), a neural network approach (TSAR) and a pharmacophore search method (CATALYST™). The techniques employed allowed reliable activity predictions and confirmed the heterogeneity of this series of compounds, which was previously established in biochemical experiments.

Quantum chemical calculations using the density functional theory were performed to model the mechanism of selective catalytic reduction of NO by NH3 on a supported vanadium oxide monolayer. In the first step, the adsorption of NH3 on a bimetallic cluster representative of vanadium oxide, containing a terminal VO adjacent to a VOH group, was investigated. The calculations indicate that NH3 may be strongly adsorbed on VOH (Brönsted acid site) as NH+4(ads); subsequently, NO reacts with this activated NH3 to yield the reaction products N2 and H2O. The present results give support to a dual-site Eley-Rideal-type mechanism involving a Brönsted site.

The purpose of the present work was to develop a method allowing one to extract the information needed for the construction of the internal chemical hardness tensor at the molecular orbital level from standard density functional calculations. This method is based on the Janak theorem and on the extension of the Slater transition-state concept. A detailed discussion of the current ideas about the validity of the Janak theorem is presented as well as of the established relations of this subject with the ensemble V-representability problem. The internal chemical hardness tensor has been obtained for water molecule as an example system. Its structure is consistent with the criteria for the internal molecular stability.

The antimalarial activity of a series of synthetic 1,2,4-trioxanes is correlated with molecular structure by using a pharmacophore search method (CATALYST). The technique is shown to have predictive accuracy and confirms that docking between an active trioxane and the receptor, heme, is the crucial step for drug action.

We analyze differences between ground-state electron densities of a model molecular complex obtained by solving the Kohn-Sham equations with constrained electron density (KSCED) [Wesolowski and Warshel, J. Phys. Chem. 97, 8050 (1993)] and as a solution of the standard Kohn-Sham equations applied to the whole complex. The differences between KSCED and KS electron densities which result from the inaccuracy of the kinetic energy functional applied in the KSCED equations are discussed. The model molecular complex consists of collinear H₂ and HCN molecules separated by a short distance. Several kinetic energy functional approximations are applied within the KSCED framework and the differences between resulting electron densities are studied. The KSCED electron densities depend directly on the functional derivative of the kinetic energy functional applied in the KSCED equations. Among the studied functionals, the one proposed by Zhao et al. [Phys. Rev. A 47, 918 (1993)] and the gradient-dependent functional proposed by Perdew and Wang [Phys. Rev. B 33, 8800 (1986)] led to the smallest differences between KS and KSCED electron densities, thus having the most accurate functional derivative. Both functionals allow one to extend the range of applicability of the KSCED equations to system geometries where an overlap of electron densities of the partners of the complex is significant.

The iron(II) spin-crossover compound [Fe(ptz)₆](PF₆)₂ (ptz = 1-propyltetrazole) crystallizes in the triclinic space group P†, with a = 10.6439(4) Å, b = 10.8685(4) Å, c = 11.7014(4) Å, α = 75.644(1)°, β = 71.671(1)°, γ = 60.815(1)°, and Z = 1. In [Fe(ptz)₆](PF₆)₂, the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, ΔV°_HL, is 24(2) ų/molecule. The interaction constant Γ, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm^-1. In contrast to the case of the related compound [Fe(ptz)₆](BF₄)₂ (Decurtins et al. Inorg. Chem. 1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn_1-xFe_x(ptz)₆](BF₄)₂, x = 0.1 (Jeftić et al. J. Phys. Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)₆](PF₆)₂, the interaction constant Γ is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin → low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation by 1 order of magnitude.

In the iron(II) spin-crossover compound [Fe(ptz)₆](BF₄)₂, the thermal spin transition is accompanied by a crystallographic phase transition showing a hysteresis with T_c^↓ = 128 K and T_c^↑ = 135 K at ambient pressure [Franke, P. L.; Haasnot, J. G.; Zuur, A. P. Inorg. Chim. Acta 1982, 59, 5]. The hysteresis is due to an interplay between the spin-transition and the R³ → P† crystallographic phase transition with a large low-spin fraction stabilizing the P† phase at low temperatures. In the mixed crystal [Zn_1-xFe_x(ptz)₆](BF₄)₂, x = 0.1, with the iron complexes imbedded into the isomorphous zinc lattice, the crystallographic phase transition can be induced by an external pressure [Jeftić, J.; Romstedt, H.; Hauser, A. J. Phys. Chem. Solids 1996, 57, 1743]. Thus the P† phase is additionally stabilized by external pressure. The interaction constant Γ, which describes cooperative effects between the spin-changing complexes, differs for the two crystallographic phases. Values for Γ(P†) of 144(8) cm^-1 and the volume difference ΔV⁰_HL of 29(4) ų are determined from a simultaneous fit to a series of transition curves for different pressures and iron content x in the P† phase. These values are compared to the corresponding values for the R³ phase, viz. Γ(R³) of 170(9) cm^-1 and ΔV⁰_HL(R3) of 26(3) ų. Surprisingly Γ(R³) is larger than Γ(P†) despite the fact that ΔV⁰_HL(R3) is smaller than ΔV⁰_HL(P1). The high-spin → low-spin relaxation at temperatures above ~80 K is thermally activated, while below ~40 K temperature independent tunnelling takes place. An external pressure of 1 kbar accelerates the high-spin → low-spin relaxation exponentially by 1 order of magnitude in the tunnelling region in both crystallographic phases and regardless of x. In the concentrated material the high-spin → low-spin relaxation is self-accelerating due a buildup of an internal pressure [Hauser, A. Chem. Phys. Lett. 1992, 192, 65]. Both cooperative effects and external pressure result in a shift of the maximum of the ¹A₁ → ¹T₁ absorption band.

A spectroscopic and kinetic study of the photoinduced electron transfer (ET) reaction between 9,10-dicyanoanthracene (DCA) and 1-methylnaphthalene (MNA) in acetonitrile using the transient grating technique is reported. Apart from the bands assigned to ¹DCA* and DCA^•-, the transient spectrum exhibits a band located at 580 nm and ascribed to MNA₂^•+. This species is generated upon reaction of a second donor molecule with a 1:1 complex to form a 1:2 complex (DCA^•-·MNA₂^•+). Using chloranil as electron acceptor, the rate constant of this reaction has been measured to be 6.8 × 10⁹ M^-1 s^-1. From the donor concentration dependence of the kinetics of DCA^•- diffraction intensity and of the free ion yield, the rates of back ET to the ground state within the 1:1 and the 1:2 complexes have been determined to be equal to 1.8 × 10⁸ and 16.9 × 10⁸ s^-1, respectively, while their rate constants of separation into free ions are 1.6 × 10⁸ and 1.1 × 10⁸ s^-1, respectively. These values have been obtained assuming a reaction scheme in which both forward and backward ET essentially take place at contact distance. In the case of the 1:1 complex, however, charge recombination within a loose ion pair cannot be ruled out.

The luminescence of the scorpion's outer shell has been shown to be due to fluorescence of very short lifetime (nanoseconds). The emission and excitation spectra have been determined, and the potential biological significance of this photoluminescence is discussed.

The reliability of the transient grating technique to determine the energetics of processes following second order kinetics, such as free ion recombination, is investigated theoretically. As the second order process takes place, the grating of the reactant population becomes anharmonic, complicating the determination of the population dynamics from the time profile of the diffracted intensity. However, the thermal phase grating is anharmonic from the beginning of the reaction, but is harmonic when the process is completed. As a consequence, the energetics of the reaction can be deduced accurately from the maximum amplitude of the diffracted intensity without having to take the anharmonicity of the grating into account, as long as thermal diffusion is slow. To achieve this, the crossing angle of the pump pulses on the sample has to be smaller than 1°. Otherwise, an arduous fitting procedure is required to extract the energetic parameters from the experimental data.

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe^•+), of perylene radical anion (Pe^•-), and of anthraquinone radical anion (AQ^•-) is reported. In boric acid glass, the excited-state lifetime of Pe^•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe^•+, Pe^•-, and AQ^•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe^•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA^•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA^•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe^•+* could be observed.

Cyclic voltammetry of phosphafulvene and dibenzophosphafulvene shows that in DMF these compounds are reduced at -1.200 and -1.349 V, respectively. The EPR spectra of the corresponding radical anions, formed by electrochemical reduction or by reaction on a potassium mirror, are Recorded between 110 K and room temperature. The g and 31P hyperfine tensors are measured and compared to those previously obtained for a phosphaalkene radical anion. Abinitio investigations on model phosphaalkene and phosphafulvene radical anions show that, in accord with the experimental results, the electronic structure of these two species are quite different: whereas the unpaired electron is delocalized on the whole PC(H)R moiety in the phosphaalkenic anion, it is markedly localized on the phosphorus atom in the phosphafulvene anion. Calculated spin densities and charge distributions for phosphafulvene and azafulvene anions are compared.

Two phosphaalkenes containing either a furane or a thiophene ring bound to the carbon atom of the −Pdouble bond; length as m-dashC < bond have been synthesized. The crystal structure of the furane derivative has been determined and the electrochemistry of both compounds has been investigated. THF solutions of these compounds react at 255 K with a potassium mirror to yield the corresponding radical anions which have been studied by EPR in both the liquid and solid states. The resulting hyperfine constants are compared with the values predicted by ab initio calculations on radical anions formed from model phosphaalkenes.

The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn–Teller distortion of allene. The calculated hyperfine constants support the EPR results.

