The previously proposed model for reactant–modifier interaction in the enantioselective hydrogenation of activated carbonyl compounds over platinum chirally modified by cinchona alkaloids has been extended to platinum modified by synthetic pyrrolidinyl–naphthyl–ethanol modifiers. As in the case of cinchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is achieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one interaction between modifier and reactant was calculated at the ab initio level. A comparison of the results for different modifiers leads to the conclusion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of the (R)-product is traced to the fact that the pro-(S) complex leading to the (S)-product upon hydrogenation is more destabilised due to repulsive interaction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanism of enantiodifferentiation.

The sulfidation behavior of alumina-supported W catalysts was investigated by means of temperature-programmed sulfidation, quick extended X-ray adsorption fine structure measurements, and X-ray photoelectron spectroscopy of two series of tungsten catalysts, one made from ammonium metatungstate and the other from ammonium tetrathiotungstate. The effect of the fluorination of the alumina support on the sulfidation behavior of W on these two series of catalysts was also studied. The sulfidation of catalysts prepared with ammonium metatungstate passes through intermediates of W oxysulfides; the sulfided catalysts are mixtures of W oxysulfides and WS₂. After sulfidation at 400°C and atmospheric pressure for 4 h, the degree of sulfidation is only about 50%. Fluorination slightly increases the degree of sulfidation. When ammonium tetrathiotungstate is used as the precursor, fully sulfided catalysts can be obtained. Fluorination accelerates the transformation of WS₃ sulfide to WS₂.

Platinum particles supported on graphite have been investigated by scanning tunneling microscopy (STM). For one monolayer thick Pt particles the individual Pt atoms form a characteristic intensity pattern due to a mismatch between the Pt and graphite lattice. Based on density functional theory calculations and model structures of Pt on graphite it is argued that the observed STM imaging contrast has its origin in the tip induced elastic deformation of the graphite underneath the Pt particle. The Pt–graphite potential is much stiffer than the graphite–graphite potential. The calculations furthermore indicate that Pt adsorption is favored over top rather than hole sites and that the barrier for diffusion is very low.

Nondestructive immobilization of cobalt and copper Schiff base complexes in silica aero- and xerogels was achieved via the sol−gel method using a precursor N,N‘-ethylenebis(salicylidenaminato) (salen) ligand modified with pendant silyl ethoxy groups. Aerogels were obtained by semicontinuous extraction of the wet gels with supercritical CO₂ and xerogels by conventional drying. Cobalt and copper(salen) containing silica gels were characterized by FTIR, UV−vis, and XPS spectroscopy, laser ablation-ICP-MS, and EPR studies. Aero- and xerogel incorporated salen compounds exhibited similar spectroscopic properties to cobalt/copper(salen) precursors and known metal(salen) compounds. BET measurements confirmed the importance of supercritical CO₂ drying in maintaining the mesoporous structure of the aerogel. Laser ablation-ICP-MS and EPR studies of the aerogels showed that a uniform distribution of the isolated metal(salen) complex was achieved via molecular mixing using the sol−gel method. Stability of these materials was demonstrated by thermogravimetric analyses in air and leaching studies conducted under typical liquid-phase oxidation conditions. XPS analyses showed surface relative atomic concentrations in the modified gels to be similar before and following leaching studies.

Enantioselective hydrogenation of the pseudo-aromatic 4-hydroxy-6-methyl-2-pyrone to the corresponding 5,6-dihydropyrone has been studied over cinchonidine-modified Pd/Al₂O₃ and Pd/TiO₂ catalysts. A mechanistic model for enantiodifferentiation is proposed, involving two H-bond interactions (N–H···O and O–H···O) between the deprotonated reactant and the protonated chiral modifier. The model can rationalize (i) the sense of enantiodifferentiation, i.e., the formation of (S)-product in the presence of cinchonidine as modifier; (ii) the complete loss of enantioselectivity when the acidic OH group of the reactant is deprotonated by a base stronger than the quinuclidine N of the alkaloid; and (iii) the poor enantiomeric excesses obtained in good H-bond donor or acceptor solvents. NMR and FTIR investigations, and ab initio calculations, of reactant–modifier interactions support the suggested model. Several factors, such as catalyst prereduction conditions, trace amounts of water, presence of strong bases and acids, and competing hydrogenation of acetonitrile to ethylamines, were found to affect the efficiency of this catalytic system.

