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.