Several radiation defects have been detected by EPR in a single-crystal of Pt(dmimt)4Cl2.4H2O (dmit = 1,3-dimethyl-imidazoline-2-thione). In order to identify these rediogenic species, the structure of the crystal has been resolved and the angular dependence of the EPR signals has been analysed. The resulting g tensors and 195Pt hyperfine tensors have been determined and the orientations of their principal axes have been compared to those of the bond directions of the precursor. It is shown that both Pt(I) and Pt(III) complexes are trapped, whereas Pd(dmimt)42+ present as an impurity, leads only to the Pd(I) species. The temperature dependende of the EPR spectra gives information about the relative stability of the paramagnetic species and shows that the formation of some species, especially the Pt(III) complexes, requires drastic modifications of the parent Pt(II) cation.

We study the ionic conductivity versus temperature and frequency of large Na²S single crystals by using a calibrated impedance apparatus. The experimental setup used for the ionic conductivity measurements up to 1350 K and its calibration are described. The apparatus allows to measure complex impedances between 0.1 Ω and 10 GΩ. The high temperature conductivity data were analyzed in terms of the conventional Frenkel defect model. We assume that cation vacancies and cation interstitials are the dominant intrinsic defects. The energy of motion was found to be 0.61 ± 0.05 eV for a cation vacancy. The energy of formation of a Frenkel defect pair is 2.51 ± 0.05 eV. Results are given that show clear evidence of a superionic behaviour close to the melting point, similar to the one found in alkaline earth fluorides and several halides. Furthermore, X-ray diffraction experiments on a high optical quality single crystal were performed. The cell parameter and the population parameter of Na⁺ were accurately determined (6.5373 Å and 0.988, respectively).

The results of a detailed optical and paramagnetic-resonance study performed on copper in CaF₂ and silver in BaF₂ are presented. Two different Ag⁺ centers were identified in BaF₂. One is associated with an interstitial F^- ion whereas the other one has a cubic surrounding. The Cu²⁺ ion in CaF₂ was shown to reorient at 4.2 K between 6 equivalent minima of D_2h symetry. This fact is interpreted with the aid of a T_2gx(T_2g+E_g) type Jahn-Teller effect. The nonlinear mixed coupling terms are shown to play an important role. The Cu⁺ impurity in CaF₂ is presumably off-center in the F^- sublattice without associated defect or impurity.

A detailed analysis of Zeeman splittings of highly resolved spin-forbidden transitions in [Cr(bpy)₃](PF₆)₃ is presented. Assignments of vibronic bands are made based on low-temperature absorption, emission, and infrared spectra. The pattern of doublet states, obtained for H = 0 and H = 5 T, is consistent with angular overlap model (AOM) calculations, which allow one to consider σ- and π-interactions between the metal-d and relevant ligand orbitals and the particular angular geometry of the chromophore simultaneously. The observed level splittings are found to result from the combined effect of trigonal distortion and contributions of the symmetry adapted d_π-orbitals involved due to coupling with corresponding counterparts from the bidentate ligand (phase coupling). The larger splitting of the lowest excited state ²E_g(O_h) in the analogous ClO₄^- salt is due to the more distorted geometry of the [CrN₆] moiety. Related properties of the bipyridine ligand, which turn out to show donor behavior in the present compounds, and the acetylacetonate ligand are discussed, and AOM parameters for the metal−ligand π-interaction are correlated with results of MO calculations.

Complete Active-Space Self-Consistent-Field (CAS-SCF) calculations for cubic N8 are presented. We studied the N8↔4N2 reaction inD 4h symmetry and found its energy release and activation barrier with three different atomic basis sets. The energy release for this reaction is predicted to be around 526 kcal/mol, while the energy barrier to dissociation is estimated about 159 kcal/mol. These results are in substantial agreement with previousab initio estimates.

Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and iron sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.

A new member belonging to the binary phase diagram of BaF₂ and BaCl₂ was synthesized. The single domain crystals of Ba₁₂F₁₉Cl₅ can be prepared from a nonstoichiometric flux with molar ratio of 1 : 1 between BaFCl and BaF₂. The compound crystallizes at room temperature in the non-centrosymmetric hexagonal space group P62m with a = b = 1408.48(14) and c = 427.33(5) pm. Three different barium environements with coordination number of nine are found. The barium fluorine distances vary between 250.59(6) - a short distance compared to other Ba — F distances - and 302.7(1) pm and barium chlorine distances between 331.55(3) and 336.19(15) pm. This compound is further characterized using Raman spectroscopy.

In the structure Ba₁₂F₁₉Cl₅ [hexagonal space group P62m] the two chlorides on the sites Cl(1) and Cl(2) can partially be replaced by bromide ions. Single crystals of the type Ba₁₂F₁₉Cl_δ Br_5-δ  with a chloride to bromide ratio up to 2 : 3 could be obtained by cooling a flux of 75 mol% BaF₂ and 25 mol% BaX₂ with X = Cl, Br. The crystal quality decreases with increasing bromide concentration. Structural parameters of five selected single crystals with different chloride/bromide ratio were studied by single crystal X-ray diffraction methods. The refined total Cl^-/Br^- population ratio in the crystals is close to the one of the flux. The lattice parameters and interatomic distances change in various ways, when the smaller chloride ion is replaced by the bigger bromide ion. The refinements show a statistical disorder on the halide sites with preferential bromide substitution on site Cl(1).

Resonant fluorescence line narrowing of the R₁ line of the [Cr(ox)₃]³⁻ chromophore in [Rh(bpy)₃][NaCr(ox)₃]ClO₄ at 1.6 K neither gives rise to the usual three-line pattern nor to spectral diffusion. Instead multi-line spectra with spacings equal to the zero-field splitting of the ground state are observed. This phenomenon is attributed to efficient non-radiative resonant energy transfer within the R₁ line.

An overview of the possibilities brought by the density functional theory to the coordination chemistry is given. The new trends in the development of the theory are outlined.

A bis-cyclometallating ligand bearing two different terdentate coordination sites (N---C---N: dipyridyI-benzene; P---C---P: diphosphaalkenebenzene moieties) has been synthesised. Selective reactions of appropriate metal complex precursors afforded a heterodinuclear ruthenium(II)-palladium(II) complex characterised by 1H, 13P NMR spectroscopy and FAB-MS techniques. We have compared its electrochemical and spectroscopic properties (absorption and emission) with the individual ruthenium(II) and palladium(II) subunits.

In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble.

The characteristics of natural organic matter (NOM), which affect its interactions with and therefore the transport of colloids and particles, depend on the evolution of its individual components. Information on the temporal variations of the components of the organic matter of a well-studied eutrophic lake (Bret, VD, Switzerland) was extracted using pyrolysis/gas chromatography/mass spectrometry. The influence of aquagenic organic matter was found to be at a maximum in summer. The soil-derived organic matter, on the other hand, was found in larger proportions in winter and spring. The appearance of hydroxypropanone in the pyrolysis fragments in the summer months indicates that this portion of the organic matter is aquagenic and very fresh. The dominance of furaldehyde in pyrolysates during the rest of the year indicates the presence of polysac charides may be of either aquagenic or pedogenic origin. The absence of lignin fragments found in the NOM in this lake suggests that lignin-containing components of terrestrial organic matter are not leached out from the soil in significant quantities. A correlation was found between 3-day cumulative rainfall and the proportion of terrestrial components in the identified organic matter in the spring. This correlation disap peared in the summer, probably because of a higher vegetal cover and the masking effect of the high aquagenic productivity. These factors are also likely to be of importance in more complicated lacustrine systems.

The hydrogen exchange reaction H+CNH->HCN+H may be a key step in gas-phase interstellar nitrogen chemistry. It is one of the reactions supposed to cause increasing HNC depletion with increasing temperature in dense interstellar clouds. In this paper we report the results of extensive ab-initio calculations on the H+CNH<->H+HCN system that partially confirm this hypothesis. It is shown that both forward and reverse reactions possess activation barriers. However, the activation energy of the H+CNH channel (4.2+/-1.0kcal/mol.) is four times smaller than for the endothermic HCN+H path. Calculations on the rate of the forward reaction show that tunneling under the entrance channel barrier allows a small rate coefficient at the temperatures under 100K and that the rate coefficient increases steadily with increasing temperature for T>100K.

Hydroxyurea (HU), a ribonucleotide reductase inhibitor has been used in the treatment of some malignant and viral diseases and seems now to be promising, in association with 2,3dideoxynucleosides, for the management of AIDS. In an attempt to increase the specificity of action of this radical scavenger, or at least, to study the topological aspects of its reactivity, we introduced the N-hydroxyureido group into nucleosides by using Mitsunobu reaction or by reacting a nucleoside nitrogen nucleophile with a carbonyl electrophile. From the currently available antiviral testing results, concerning the nucleoside analogues it appears that the most noticable activity exert against Varicella Zoster virus (VZV). One acyclonucleoside derivative was found to be very active against the virus HIV-1, its therapeutic index is better than 100.000. We prepared peptid-like dinucleotide analogues33,36 also in which the internucleosidic bridge consists of a spacer of approximately the same length as in the natural compounds. These compounds could be tested as inhibitors of nucleotide-protein interactions, we supposed that they are able to disrupt zinc finger parts of nucleocapsid. Antiviral activity of these dinucleotides were tested in vitro against HIV-1, HIV-2, HSV-1, HSV-2, CMV, VZV and EBV but in no case EC50 values inferior to 10 µM was found.

The pseudo rotation of PF5 has been investigated using both static and dynamic density functional theory (DFT) methods. The lowest energy path is the Berry pseudorotation, corresponding to the concerted exchange of two apical and two equatorial ligands. The potential energy surface has been derived and the transition state localised. In ab initio molecular dynamics the Berry pseudorotation has been observed and occurs with a typical period of 0.6 ps at 750 K. Analysis of the trajectories and comparison of the spectral density with the vibrational frequencies is presented.