Model platinum catalysts have been designed to study the platinum−solvent interface in situ using attenuated total reflection (ATR) infrared spectroscopy. Pt and Pt/Al₂O₃ thin films were evaporated on a Ge internal reflection element (IRE) and characterized by XRD, XPS, AFM, STM, and IR spectroscopy. Changes within the adsorbate layer of the Pt catalyst during cleaning with O₂ and H₂ were followed. After cleaning, the catalyst surface was probed by CO adsorption from CH₂Cl₂. For the Pt/Al₂O₃ film the spectrum of adsorbed CO showed a band at 2000 cm^-1, which is typical for Pt/Al₂O₃ catalysts. The stretching vibration of linearly bonded CO exhibited a coverage-dependent frequency shift due to vibrational coupling, thus showing the existence of large clean domains on the reactive catalyst surface even in the presence of an organic solvent. CO adsorption from CH₂Cl₂ was slow before the cleaning process. However, subsequent admission of H₂ resulted in an instantaneous and drastic increase of the CO absorption signal. The origin of this effect is a structural change of the Pt particles induced by dissolved hydrogen, which was directly monitored by ATR spectroscopy using CO as probe molecule. STM investigations showed sintering of the Pt particles upon hydrogen treatment in CH₂Cl₂ at room temperature, which leads to a surface-enhanced infrared absorption (SEIRA).

The sulfidation behavior of alumina-supported Ni–W catalysts was investigated by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), quick extended X-ray absorption fine structure (QEXAFS), and X-ray absorption near-edge structure spectroscopy (XANES). Either ammonium tetrathiotungstate or ammonium metatungstate was used as the precursor of tungsten, and nickel nitrate was the source of nickel. The effect of fluorination of the alumina support on the sulfidation behavior of tungsten and nickel on these two series of catalysts was studied as well. The sulfidation of the catalysts prepared from ammonium metatungstate passes through W(VI) oxysulfide intermediates. Fluorination of the alumina support aids the sulfidation of tungsten and nickel at low temperature and promotes the transformation of the W(VI) oxysulfide intermediates to WS₂. After sulfidation at 400°C and atmospheric pressure for 4 h, about 50% of tungsten and 60% of nickel in the catalysts prepared from ammonium metatungstate were sulfided. EXAFS showed that ammonium tetrathiotungstate supported on alumina decomposes to oxidic tungsten during the second impregnation with nickel nitrate. Nevertherless, sulfidation of the catalysts prepared from ammonium tetrathiotungstate is much easier. It also passes through W(VI) oxysulfide intermediates, and fluorination aids the formation WS₂. In the sulfided catalysts prepared from ammonium tetrathiotungstate and nickel nitrate, 100% of tungsten and nickel is in the sulfided state, but a small amount of tungsten is in a {WS₃} state, with fully sulfided W(VI), rather than in the WS₂ state. The fluorine-containing catalyst contains a larger fraction of WS₂ than the fluorine-free catalyst.

Attenuated total reflection (ATR) spectra of adsorbates and solvent on thin metal films were investigated with emphasis on the band-shape of absorption bands. Distorted band-shapes are found even far from the critical angle. Strong absorption bands are more distorted. The band-shape strongly depends on the optical constants of the metal film and its thickness. The distortion increases with increasing thickness and increasing refractive index of the thin metal film. For a 10 nm thick Pt film the measured band-shapes for liquid water and ethanol are in good agreement with theoretical predictions using the bulk optical constants for Pt. For CO adsorbed on a 1 nm Pt film a distorted band-shape is observed whereas calculations assuming bulk optical constants predict band-shape distortion only for considerably thicker Pt films. The effective optical constants for very thin metal films deviate considerably from the bulk values, due to the island structure of the film and non-adiabatic effects can lead to distorted band-shapes. Structural changes within a Pt film, induced by hydrogen treatment, leads to a change in the band-shape for adsorbed CO. The results show that band-shape analysis is a valuable tool for in situ ATR IR spectroscopy of metal films.