Methane concentrations were measured in the water column of the northern basin of Lake Lugano in 1993 and 1994. Methane profiles show three distinct zones: (1) low concentrations generally below 0.1 mmol m−3 from the surface to 80–90-m depth; (2) a. sharp rise in concentration up to 50 mmol m−3 from 80–90- to not, vert, similar 150-m depth; and (3) from not, vert, similar 150-m depth to the bottom with a lower concentration gradient and with the highest methane concentrations in near-bottom water was not, vert, similar 80 mmol m−3. These profiles result from a combination of several factors such as fluxes from sediments, spatially variable vertical mixing, and aerobic and anaerobic bacterial oxidation in the water column. δ13C values indicate a biogenic origin for the methane. The methane inventory in the anoxic hypolimnion is not, vert, similar 2000 metric tonnes, yet its transfer to the surface water is almost completely cancelled by oxidation at the permanent redox interface situated at not, vert, similar 90-m water depth. Increased methane concentrations in near-surface water are not related to the deep-water methane reservoir but probably result from bacterial production of methane in the photic zone. Surface water is over saturated with methane in respect to the atmosphere. Anthropogenic eutrophication in temperate zone lakes has affected the carbon budget and methane production and storage. The potential impact of these changes on global emissions to the atmosphere should be evaluated.

For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNHright harpoon over leftHCNHright harpoon over leftHCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2±1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions.

The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN+ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH ⁺ ₂. On many points, the differences relative to the SiNH ⁺ ₂ isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH ⁺ ₂. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN+ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH2, H2SiNH, and SiNH ⁺ ₂ species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm–1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 eV

The experimental and the theoretical interests for the silicon chemistry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH2 and HNSiH2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH+ cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G** atomic orbital expansion. Accurate geometries and Be rotational constants have been determined at the Möller-Plesset MPn(n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH2, HNSiH2, and finally HSiNH2 in its floppy linear singlet ground state and in its lowest cis-bent a3A′ state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH+ entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species.

The liquid Mg - Bi system exhibits strong compound formation at the `octet' composition . We present results of first-principles molecular dynamics simulations of this alloy system at different compositions: the pure Mg and Bi liquid components, the stoichiometric liquid, and a Mg-rich composition . For the pure liquids, our results are in excellent agreement with experimental diffraction data. For , a significant modification of the characteristics of the local ordering is found w.r.t. the crystalline -phase: the ordering in the liquid is much more ionic. This structural modification is consistent with the structure of the superionic -phase, that was reported recently by Barnes et al 1994 J. Phys.: Condens. Matter 6 L467. Our simulations cannot reproduce the `reverse' metal - nonmetal transition observed upon melting, the computed conductivity being much larger than found in experiments. Instead, for the Mg-rich alloy, the calculated conductivity approaches closely to the experimental value.

Standard ab initio and density functional calculations have been applied to the reaction O•- + H2O → [H2O2]•- → OH- + OH•. While two intermediates are found as minima on the anionic potential energy surface, neither of them is directly related to the structure of neutral hydrogen peroxide. The results of different combinations of exchange and correlation functionals are systematically compared to each other and to MP2, MP4SDTQ, and CCSD(T) calculated data. The role of the basis set and the Hartree−Fock exchange in the hybrid DFT scheme is discussed. While for the two minima a reasonable agreement between all the methods was found, the geometries of the located transition structures strongly depend on the method and basis set used.

The electroaffinity of the O2 molecule is revisited using density functional theory (DFT) and perturbation treatments built on a MCSCF wavefunction that includes most of the non-dynamic correlation effects (MC/P approach). Using a standard 6–31 + G* basis set, DFT treatments based on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/perturbation treatments using an Epstein-Nesbet partition of the molecular Hamiltonian give a more accurate value of +0.492 eV in excellent agreement with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far less tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the dynamic correlation of the electron pair describing the σO---:O bond.

Some properties of small Lin clusters (n up to 20) are theoretically investigated, within the density functional theory formalism. The structural properties are examined at the so-called local level of approximation. For very small clusters (n<=8), the Lin conformations which are well known from ab initio calculations are found at very low computational cost. For n>8, optimal starting geometries are generated from two growth patterns, based on the increase of the number of pentagonal subunits in the clusters by adsorption of one or two Li atoms. Several new stable structures are proposed, for which the corresponding vibrational analysis is performed for n up to 18. The study of energetic properties and stability requires the use of gradient-approximated functionals. Such functionals are used for the determination of the relative stability of these clusters. For example, we show that the icosahedral structure is the most favorable geometry for Li13, whereas this is not the case for Na13. Ionization potentials and binding energies are also investigated in regard to the size and the geometry of the clusters. Comparison with experimental results and other theoretical approaches (such as nonspherical jellium model) suggests that some combinations of gradient-corrected functionals are more adapted than others to describe Lin energetic and structural properties.

We present density functional calculations of the bonding structures and diffusion barriers for a Cl adatom on Si(001)-(2×1). Besides the stable adsorption site at the dangling bond (DB), a metastable bridge-bonded state breaking a surface dimer bond and ~1.1 eV higher in energy than the DB is found. The calculated properties of this state agree with recent ESDIAD and HREELS observations. This bridge-bonded site is not along the Cl intradimer diffusion pathway of lowest energy. For this path a transition state also having a bridging structure (but not breaking the dimer bond) and rather low in energy ( ~0.6 eV with respect to the DB) is determined. The low intradimer barrier is consistent with the facile switching of Cl recently observed in scanning tunneling microscopy experiments.

The recently developed frozen density functional theory (FDFT) is extended to ab initio free energy calculations of chemical reactions in solution. This method treats the solute−solvent system as a supermolecule but constrains the electron density of the solvent molecules. Unlike hybrid quantum mechanical/molecular mechanics techniques, FDFT represents the solvent quantum mechanically. The quality of the solute−solvent interaction potential is examined by generating clusters of a reacting system and several solvent molecules and comparing the supermolecule DFT energies to the corresponding FDFT energies. The FDFT potential surfaces for solute−solvent systems provide a good approximation of the supermolecule DFT surfaces and require, in some cases, several orders of magnitude less computation time (in particular if one treats many solvent molecules quantum mechanically). The ab initio free energy surface for the F^- + HF → FH + F^- proton transfer reaction in solution is calculated using the corresponding “classical” empirical valence bond (EVB) potential surface as a reference potential. The encouraging results indicate that FDFT can be used to study chemical reactions in solution, capturing the quantum mechanical aspects of the solvent, which is not possible using hybrid quantum mechanical/molecular mechanics approaches. Furthermore, the use of EVB as a reference potential is found to be an extremely effective way of obtaining ab initio free energies for chemical processes in solution or in clusters.

From our combined experimental and computer modeling study we found a structurally and kinetically well-defined second coordination shell around chromium(III) ions in aqueous solution. Strong hydrogen binding due to polarization of first coordination sphere water molecules leads to a mean coordination number of 12.94 water molecules in the second shell and to short first shell hydrogen−second shell oxygen distances of about 1.4 Å. The experimentally measured exchange rate constant of = (7.8 ± 0.2) × 109 s-1 (ΔH = 21.3 ± 1.1 kJ mol-1, ΔS = +16.2 ± 3.7 J K-1 mol-1) corresponds to a lifetime of 128 ps for one water molecule in the second coordination shell and compares very well with a lifetime of 144 ps as observed from molecular dynamics simulation of a [Cr(H2O)6]3+ complex in aqueous solution. The geometry and the partial atomic charges of [Cr(H2O)6]3+ were determined by density functional theory (DFT) calculations. Water exchange from the second coordination shell to the bulk of the solution proceeds between a H2O sitting in the second shell and an adjacent one which just entered this shell from the bulk. By a small rotation of the first coordination shell water molecule, one of its two hydrogen bonds jumps to the entered water molecule and the one which lost its hydrogen bond leaves the second shell of the [Cr(H2O)6]3+. This associative reaction mode is a model for water exchange between water molecules which are bound by strong hydrogen bonds, as in the case for strongly polarizing 3+ ions such as Al3+ or Rh3+. Furthermore, the exchange phenomenon between second sphere and bulk water involving only two adjacent water molecules is strongly localized and independent of other water molecules of the second shell. In this respect it may be considered as a starting point for a study of water exchange on a protonated metal oxide surface.

Ground‐state properties of a linear hydrogen‐bonded FH...NCH complex are studied by means of the ‘‘freeze‐and‐thaw’’ cycle of Kohn–Sham Equations with constrained electron density (KSCED) [T. A. Wesolowski and J. Weber, Chem. Phys. Lett. 248, 71, (1996)]. For several geometries of the complex, the electron density and the total energy are compared to the ones obtained by means of the standard Kohn–Sham calculations. The comparisons are made to assess the accuracy of several gradient dependent approximate kinetic energy functionals applied in the KSCED equations. It was found that the closest results to the Kohn–Sham ones were obtained with the functional whose analytical form was proposed by Perdew and Wang for exchange energy [J. P. Perdew and Y. Wang in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrig (Academie Verlag, Berlin, 1991), p. 11] and parametrized by Lembarki and Chermette for kinetic energy [A. Lembarki and H. Chermette, Phys. Rev. A 50, 5328 (1994)]. Around the interaction energy minimum as well as for larger intermolecular distances, the ‘‘freeze‐and‐thaw’’ cycle of KSCED leads to very similar potential energy surface as the standard supermolecule Kohn–Sham calculations. 

The ground and excited state properties of the Cr3+ ion doped into the cubic host lattices Cs2NaYCl6 and Cs2NaYBr6 have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets 4A2g, 4T2g, 2Eg, and 4T1g. The geometry of the CrX^{3 -} ₆ cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Å for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm–1 too low. An energy lowering is obtained in the first 4T2g excited state when the octahedral symmetry of CrX^{3 -} ₆ is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the 4T2g geometry derived from high-resolution optical spectroscopy of Cs2NaYCl6:Cr3+. It corresponds to an axially compressed and equatorially elongated CrX^{3 -} ₆ octahedron.

A new method for calculating the ground state electron density of interacting molecules is presented. The supermolecule electron density is obtained using an iterative procedure. At each step the electron density of one molecule is calculated using previously introduced Kohn-Sham equations with constrained electron density. These equations contain terms representing the coupling between constrained and non-constrained electron densities. The coupling terms also involve a new functional, namely the non-additive kinetic energy functional that is not present in the original Kohn-Sham method. Its first-principles analytical form in not yet known. We examine the analytical form of this functional derived from Thomas-Fermi theory. The electron density obtained is compared with that calculated using the original Kohn-Sham method applied to the supermolecule. Good agreement has been found for a broad range of electron density overlaps.

In resonant fluorescence line narrowing (FLN) experiments in the R₁ transition of the [Cr(ox)₃]^3- chromophore in [Ru(bpy)₃][NaAl:Cr(1%)(ox)₃] and [Rh(bpy)₃][NaCr(ox)₃]ClO₄ multiline spectra are observed at 1.8 K, (ox=oxalate, bpy=2,2’-bipyridine). For [Rh(bpy)₃][NaCr(ox)₃]ClO₄ the number of lines and their relative intensities depend critically upon the excitation wavelength within the inhomogeneous distribution, and in time-resolved FLN experiments additionally upon the delay. This behavior is clear evidence for a resonant energy-transfer process. At 4.2 K the more common phonon-assisted process becomes dominant, manifesting itself as spectral diffusion.

Chemical variation and combination of metal ions of different valencies in the oxalate backbone as well as in the tris-bpy cation of the three-dimensional network structures of the type [M^II₂(ox)₃][M^II(bpy)₃] (bpy = 2,2'-bipyridine, ox = C₂O₄^2-), [M^IM^III(ox)₃][M^II(bpy)3] and [M^IM^III(ox)3][M^III(bpy)3]ClO₄ offer unique opportunities for studying a large variety of photophysical processes. Depending upon the relative energies of the excited states of the chromophores, excitation energy transfer either from the tris-bipyridine cation to the oxalate backbone or vice versa is observed, as for instance from [Ru(bpy)₃]²⁺ as photo-sensitiser to [Cr(ox)₃]^3- as energy acceptor in the combination [NaCr(ox)₃][Ru(bpy)₃], or from [Cr(ox)₃]^3- to [Cr(bpy)₃]³⁺ in [NaCr(ox)₃][Cr(bpy)₃]ClO₄. In addition efficient energy migration within the oxalate backbone is observed. Furthermore, depending upon the excited state redox potentials, light-induced electron transfer processes may be envisaged.

In analogy to the [M^II(bpy)₃]²⁺ cations, where M^II is a divalent transition-metal and bpy is 2,2‘-bipyridine, the tris-chelated [M^III(bpy)₃]³⁺ cations, where M^III is Cr^III or Co^III, induce the crystallization of chiral, anionic three-dimensional (3D) coordination polymers of oxalate-bridged (μ-ox) metal complexes with stoichiometries [M^II₂(ox)₃]_n^2n- or [M^IM^III(ox)₃]_n^2n-. The tripositive charge is partially compensated by inclusion of additional complex anions like ClO₄^-, BF₄^-, or PF₆^- which are encapsulated in cubic shaped cavities formed by the bipyridine ligands of the cations. Thus, an elaborate structure of cationic and anionic species within a polymeric anionic network is realized. The compounds isolated and structurally characterized include [Cr^III(bpy)₃][ClO₄] [NaCr^III(ox)₃] (1), [Cr^III(bpy)₃][ClO₄][Mn^II₂(ox)₃] (2), [Cr^III(bpy)₃][BF₄] [Mn^II₂(ox)₃] (3), [Co^III(bpy)₃][PF₆][NaCr^III(ox)₃] (4). Crystal data:  1, cubic, P2₁3, a = 15.523(4) Å, Z = 4; 2, cubic, P4₁32, a = 15.564(3) Å, Z = 4; 3, cubic, P4₁32, a = 15.553(3) Å, Z = 4; 4, cubic, P2₁3, a= 15.515(3) Å, Z = 4. Furthermore, it seemed likely that 1,2-dithiooxalate (dto) could act as an alternative to the oxalate bridging ligand, and as a result the compound [Ni^II(phen)₃][NaCo^III(dto)₃]·C₃H₆O (5) has successfully been isolated and structurally characterized. Crystal data:  5, orthorhombic, P2₁2₁2₁, a = 16.238(4) Å, b = 16.225(4) Å, c = 18.371(5) Å, Z = 4. In addition, the photophysical properties of compound 1 have been investigated in detail. In single crystal absorption spectra of [Cr^III(bpy)₃][ClO₄][NaCr^III(ox)₃] (1), the spin−flip transitions of both the [Cr(bpy)₃]³⁺ and the [Cr(ox)₃]^3- chromophores are observed and can be clearly distinguished. Irradiating into the spin-allowed ⁴A₂ → ⁴T₂absorption band of [Cr(ox)₃]^3- results in intense luminescence from the ²E state of [Cr(bpy)₃]³⁺as a result of rapid energy transfer processes.

The high-spin → low-spin relaxation dynamics of the Fe(III) spin-crossover complexes [Fe(Sal₂tr)]PF₆ (H₂Sal₂tr = Bis(salicylaldimino)triethylenetetramine) and [Fe(acpa)₂]PF₆(Hacpa = N-(1-acetyl-2-propylidene)-2-pyridylmethylamine) are discussed within the theory of nonadiabatic multiphonon relaxation. A Huang−Rhys factor S of ≈25, estimated on the basis of average metal−ligand bond length differences Δr_HL of ≈ 0.12 Å, explains the observed low-temperature tunneling rate constants k_HL(T→0) of ≈ 10² s^-1 as well as the thermally activated process at T > ≈100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover compounds.

At low temperatures an external pressure of 1 kbar accelerates the high-spin → low-spin relaxation in the [Zn_1−xFe_x(ptz)₆](BF₄)₂, X = 0.1, spin-crossover system by one order of magnitude. This is due to the large difference in volume between the high-spin and low-spin states of 26 ų/molecule. The relative vertical and horizontal shifts of the potential wells of the two states as a function of pressure are estimated to be 130 cm⁻¹/kbar and 10⁻³ Å/kbar, respectively.

In the [Fe(ptz)₆](BF₄)₂ (ptz = 1-propyltetrazole) spin-crossover system, the thermal spin transition is accompanied by a first order crystallographic phase transition (T_c^↓ = 128 K and T_c^↑ = 135 K) from R³ above T_c^↓ to P¹ at low temperatures (Wiehl L., Acta Cryst. B49, 289 (1993)). The high-symmetry phase can be super-cooled, in which case the spin transition is still complete and quite steep (T_1/2 = 125 ± 2 K) but now without a hysteresis. The corresponding interaction constant Γ is 170 cm⁻¹. In the diluted system [Zn_1−xFe_x(ptz)₆](BF₄)₂, X = 0.1, the spin transition is gradual with T_1/2 = 95 ± 2 K. From the shift of T_1/2 towards high temperatures with external pressure a value for ΔV_HL⁰ of 26 ų molecule⁻¹ is obtained. Pressures above 250 bar induce a crystallographic phase transition even in the diluted system, as a result of which the spin transition is discontinuous. The interplay between the thermal spin transition and the crystallographic phase transition in the neat and the diluted system is discussed consistently.

In the [Fe(etz)₆](BF₄)₂ spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high-spin (HS) → low-spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light-induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS → LS conversion on site B reveal a light-induced HS→LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B-site complexes with a critical temperature T^↑_c K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first-order kinetics (self-acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self-retardation.

The [Fe(etz),](BF,), spin-cross-over system (etz = 1-ethyl-1 _H-tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a10.419(3), b=15.709(1), c = 18.890(2) Å, α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) ų and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites n_A:n_B is 2:l. Iron(II) on site A undergoes a thermal low-spin (LS) → high-spin (HS) transition with T_1/2I, = 105 K. whereas that on site B remains in the high-spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low-spin state on site B. On site A the high-spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light-induced population of the low-spin state can be achieved with λ = 820 nm.

Rearrangement of 5α- and 5β-cholesta-6,8(14)-dienes (13a and 13b, resp.) in the presence of anhydrous toluene-4-sulfonic acid in acetic acid leads to 5α- and 5β-12(13 → 14)-abeo-cholesta-8,13(17)-dienes (15a and 15b, resp.) via 5α- and 5β-cholesta-8,14-dienes (14a and 14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular-mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.

To determine the limitations of electrospray mass spectrometry for the study of condensed-phase chemistry, it is important to understand the origin of cases for which the electrospray mass spectra, which are a measure of the relative abundances of gas-phase ions, do not reflect the equilibrium ion abundances in the solution electrosprayed. One such divergent case is that of free-base octaethylporphyrin. Under conditions for which this porphyrin is present in solution predominantly as the doubly charged, diprotonated molecule, the predominant ionic species observed in the electrospray mass spectrum is the singly charged, monoprotonated molecule. In this paper, direct optical spectroscopic measurements of the ions in solution (absorption spectra) and in the electrospray plume (fluorescence excitation spectra) are correlated with the ion distribution observed in the gas-phase (as reflected in the electrospray mass spectra) to determine at what point in the electrospray process and by what mechanism(s) the transformation from dication to monocation occurs. The data indicate that the major portion of the doubly protonated porphyrin species originally present in solution are converted to singly protonated species relatively late in the electrospray process, during the latter stages of droplet desolvation in the atmospheric/vacuum interface of the mass spectrometer, via the loss of a charged solvent molecule/cluster.