Adsorption of cinchonidine on a platinum model catalyst studied by in situ ATR-IR spectroscopy revealed that the adsorption mode depends on surface coverage and is affected by concomitant adsorption and fragmentation of solvent molecules.

Base-catalyzed H/D-exchange for α- and β-isophorone (1 and 2, resp.) was monitored by NMR spectroscopy to identify the number and nature of reactive sites. Results show that α-isophorone (1) undergoes H/D exchange at up to four different sites depending on reaction conditions. β-Isophorone (2), on the other hand, exhibits activity at two sites, predominantly at the α-position, under comparable conditions. Quantum-chemical calculations indicate that the thermodynamically more-stable anions formed upon proton abstraction from isophorone are not favored kinetically in all cases. Thermodynamically unfavorable H/D-exchange at the α-position in 1, which is observed experimentally, is explained via intermediate formation of γ-isophorone (3) with subsequent conjugation to the α-isomer. Differences observed in the reactivities of the two isomers and differences in reactivity of 1 under various conditions in reactions involving proton abstraction as an initial step may be partly explained on the basis of these results.

The reactions of hydrogen atoms adsorbed on a Ni(111) surface (surface-bound H) and hydrogen atoms just below the surface (bulk H) with coadsorbed acetylene are probed under ultrahigh vacuum conditions. Bulk H is observed to react with acetylene upon emerging onto the surface at 180 K. Gas-phase hydrogenation products, ethylene and ethane, are produced as well as an adsorbed species, ethylidyne. Ethylidyne is identified by high-resolution electron energy loss spectroscopy. Surface-bound H reacts with adsorbed acetylene above 250 K to produce a single product, adsorbed ethylidyne. No gas-phase hydrogenation products, such as ethylene or ethane, are observed. The reaction of surface-bound H is extremely slow, with a rate constant determined from measurements of the initial reaction rate to be in the range of 10^-5−10^-3 (ML s)^-1 for a temperature range of 250−280 K. The activation energy for the rate-determining step, which is shown to be the addition of the first surface-bound H to acetylene to form an adsorbed vinyl species, increases from 9 to 17 kcal/mol as the total coverage decreases from 0.92 to 0.74 ML. The reaction rate cannot be described by a simple first-order dependence on the coverage of either reactant, indicating the presence of strong interactions between reactants. Measurements of the equilibrium constant reveal strong interactions between the reactant surface H and the product ethylidyne, possibly resulting in island formation. Mechanisms for the formation of ethylidyne by the reactions of both surface-bound and bulk H are proposed, as well as mechanisms for the formation of ethylene and ethane by bulk H. The different product distributions resulting from the reaction of acetylene with the two forms of hydrogen are discussed in terms of the large energy difference between bulk and surface-bound H.

An in situ attenuated total reflection study of the chiral solid−liquid interface created by cinchonidine adsorption on a Pt/Al₂O₃ model catalyst is presented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydrogenation of α-functionalized ketones. Cinchonidine adsorbs via the quinoline moiety. The adsorption mode is coverage dependent and several species coexist on the surface. At low concentration (10^-6M) a predominantly flat adsorption mode prevails. At increasing coverage two different tilted species, α-H abstracted and N lone pair bonded cinchonidine, are observed. The latter is only weakly bound and in a fast dynamic equilibrium with dissolved cinchonidine. At high concentration (10^-4−10^-3 M) all three species coexist on the Pt surface. A slow transition from an adsorbate layer with a high fraction of α-H abstracted cinchonidine to one with a high fraction of N lone pair bonded cinchonidine is observed with the cinchonidine concentration being the driving force for the process. The reverse transition in the absence of dissolved cinchonidine is fast. Cinchonidine competes with solvent decomposition products for adsorption sites on the Pt, which may contribute to the observed solvent dependence of the heterogeneous enantioselective hydrogenation of ketones by cinchonidine-modified Pt.