A study of the energetics of electron transfer (e.t.) quenching of 9-cyanoanthracene fluorescence by aromatic (π) and aliphatic (n) donors in n-hexane using picosecond transient thermal phase grating spectroscopy is reported. The results show that the enthalpy of exciplex formation is well correlated with the redox potentials of the partners and independent of the nature of the donor, as long as exciton interaction is small. The main difference between exciplexes with donors of the two classes is the larger ground-state repulsion energy with n-donors. However, if the adiabatic ionization potentials are considered, the enthalpy of exciplex formation with n-donors is larger than expected by about 0.4 eV. These two contradictory results reflect an inconsistency between the values for adiabatic ionization potentials and oxidation potentials of n-donors. The occurrence of two distinct Rehm−Weller plots for e.t. quenching with π- and n-donors shifted by about 0.8 eV on the horizontal axis seems to originate to some extent from the use of adiabatic ionization potentials.

The rate constants of quenching of the triplet state T₁ of four aromatic ketones by triethylamine in acetonitrile show no correlation with the Gibbs energy of the reaction, but depend on the nature of T₁, i.e. n–π*, π–π* or charge transfer (CT) type. For example, the ratio of rate constants between two systems of n–π* and CT type, with the same driving force, is over 10⁸. It is concluded that these reactions are kinetically controlled, the decisive factor for the activation energy being the electrostatic charge distribution of the carbonyl group in the triplet state. 

The effect of excess excitation energy on the rotational dynamics of rhodamine 6G in the ground and the first singlet excited state has been investigated in series of n-alcohols and alkanenitriles using the picosecond polarization grating technique. In nitriles, the reorientation times are the same for excitation at the S₁ ← S₀ and S₂ ← S₀ transitions, and no state dependence could be detected. In alcohols, the rotational dynamics of rhodamine 6G in the excited state is about 25% faster when formed with 1.15 eV excess excitation energy. This effect is ascribed to a decrease of the hydrodynamic volume due to dissociation of solute/solvent hydrogen bond following intramolecular vibrational redistribution. An accompanying perturbation of the solvent shell structure caused by the fast local temperature jump is not excluded.

A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik’s coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared.

The synthesis and crystal structure of 1,2-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzene, L, are reported as well as the preparation and conformation of the novel seven-membered ring complex [PdLCl2]; this complex reacts with alcohols (MeOH, EtOH) to give a chiral cyclometallated complex [rac(R)P, (R)C; (S)P, (S)C] where the metal is bound to both a phosphaalkene and a phosphite phosphorus atom.

To measure the rotation barrier around an R3C−•PH bond in the solid state, 9-phosphinotriptycene50 2 has been synthesized and its crystal structure has been determined. It is shown, by EPR, that the radiogenic radical 3, which results from a homolytic scission of a P−H bond, can indeed be trapped in the crystal matrix. Its g-tensor together with its 31P and 1H hyperfine coupling have been measured at 300 and 77 K. These tensors show that free rotation around the C−P bond occurs at room temperature but is blocked at liquid nitrogen temperature. The temperature dependence of the EPR spectra has been analyzed using the density matrix formalism and has led to a rotation barrier of about 2.5 kcal·mol-1 . This result and the various hyperfine couplings have been compared with the values predicted by ab initio methods for two isolated model radicals: the tert-butylphosphinyl radical 4 and the barrelenophosphinyl radical 5.

An intermolecular potential energy surface was derived for the hydrogen‐bonded water trimer as a function of the three torsional angles ω₁, ω₂, ω₃, for energies up to 1300 cm⁻¹ (3.7 kcal/mol) above the global minimum. The O...O distances and the intramolecular geometry of the H₂O molecules are held fixed. This surface is based on the ab initio calculations presented in a companion paper [W. Klopper et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large basis sets and the most extensive treatment of correlation energy for calculations of (H₂O)₃ so far. The 70 ab initio interaction energies, multiplied by six due to the S₆ symmetry of the surface, were fitted using an analytical potential function, with an average error of ≊11 cm⁻¹. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and ‐values and other properties of this cluster. Also given is a classification of the low‐lying torsional wave functions according to nodal properties.

We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet‐cooled phenol⋅(H₂O)₃ and d₁‐phenol⋅(D₂O)₃ in the S₀ and S₁ electronic states. Two‐color resonant two‐photon ionization combined with time‐of‐flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass‐selective vibronic spectra of both isotopomers in both electronic states. In the S₀ state, eleven low‐frequency intermolecular modes were observed for phenol⋅(H₂O)₃, and seven for the D isotopomer. For the S₁ state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol⋅(H₂O)₃ were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H‐bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal‐mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S₀ state vibrational frequencies and isotope shifts.

Mass‐selective ground‐state vibrational spectra of jet‐cooled carbazole⋅R (R=Ne, Ar, Kr, and Xe) van der Waals complexes were obtained by populating ground‐state intra‐ and intermolecular levels via stimulated emission pumping, followed by time delayed resonant two‐photon ionization of the vibrationally hot complex. By tuning the dump laser frequency, S₀ state vibrational modes were accessed from ≊200 cm⁻¹ up to the dissociation energy D₀. Upon dumping to ground‐state levels above D₀, efficient vibrational predissociation of the complexes occurred, allowing us to determine the S₀ state van der Waals binding energies very accurately. The D₀(S₀) values are <214.5±0.5 cm⁻¹ (R=Ne), 530.4±1.5 cm⁻¹ (R=Ar), 687.9±4.0 cm⁻¹ (R=Kr), and 890.8±1.6 cm⁻¹ (R=Xe). In the S₁ state, the corresponding binding energies are larger by 9% to 12%, being <222.9±1.0 cm⁻¹, 576.3±1.6 cm⁻¹, 756.4±4.5 cm⁻¹, and 995.8±2.5 cm⁻¹, respectively.

Mass‐selective ground‐state vibronic spectra of molecular van der Waals complexes carbazole⋅S, S=N₂, CO, and CH₄, were measured by stimulated emission pumping followed by resonant two‐photon ionization of the vibrationally hot complexes. S₀‐state vibrational modes were accessed from ≊200 cm⁻¹ up to the ground‐state dissociation limit D₀(S₀) of the van der Waals bond. Above D₀, efficient vibrational predissociation of the complexes occurs, allowing accurate determination of the van der Waals dissociation energies as 627.2±7.9 cm⁻¹ for N₂, 716.5±29.8 cm⁻¹ for CO, and 668.6±15.1 cm⁻¹ for CH₄. In the S₁ excited state, the van der Waals binding energies increase to 678.5±8.0, 879.2±29.9, and 753.8±15.2 cm⁻¹, respectively. The relative increases upon electronic excitation are about 8% and 13% for N₂ and CH₄, similar to the analogous rare gases Ar and Kr. For CO, the relative increase of van der Waals binding energy is 23%. The differences are primarily due to electrostatic interactions.

Accurate hydrogen-bond dissociation energies were determined for gas-phase hydrogen-bonded complexes between 1-naphthol or 1-naphthol-d₃ and H₂O, CH₃OH, NH₃ and ND₃, using the stimulated emission pumping-resonant two-photon ionization spectroscopy technique in supersonic jets. The hydrogen-bond dissociation energies obtained for the electronic ground state are D₀ = 2035 ± 69 cm⁻¹ for 1-naphthol · H₂O, 2645 ± 136 cm⁻¹ for 1-naphthol · CH₃OH, 2680 ± 5cm ⁻¹ for 1-naphthol · NH₃ and 2801 ± 14 cm⁻¹ for 1-naphthol-d₃ · ND₃, respectively. Upon electronic excitation to the S₁ state the binding energies increase by approximately 8%.

A coupled three-dimensional model calculation of the low-frequency large-amplitude intermolecular torsional states in (H₂O)₃ and (D₂O)₃ is presented, based on the analytical modEPEN intermolecular potential surface and a three-dimensional discrete variable representation approach. The lowest torsional levels of both (H₂O)₃ and (D₂O)₃ lie above the sixfold (upd) torsional barrier. The first eight (eleven) torsions of (H₂O)₃ ((D₂O)₃) are pseudorotational states. The ‘radial’ and ‘polar’ torsional fundamental frequencies are predicted at 151 and 160 cm⁻¹ for (D₂O)₃, and for (H₂O)₃ at 185.0 and 185.3 cm⁻¹, respectively. Each of these in turn support a ladder of pseudorotational levels.

The 1.3 S biotinylatable subunit of Proprionibacterium shermanii transcarboxylase complex was fused to the C-terminus of the human neurokinin 1 receptor gene and introduced into the Semliki Forest virus expression vector pSFV1. RNA transcribed from pSFV1-NK1-biot and pSFV-Helper2 was coelectroporated into BHK cells permitting in vivo packaging of recombinant virus. Infection of BHK and CHO cells with SFV-NK1-biot virus yielded high level of the fusion receptor as detected by metabolic labeling, immunoblotting with streptavidin alkaline phosphatase and binding to substance P. Like native receptor, the biotinylated receptor fusion was able to stimulate Ca2+ mobilization in infected CHO cells,indicating functional coupling to guanine-nucleotide-binding proteins.