A series of titania–silica aerogels with 0–100 wt% TiO₂ content were synthesized and characterized by N₂ physisorption, DRIFT, UV-Vis, XPS, and ²⁹Si CP/MAS NMR analysis. It is shown that kinetic analysis of the epoxidation of 2-cyclohexene-1-ol (1) with TBHP is an informative test reaction providing insight in the nature of active sites. The surface area, pore volume, hydrophobicity, and relative abundance of Ti–O–Si linkages in the aerogels decreased with increasing Ti/Si ratio. Parallel to these changes, the initial rate of epoxide formation per Ti site (TOF) and the epoxide selectivity decreased but the productivity of the catalysts went through a maximum at 10 wt% TiO₂. We propose that due to kinetic effects in the sol–gel synthesis the whole range of active Ti sites may be present in the mixed oxides, spanning from tetrahedral Ti isolated by four SiO groups to octahedral Ti surrounded by six TiO groups in titania nanodomains. Ether formation from 1 was catalyzed by Brønsted sites present only on high titania-containing aerogels. Oligomerization was a major side reaction on all catalysts including Ti-free silica. Si-free titania was the most active in allylic oxidation of 1 to cyclohexenone. Silylation, or amine (Me₂BuN) addition to the reaction mixture, eliminated ether formation and suppressed oligomerization.

We report a theoretical density functional analysis of the exchange interactions in (VO)₂P₂O₇ using molecular fragments. The calculations confirm that the magnetic structure must be decribed on the basis of linear dimer chains. The strongest exchange interaction is found through O-P-O bridges. The magnitude of the exchange parameters is governed not only by V-V distance but also by the whole structure along the superexchange pathway. The two chains present in the structure of (VO)₂P₂O₇ are magnetically inequivalent. For the monoclinic phase of (VO)₂P₂O₇, important variations in the calculated parameters for dimers with identical bridges are observed within one chain. The magnetic structure of this chain should be described not by two but by three or even four coupling constants.

This article presents a numerical quadrature intended primarily for evaluating integrals in quantum chemistry programs based on molecular orbital theory, in particular density functional methods. Typically, many integrals must be computed. They are divided up into different classes, on the basis of the required accuracy and spatial extent. Ideally, each batch should be integrated using the minimal set of integration points that at the same time guarantees the required precision. Currently used quadrature schemes are far from optimal in this sense, and we are now developing new algorithms. They are designed to be flexible, such that given the range of functions to be integrated, and the required precision, the integration is performed as economically as possible with error bounds within specification. A standard approach is to partition space into a set of regions, where each region is integrated using a spherically polar grid. This article presents a radial quadrature which allows error control, uniform error distribution and uniform error reduction with increased number of radial grid points. A relative error less than 10^−14 for all s-type Gaussian integrands with an exponent range of 14 orders of magnitude is achieved with about 200 grid points. Higher angular lquantum numbers, lower precision or narrower exponent ranges require fewer points. The quadrature also allows controlled pruning of the angular grid in the vicinity of the nuclei.

The results of a study on the ground states of tricarbonato complexes of dioxouranate using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to uranium in the formal oxidation states VI and V, [UO₂(CO₃)₃]^4- and [UO₂(CO₃)₃],^5- have been fully optimized in D_3h symmetry at second-order perturbation theory (MBPT2) level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. The uranyl fragment has also been optimized at CASSCF/CASPT2, to obtain an estimate of the MBPT2 error. Finally, the effect of distorting the D_3h symmetry to C₃ has been investigated. This study shows that only minor geometrical rearrangements occur in the one-electron reduction of [UO₂(CO₃)₃]^4- to [UO₂(CO₃)₃],^5- confirming the reversibility of this reduction.

The results of a theoretical study on the formation of the nitrogen cluster N₁₀ from the ionic species N₅⁺ and N₅^− are presented. The possibility to form N₈ from N₅⁺ and N₃^−has also been studied but no stable form was found. Structural and vibrational data are given for the different clusters. It is suggested that the anion N₅^− might be stable enough to be synthesized. The calculations have been carried out using multiconfigurational self-consistent-field wave functions and second-order perturbation theory.

An implementation of spin–orbit coupling within a two-component generalization of the density functional code MAGIC is described. The spin–orbit operator is represented in the effective one-electron mean-field approximation and included into the Fock matrix within an iterative self-consistent scheme. First tests have been carried out for the spin–orbit splitting of several atoms. The spin–orbit effect on the bond distance and harmonic frequency of some diatomics has also been determined. This scheme allows to include spin–orbit in a simple way and can be efficiently used to treat large systems.

Three all-nitrogen chemical species in bulk compounds are experimentally known from the three last centuries: N₂, N₃^-, and N₅⁺. The last one was predicted in 1991. Furthermore there is evidence for tetrahedral N₄ in matrixes. Could further "nitrogens" exist? In recent years, the hypothetical existence of poly-nitrogen clusters has been the object of several theoretical investigations (refs 5−16 and references therein). Besides their theoretical interest, these structures have drawn attention because of their possible use as high energy-density materials (HEDM), that is, the large ratio between the energy released in a fragmentation reaction and the specific weight.

The structure and vibrational frequencies of the UO₂ molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin−orbit coupling. The molecule has been found to have a (5fφ)(7s), ³Î¦_u, Ω = 2 ground state with a U−O bond distance of 1.77 Ã…. The computed antisymmetric stretching σ_u frequency is 923 cm^-1 with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm^-1 and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.

The ¹H and ¹⁵N resonances of the carbon monoxide complex of ferrocytochrome c‘ ofRhodobacter capsulatus, a ferrous diamagnetic heme protein, have been extensively assigned by TOCSY−HSQC, NOESY−HSQC, and HSQC−NOESY−HSQC 3D heteronuclear experiments performed on a 7 mM sample labeled with ¹⁵N. Based on short-range and medium-range NOEs and H^N exchange rates, the secondary structure consists of four helices: helix 1 (3−29), helix 2 (33−48), helix 3 (78−101), and helix 4 (103−125). The ¹⁵N, ¹H^N, and ¹H^αchemical shifts of the CO complex form are compared to those of the previously assigned oxidized (or ferric) state. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. The χ-tensor is axial, and the orientation of the z-axis is approximately perpendicular to the heme plane. The paramagnetic chemical shifts of the protons of the heme ligand have been determined and decomposed into the Fermi shift and dipolar shift contributions. Magnetic susceptibility studies in frozen solutions have been performed. Fits of the susceptibility data using the model of Maltempo (Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540−2547) are consistent with a rather low contribution of the S = ³/₂ spin state over the range of temperatures and confirm the value of the axial anisotropy. Values in the range 10.4−12.5 cm^-1 have been inferred for the axial zero-field splitting parameter (D). Analysis of the contact shift and the susceptibility data suggests that cytochrome c‘ of Rb. capsulatus exhibits a predominant high-spin character of the iron in the oxidized state at room temperature.

The first-order Raman spectrum of K₂S was measured over the temperature range from 10 to 742 K. The temperature dependence of the linewidth can be explained using results from the anharmonic lattice dynamics approach. Both the cubic and the quartic anharmonic interactions are of importance for this system. At 293 K, the Raman line is at ω_T2g=128 cm^−1 with a full width at half maximum Γ_T2g=2.9 cm^−1.