The spectroscopic properties of several Ag⁺ activated strontium fluoride crystals—originating from different batches—were examined. Absorption, emission and excitation experiments including luminescence dynamics and polarization studies were performed at various temperatures. Four Ag⁺ related photoluminescent centers were found. Their properties are described. The predominant species in the asgrown crystals emits in the UV (at 315 nm) and was shown to be the single Ag⁺ ion in cubic surroundings. Two other centers emitting in the violet (400 nm) and in the yellow (550 nm), respectively, were attributed to (Ag⁺)₂pairs with different geometries. Models of these pseudo-molecular clusters are proposed. The nature of the system which produces a weak green (500 nm) luminescence remains open to questions.

We performed CAS-CI calculations on Li2 using a set of molecular orbitals (MO) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO). The dimension of the CAS-CI space was of about 12 million symmetry-adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results.

We present molecular dynamidfree energy calculations on the molecules acetamide, N-methylacetamide, N,N-dimethylacetamide, ammonia, methylamine, dimethylamine, and trimethylamine. Unlike the experimental data, which suggest a very non-additive solvation free energy (N-methylacetamide and methylamine having the most negative free energy of solvation), the calculations all find that the free energy of solvation monotonically increases as a function of methyl addition. The disagreement with experiment is surprising, given the very good agreement (within 0.5 kcal/mol) with experiment for calculation of the solvation free energy of methane, ethane, propane, water, methanol, and dimethyl ether.

First principles molecular dynamics studies of the low, intermediate, and high temperature phases of Ge(111) are reviewed. The atomic structure and electronic properties of the c(2 × 8) reconstruction, the diffusion of Ge adatoms at the c(2 × 8) → (1 × 1) disordering transition at T ~ 300°C, and the behavior of Ge(111) close to the bulk melting temperature are discussed.

The influence of electronic and structural effects in FeCL and FeCl6 entities has been investigated through X-ray absorption spectroscopy and theoretical calculations of electronic transition energies using the MSLSD method. The relative importance of the formal oxidation degree of the metal, the metal-ligand distance, and the symmetry of the site on the energy of near-edge structures are studied. The principal effect is the stabilization of the iron 1s orbital in the ground state when the oxidation degree increases. The accuracy of the theoretical determination regarding the experimental spectra is discussed. The incidence of the same electronic parameters on the intensities of near-edge structures is also investigated through a transition cross section calculation.

Thermally stimulated depolarization current (TSDC) measurements of CaF2:Cu+ samples show three different dipolar relaxation bands, with temperature peaks at 52, 144 and 187 K. The two higher temperature bands are more intense and reveal the presence of Cu+ in probably two different lattice sites. The band at 144 K suggests it is related to the Cu+ off-center effect, observed through an optical absorption at 328 nm, and it is likely temperature independent. To this Cu+ off-center effect it is also associated an impurity-vacancy defect, that result from needed charge compensation in CaF2. The observed TSDC band at 187 K is attributed to Cu+i—Fi- pair, both ions in interstitial positions. The weakest TSDC at 52 K is assumed to be from a low un-intentional Mn2+ doping, sitting in an off-center substitutional position. It is proposed a model to explain the several positions available for the substitutional Cu+ that response for the TSDC result.

A local density functional study of the structure and energetics of offretite, with a Si4+ ion substituted by (A13+, M+) (M = Na, K), is presented. The calculations have been performed within the framework of the first principles molecular dynamics method using a periodically repeated unit cell with 55 atoms. It is found that the preferred site for K+ cations lies inside the cancrinite cage, in agreement with experiment. This site is related to A1 substituting a Si atom at a TI site, which is one of the two inequivalent tetrahedral sites of offretite, and belongs to the hexagonal prism, Site selectivity is quite pronounced in the case of K+, as the large size of this cation gives rise to important steric effects in the offretite framework. For Na+, instead, the lowest energy site is located in the channel, near a window of the cancrinite cage, and corresponds to an A1 in Tz. For this cation the spread in the relative substitution energies of the different binding sites is very small, the four sites of lower energies being in a range of -2.5 kcal/mol.

Neoglycolipids bearing a paramagnetic probe in their lipophilic aglycon have been prepared. All belong to the Image -glucose series, both anomers for the glucoside representatives, respectively β and α anomers in the S- and C-glucosyl series. Two different types of radical sites have been used, a relatively short-lived imino N-oxyl group for glucosides and a more stable N-acylamino N-oxyl moiety in the other cases. EPR spectra of these radical species afforded information on the conformation of the lipophilic chain in the vicinity of the paramagnetic probe.

A series of 2,3-O-cyclopentylidene C-glycoside analogs in which the furanose ring has been replaced with a N-hydroxypyrrolidine have been prepared. A structural study of these tricyclic compounds and the aminoxyl radicals thereof has been carried out using variable temperature 1H NMR, X-ray diffraction, molecular dynamics and EPR spectroscopy. Both types of compounds, N-hydroxypyrrolidines and pyrrolidine N-oxyls, fundamentally prefer - in solutions- N-endo conformations over the alternate, N-exo forms found by X-ray diffraction studies and computed to be more stable by molecular dynamics.

Optical hole burning, a potential technique for spectrally selective recording, was demonstrated in Sm-doped MBE-grown thin films of CaF₂/Si(111). The inhomogeneous broadening of the corresponding Sm²⁺ 5d(T_1u) ↞ 4f(⁷F₀, A_1g) transition (690 nm) was investigated as a function of substrate temperature and film thickness. The MBE apparatus is briefly described as well as the thin film growth procedure.

The state-of-the-art computational chemistry software, visualization tools and high performance computing infrastructure are employed to assist the design of new and more effective drugs. At present a project focuses on the quantitative structure activity relationships of antimalarial drugs.

Bis(N-methylimidazolidinethi-2-one)copper(I) chloride has been synthesized and its crystal structure determined. X-Irradiation of a single crystal of this compound leads to the formation of a CuII complex which was studied by EPR: it was shown that this species results from the addition of a radiogenic Cl atom on the CuI precursor. The structural changes induced by this reaction are revealed by the g-tensor and by the hyperfine tensors of one copper and two chlorine nuclei. The structure of this S2Cl2Cu type complex was compared with other sulfur- or chlorine-containing CuII complexes.

The fundamental aspects of N2 and 0 2 adsorption in zeolites have been investigated by density functional calculations on different models. Simple systems where these molecules interact with a positive point charge or with isolated Li+ and Na+ cations have led to a qualitative explanation for the N2/02 separation process. A classical description involving electrostatic and induction energies is adequate to explain the basic reason for a stronger N2 adsorption. At short distances (bonding interaction), the electronic structure of the cation has to be taken into account. The presence of core electrons in large cations limits the stabilizing contribution of the electrostatic and induction terms to the total energy, implying that Li+ is more efficient than Na+ in the adsorption process. The presence of zeolite clusters decreases the binding energies for both N2 and 0 2 , but the main trends remain valid. Moreover, due to a larger screening of 0 2 adsorption, it improves the efficiency of Li+ with respect to Na+ for the N2/02 separation.

In density-functional theory (DFT), Perdew, Parr, Levy, and Balduz [Phys. Rev. Lett. 49, 1691 (1982)] have shown that for all the electronic systems, the energy of the highest occupied molecular orbital (HOMO) is equal to the negative of the ionization potential. This equality is not recovered within the different approximations of the exchange-correlation functional proposed in the literature. The main reason is that the exchange-correlation potentials of various functionals used in DFT calculations decay rapidly to zero whereas they should exhibit a Coulombic asymptotic -1/r behavior. In this work we propose a gradient-corrected (GC) exchange potential with a correct asymptotic -1/r form for large values of r. The energy of the HOMO calculated with this potential is improved compared to the local-density approximation (LDA) and to the GC functionals widely used in the DFT. Our HOMO eigenvalues are compared to the optimized-potential-model eigenvalues which are the exact values for the exchange-only potential. Using the fact that the LDA satisfies the virial theorem, the exchange energy corresponding to this GC exchange potential can be calculated under a simple assumption.

Single crystals X-ray diffraction was performed on five crystals with nominal compositions Sr1/2Sm1/2FCl, Sr1/2Ca1/2FCl, Sr2/3Ba1/3FCl1/3Br2/3, Sr1/2Sm1/2FCl1/2Br1/2 and Sr0.6Sm0.1Ca 0.3FCl. Population refinements confirm the presence of correlations between lattice constants and composition. A new correlation between the heavy halogen z value and the unit cell volume is found. Luminescence spectra of bivalent samarium in these crystals contribute to the structural characterization of these compounds.

Density functional calculations of ground and excited states of Li n (nle8) clusters have been performed, within two different approaches. Using a set of Kohn-Sham orbitals to construct wave functions, the calculation of the oscillator strengths of the electric dipole transitions is performed. Our results have been tested at two levels: first the necessary comparison with the experimental data, second the comparison with high level CI (MRD-CI) calculations. This last point is not a trivial challenge, because such anab initio method leads for small clusters to a highly accurate description of the electronic structure and optical absorption spectra. Finally, this is also a new test for the capability of using Kohn-Sham orbitals to construct physically meaningful wave functions. Transition energies, oscillator strengths and finally optical absorption spectra presented here are in general in reasonable agreement with the experimental data and the MRD-CI calculations. That is very promising for bigger systems, with regard to the modest computational effort (CPU time and memory size) of density functional calculations.

The new diphosphaalkene 1,3-bis[2-(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzenLe,, h as been synthesized. Due to the presence of two P=C bonds three isomers (EE, EZ, ZZ) were observed by 31P NMR, and the crystal structures of two of them could be determined (EE, ZZ). The electrochemical behavior of L has been studied by cyclic voltametry: a quasi-reversible reduction occurs at -1.89 V/SCE and corresponds to the formation of a radical anion which has been studied by ESR at variable temperature. The experimental 31P and 'H hyperfine constants are consistent with free rotation about the P=C and Cphosphaalkene-Cbenzene bonds at room temperature and agree with ab initio predictions. One of the isomers of L forms complexes with palladium(I1) and platinum(I1) ions. The crystal structures show that L is orthometalated and acts as a terdentate ligand by coordinating the metal with each phosphorus atom. These complexes are electrochemically reduced between -0.92 and - 1.29 V, and the resulting paramagnetic species are studied by ESR in liquid and frozen solutions. This reduction process was shown to be a ligand-centered process, an appreciable part of the unpaired electron is localized on each of the phosphaalkene carbons (20%) and phosphorus atoms (5%).

The possibilities and limits of the molecular orbital theory to deal with the problem of determining electronic structure of solids have been explored. A cluster model based on the charge neutrality in the solid has been used in test calculations on some III-V semiconductors and have given quite satisfactory results. Recommendations are given to widen the field of applications of this procedure.

Oxygen-free strontium fluoride crystals containing single monovalent silver ions in a cubic site were grown. Our experiments showed that the Ag⁺ ion remained chemically stable upon optical irradiation at 222 nm (KrCl excimer). The ion exhibits a strong UV luminescence which presents no thermal quenching up to RT. At this temperature, the emitting level time-constant is 12 μs. An explanation is proposed for the silver photostability by relating it to the large electronic bandgap of the host (11.4 ev). The 222 nm absorbing level lies below the conduction band in a way such that photoionization of Ag⁺ is avoided, as well as other optically-induced opacity phenomena. A minimum source intensity at threshold is estimated at some 276 kW, when using a Fabry-Perot cavity. This power can normally be achieved with the excimer laser.

Two radiogenic radicals trapped in a single crystal of 1-[2,4,6-tri-tert-butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their ³¹P   hyperfine tensors, as being phosphoniumyl radical cations. The spectra modifications caused by ¹³C or ²D enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conformations of the radical. The experimental ³¹P, ¹³C, and ¹H hyperfine tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that these interactions are very sensitive to the geometry of the radical and that their measurement can yield precise structural information.

Basic properties relevant to spectral hole burning (homogeneous and inhomogeneous spectral broadening, hole burning and filling mechanisms) are investigated in MeIyMeII1 − yFXIxXII1 − x: Sm2+ (Me = Ca,Sr,Ba; X = Cl,Br,I) single crystals. The relations between the spectral characteristics of 5D0,1-7F0 transitions and the material structure are described. Hole stability is investigated up to 430 K and is shown to be determined by ionic diffusion.

The ⁶³Cu NQR spectra of five dicoordinated complex cations of Cu(I) with substituted imidazoles as ligands and six analogous complexes with substituted pyrazoles as ligands are reported. The structures of four of these complexes have been previously determined and the relationship of their ⁶³Cu resonance frequency to the average Cu---N bond length is compared to that of the analogous lutidine or collidine complexes. It is concluded that there are probably significant differences between the electronic structures of the pyridine complexes and those of the pyrazole or imidazole series.

An investigation of the influence of various gradient-corrected exchange and correlation functionals on the bond lengths and dipole moments of CO and N2O has been carried out using density functional theory. It is shown that whereas some functionals are found to be more sensitive to the basis set quality than are others, the more commonly used gradient-corrected functionals lead to properties in very good agreement with experiment provided that a sufficiently large basis set is employed.

Intersystem crossing is the crucial first step determining the quantum efficiency of very many photochemical and photophysical processes. Spin-crossover compounds of first-row transition metal ions, in particular of Fe(II), provide model systems for studying it in detail. Because in these compounds there are no competing relaxation processes, intersystem crossing rate constants can be determined over a large temperature interval. The characteristic features are tunnelling at temperatures below ∼80 K and a thermally activated process above ∼ 100 K. This, as well as the twelve order of magnitude increase of the low-temperature tunnelling rate constant on going from a spin-crossover compound with a small zero-point energy difference to a low-spin compound with a substantially larger one, can be understood on the basis of a nonadiabatic multiphonon process in the strong vibronic coupling limit.

A study of the dynamics and thermodynamics of the photoinduced electron transfer reaction between benzophenone and 1 ,Cdiazabicycl0[2.2.2]octane in acetonitrile using picosecond transient thermal phase grating spectroscopy is reported. Two heat releases were observed within the time window of the experiment (0-4 ns): a fast one corresponding to the formation of a benzophenone lower triplet state and a slow one due to the electron transfer reaction. The observed dynamics is in agreement with earlier studies using transient absorption spectroscopy. The quenching of benzophenone in the first singlet excited state at high concentration of quencher and the very rapid decay of the resulting singlet geminate ion pair are confirmed. The enthalpy of formation of the geminate ion pair was determined by comparing the amounts of heat released in the fast and slow processes. The electron transfer is more exergonic by 0.23 eV than calculated from the Rehm- Weller equation, assuming spherical ions with point charges in their centers. The difference is tentatively ascribed to the electrostatic interaction within the ion pair.

A study of the isomerisation dynamics of 3,3′-diethylthiacarbocyanine in the excited state in alkanenitriles as well as in the ground state in both alkanenitriles and n-alcohols, using ps transient grating spectroscopy and flash photolysis is reported. The intrinsic activation energy for isomerisation has been determined from isoviscosity plots. In the ground state, the activation energy amounts to 12.9 kcal/mol in both nitriles and alcohols, while it is equal to 3.0 kcal/mol in the excited state. The reduced isomerisation rate constant shows a fractional dependence on the solvent viscosity, with a fractional exponent α of 0.20 and 0.38 for the ground state isomerisation in alcohols and nitriles and of 0.65 for the excited state isomerisation. The data have been analysed within the framework of the Kramers theory using different models for friction. The best agreement was obtained with the Grote-Hynes model, which takes into account the frequency dependence of the friction. The solvent dependence of α for the ground state isomerisation is accounted for by a different average value of the infinite frequency shear modulus in alcohols and nitriles. The barrier height dependence of α is explained by a decrease of the barrier's top frequency by going from the ground to the excited state.

An application of transient thermal phase grating spectroscopy with near-infrared probing to the determination of free ion yield in photoinduced electron transfer reactions is presented. The model system is 9,10-dicyanoanthracene with various electron donors in acetonitrile. The measured yields are in good agreement with those obtained from photoconductivity, when the latter are correctly calibrated. The method is explained and its advantages and limitations are discussed.

The main geometric elements of three 3,6-substituted 1,2,4-trioxan-5-ones have been calculated by using molecular mechanics (MM2), and semiempirical (AM1, PM3) methods. The results are compared with those obtained by X-ray analysis.

The structure of the ground and lowest two excited states of H2NO have been determined in large scale configuration interaction calculations using a multiconfiguration self-consistent description of the molecular orbitals. These treatments are based on a systematic building of the correlation contribution which has been designed to account for the characteristics of the nitroxide group. This approach shows that the aminoxyl functional group is more than a three electron group shared by two atoms, but, in fact, a nine electron entity. Our best estimate of the geometry of the ground electronic state, obtained after second-order configuration interaction using a large basis of atomic natural orbitals, is pyramidal. However, since the potential depth between 0° and 40° is lower or of the same order of magnitude as the estimated inversion frequency, the conclusion that this molecule behaves like a planar system is totally justified. The structure of the excited (n−pi*) and (pi−pi*) states have been determined and the transitions energies are in accordance with the experimental results on the highly substituted stable nitroxide radicals.

The luminescence of [CrX₆]^3– X=Br^–, Cl^– has been studied through density functional theory (DFT) using both deMon and ADF codes. Multiplet energies⁴A₂,²E,⁴T₂, and⁴T₁ have been expressed as energies of non-redundant single determinants and calculated as in Ref. [1]. The influence of the metal ligand distance on the multiplet energies has been investigated. Of particular interest to this work is the Jahn-Teller effect distortion. We found that the system moves to a more stable geometry when the axial bond length is compressed and the equatorial one elongated in agreement with the experimental value.

Quantum chemical calculations based on density functional theory have been performed on Cr(CO)6, (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) at the local and nonlocal level of theory using different functionals. Good agreement is obtained with experiment for both optimized geometries and metal-ligand binding energies. In particular, a comparison of metal-arene bond energies calculated for the (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) complexes correlates well with kinetic data demonstrating that substitution of one CO group by CS leads to an important labilizing effect of this bond, which may be primarily attributed to a larger pgr-backbonding charge transfer to the CS ligand as compared with CO.

The enantioselective hydrogenation of α-ketoesters to α-hydroxyesters over Pt/Al2O3 catalysts modified by cinchona alkaloids is an interesting model reaction for the investigation of heterogeneous catalysis capable of producing optically active products. The aim of the present theoretical study is to rationalize the interaction between protonated cinchona alkaloids (modifiers) and methyl pyruvate (substrate) by investigating the possible weak complexes formed by these two species. For this purpose we use molecular mechanics and the AM1 semiempirical method. The optimization leads to two stable forms of the complexes, where the substrate is bound to the modifier via hydrogen bonding between the oxygen of the α-carbonyl of pyruvate and the quinuclidine nitrogen of the alkaloid. In such complexes the methyl pyruvate is transformed into a half-hydrogenated species which can be adsorbed on the platinum surface and, after hydrogenation, leads to methyl lactate product. The results show that adsorption of the complex leading to (R)-methyl lactate is more favorable than that of the corresponding system yielding (S)-methyl lactate, which may be the key for the enantio-differentiation.

The parametrization of the EHMO-ASED method we have recently suggested for organometallics is shown to be also applicable, in principle without any modification, to derive the major structural parameters of second-row transition metal systems such as carbonyls or metallocenes. Furthermore, this model leads to satisfactory results when used to calculate the structure of compounds as large as (N-methylindole)tricarbonylchromium(0) or (phenylo.xazoline) tricarbonylchromlum(0) with full geometry optimization of the ligands.

When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCl family, doped with samarium(II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the samarium(II) and the time evolution of the hole burning.

The tetragonal Er³⁺ ion associated with the interstitial F⁻ ion along the [100] axis in CaF₂ is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F⁻ nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported.

The absorption spectra of the ferrate (VI) ion (FeO^2-₄) in K₂MO₄ (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions ³A₂→ ¹E and ³A₂  → ¹A₁. The lowest excited state is the ¹E state, serving as initial state for ¹E → ³A₂ sharp-line luminescence at around 6200 cm^-1. Another luminescence is observed centered at 9000 cm^-1, which is assigned to the ³T₂ → ³A₂ transition. It is rather broad and three orders of magnitude weaker than the ¹E luminescence at 30K as a result of efficient non-radiative relaxation processes to the ¹E state. The temperature dependence of the total intensity and the lifetime of the ¹E → ³A₂ luminescence is understood within a complex scheme of radiative and non-radiative processes.

A spectroscopic study of supersonic jet‐cooled catechol (1,2‐dihydroxybenzene) and its d₁‐ and d₂‐isotopomers, deuterated at the hydroxy groups, was performed by resonant two‐photon ionization (R2PI) and fluorescence emission techniques, and supplemented by molecular‐beam hole‐burning experiments. The latter prove that one single rotamer of catechol is dominant under molecular beam conditions. The complicated vibrational structure in the S₀→S₁ spectrum from the 0⁰₀ band to 400 cm⁻¹ above is not due to three different rotamers, as previously thought, but is due to the excitation of a vibrational progression associated mainly with the torsion of the hydroxy groups. The torsional bands are very prominent in the R2PI spectra, but are weak in the emission spectra. Detailed analysis of the torsional bands was based on a fit to the S₁ and S₀ state frequencies and the Franck–Condon factors in absorption and emission, using a double‐minimum potential for the S₁ state and a harmonic potential for the S₀ state. In the S₁ state one of the two –O–H torsional mode frequencies is lowered from τ₂≊250 to ≊50 cm⁻¹, and the molecule is only quasiplanar with respect to the –O–H torsional coordinates.

Mass-selective ground state vibrational spectroscopy of the jet-cooled carbazole·Ar complex was performed by populating ground-state levels via a pump/dump laser pulse sequence, followed by selective resonant two-photon ionization of the vibrationally relaxed complexes. Intra- and inter-molecular van der Waals modes in the S₀ state are measurable with good signal/noise. The ground-state binding energy can be determined by detecting the negative signals resulting from loss of ground-state population via vibrational predissociation of the complex.

The cyclic water trimer shows a fascinating complexity of its intermolecular potential-energy surface as a function of the three intermolecular torsional coordinates: there are six isometric but permutationally distinct minimum-energy structures of C₁ symmetry, which can interconvert by torsional motions via six isometric transition states, also of C₁ symmetry. A second type of interconversion can occur through different torsional motions via two C₃ symmetric transition structures, and a third interconversion type via a planar C_3h symmetric transition structure. The equivalence of the six minima is broken if the ‘free’ H atom of one H₂O molecule in the cluster is chemically substituted, yielding three distinct conformers, which occur in enantiomeric pairs. Not all three conformers are necessarily locally stable minima; this depends on the substituent. The phenol–(H₂O)₂, p-cyanophenol–(H₂O)₂, 1-naphthol–(H₂O)₂ and 2-naphthol–(H₂O)₂ clusters, which are the phenyl, p-cyanophenyl and naphthyl derivatives of (H₂O)₃, were examined by resonant two-photon ionization spectroscopy in supersonic beams. These clusters exhibit S₀→ S₁ vibronic spectra with very different characteristics. These reflect the number of cluster structures formed, their low-frequency intermolecular vibrations and indirectly give information about the cluster fluxionality.

The mixed-metal ferromagnet {[P(Ph)4][MnCr(ox)3]}n, where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R3c, a=b=18.783(3), c=57.283(24) Å, α=β=90, γ=120°, Z=24 (C30H20O12PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at Tc=5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM'(ox)3]n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO6' chromophores. Their polarization patterns reflect the electric dipole selection rules for D3 symmetry. A strong site selective luminescence from the chromium(III) 2E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.

We have developed a model to describe the inhomogeneous broadening of optical spectra in the substitutionally disordered crystals. The comparison with the experimentalf–f fluorescence spectra of SrFCl_xBr_1−x:Sm²⁺ (0≤x≤1) allowed to establish, in a very detailed manner, the relationship between the inhomogeneous spectral distribution and the crystal structure around the Sm²⁺ impurity.

Single crystals of the alkaline earth fluorides doped with silver were grown successfully. This paper presents details of the methodology. The as-grown crystals consist of colorless transparent and yellowish regions. The former were shown to contain the Ag⁺ ion and the latter also silver pairs, small clusters and probably colloidal aggregates. Complex optical absorption bands were observed in the samples of the former parts after they had been X-irradiated. The samples were subsequently exposed to extended series of physico-chemical treatments with the aim to obtain information regarding the electronic structures involved. The evolution was monitored with the aid of optical absorption experiments. Factor analysis technique is presented and was applied to uncover mutually independent contributions to these absorption spectra. We identified the Jahn-Teller Ag⁺ [Bill H. et al., Solid State Commun.70, 511 (1989)] and several centers which formally involve an Ag⁻ ion.

The spectroscopic properties of Ag⁺-doped strontium fluoride crystals were investigated at various temperatures, using absorption and fluorescence spectroscopies. The system exhibits a strong ultraviolet emission upon excitation into the two principal absorption bands. The azimuthal dependence of the degree of polarization of this luminescence is analysed, as well as its dynamics. The monovalent silver ions are shown to substitute for a host cation, with cubic symmetry. This is the first reported example of a cubic coordination for the Ag⁺ ions in an insulator. This cubic field, together with the strong ionic character of the framework, confers rather original spectroscopic properties to this system. The luminescence mechanisms are interpreted on the basis of the measured decay times and with the aid of energy diagram calculations. Two closed thermalized spin-orbit levels, with symmetry A_2g and T_2g respectively, are involved in the luminescence processes. The pure spin triplet A_2g only emits at low temperatures (T<15 K), whereas the T_2g level ( approximately 2% spin singlet character) emits in turn upon warming the crystal. One-dimensional configuration coordinate diagrams are proposed to interpret the peculiar temperature dependence of the emission band maximum.

Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.

In der Übergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei ändern sich die magnetischen und optischen Eigenschaften, über die der thermische Spinübergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phänomen tritt sowohl in flüssiger Phase als auch im Festkörper auf. Eine herausragende Stellung nehmen Eisen(II) - Spincrossover - Verbindungen ein, in denen der Spinübergang im Festkörper auf sehr unterschiedliche Weise - graduell, abrupt, mit Hysterese oder stufenweise - verlaufen kann und mit Mößbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitäts- und Wärmekapazitätsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekülen kann befriedigend durch elastische Eigenschaften und durch die Änderung von Gestalt und Volumen der Komplexmoleküle beim Spinübergang erklärt werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, daß sich der Low-Spin-Zustand mit grünem Licht in den High-Spin-Zustand umschalten läßt, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht läßt sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zurückschalten. Der Mechanismus des LIESST-Effekts ist aufgeklärt, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Übergänge verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.

A study of the effect of an external electric field on spectral holes burnt at different frequencies in the inhomogeneous absorption band of a centrosymmetric squaraine dye, bis [4-(diethylamino)-2-hydroxyphenyl] squaraine (DEAH), in polymers of different polarity is presented. Average matrix induced dipole moment differences of about 1 D and 0.37 D were measured in the directions parallel and perpendicular to the long axis of DEAH. In all polymers investigated, the induced dipole moment difference decreased from the higher to the lower frequencies. Solvatochromic shift measurements were performed in order to elucidate the origin of this effect. The matrix field inducing the dipole moment is also partially responsible for the frequency shift of the absorption of DEAH. With increasing matrix field, the absorptiion frequency is shifted to the blue due to electrostatic interaction with the local dipoles of DEAH. The contribution of the electrostatic interactions to the frequency shift is smaller than the dispersion interactions by two orders of magnitude in polystyrene, but increases slightly in more polar polymers.

The free ion yield (R) resulting from the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by various electron donors in acetonitrile has been studied using ns laser photoconductivity. The influence of the chemical nature of the doors is established in a general manner. For a given oxidation potential Eox(D/D+), the rate constant of geminate ion recombination, kbac, decreases significantly as the electronic delocalization of the donor increases. As a consequence multiple Marcus plots are observed in the inverted region. These plots show decreasing curvature when going from stilbenes to amines as donors. This fan effect is tentatively explained by considering the detailed roles of the parameters V, and h in the Marcus model.

The principles of ps transient grating spectroscopy are presented. The capabilities of this technique for the study of photoinduced processes are illustrated by several new results. These include the observation of an anomalous effect in the reorientation dynamics of two ionic dyes in long-chain alkanenitriles, the measurement of the local viscosity in the interior of reverse micelles containing methanol, and the determination of the rate constant of separation of a geminate radical pair formed by photoinduced electron-transfer reaction.

The rotational diffusion time of ruthenium (II)-bis(2,2'-bipyridine) (2,2'-biquinoleine) has been measured in polar solvents of different viscosity. The rotational dynamics can be explained in terms of the Stokes—Einstein—Debye hydrodynamics theory under stick boundary condition by considering the rotating body as a prolate spheroid enclosing the complex. This overestimates the intrinsic molecular volume by a factor of 1.5. The difference can be accounted for by solvent molecules intercalated in the interligand space and stabilized by electrostatic interaction with the charge of the metal atom.