Calculated binding energies and spectroscopic properties of C₇₀ dimers are presented. The two most stable isomers of the set of conceivable [2 + 2] cycloaddition products are isoenergetic, and both are compatible with NMR, infrared, and Raman data on the product recently synthesized by Lebedkin et al.

The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)₆](BF₄)₂, by means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spin→low-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given.

The cobalt(II) complexes prepared with a series of enantiopure ligands (1-3) containing the bis(oxazolinyl)pyridine unit have been studied. The ligands form high spin octahedral complexes as shown by the X-ray crystal structure of the homochiral complex [Co(R,R-1)₂](ClO₄)₂(CH₃CN)₃. The diastereoselectivity of complex formation has been studied: equimolar mixtures of RR and SS ligands show mixtures of homochiral and heterochiral complexes for 2 and 3, but the phenyl-substituted ligand 1 shows exclusive formation of the heterochiral species. This selectivity is correlated with structural and electronic properties of the complexes.

Time-resolved X-ray diffraction is utilized to follow phase transitions in nanostructured silica/surfactant composites in real time under hydrothermal conditions. The data allow us both to obtain kinetic parameters and to observe intermediate phases. In all cases, changes in the packing of the organic component of these composites drives the transformation, indicating that surfactant packing is a dominant factor in determining the overall structure of these materials. For materials heated in pure water, however, high activation energies for transformation were measured, suggesting that large kinetic barriers can stabilize structures against surfactant-driven rearrangements. Matching between the interfacial charge density of the inorganic silica framework and the charge density of the surfactant headgroups is also found to affect the kinetics of transformation. Lamellar-to-hexagonal transitions, which complement condensation-induced changes in charge density, are observed to be continuous, while hexagonal-to-lamellar transitions, which proceed contrary to these charge density changes, are discontinuous. For materials heated in their high-pH synthesis solutions, more complex phase behaviors are observed. Hexagonal (p6mm) structures transform either to a bicontinuous cubic phase (Ia3d) or to a lamellar structure. Lamellar phases are observed at either long or short polymerization times, while cubic phases dominate at intermediate polymerization times. The production of these different phases can be understood by considering the interplay between organic packing, charge density matching, and changing activation energies. At short times, high charge on the inorganic framework favors transformation to the low-curvature lamellar structures. At very long times, silica condensation both reduces this charge density and cross-links the framework. This cross-linking raises kinetic barriers for transformation and again favors the topologically simpler hexagonal-to-lamellar transition. Transformations to the cubic phase are only observed at intermediate times, when these effects are balanced.

In this study, phase transformation of the hexagonal mesostructure MCM-41 to the cubic mesostructure MCM-48 is examined by in situ X-ray diffraction (XRD) of the transforming mesostructure and by XRD of products from bulk transformation experiments in Parr autoclaves. Transformations were studied under conditions of high pH and temperatures between 100 and 190 °C. Heating events took place after the hexagonal mesophase had assembled, but before it had fully polymerized. On the basis of these and previous results on transformations in silica−surfactant−water and surfactant−water systems, a model is proposed to explain the expected hexagonal → cubic transformation as well as the brief existence of a lamellar phase during the transformation. Additional experiments to establish synthetic parameters for the transformation included varying the silicon alkoxide source, replacing the supernatant prior to heating, and adding fluoride or aluminum to the reaction mixture. The results, taken together, illustrate the strong cooperativity between the organic and inorganic regions in controlling the assembly of the mesostructure and provide a better understanding of the effects that control phase transformations in these systems.

Results of EPR and optical spectroscopic investigation of the trigonal paramagnetic Yb³⁺ ion in SrF₂ (‘oxygen’ paramagnetic center — T₂) are presented. The energy level scheme of the center is determined from its optical spectra and the parameters of the crystal field potential are calculated.

The absorption and emission spectra of benzo[g,h,i]perylene, a six ring polycyclic aromatic hydrocarbon molecule (C₂₂H₁₂), embedded in a rare gas matrix are reported. Time dependent emission shows that this molecule exhibits sharp phosphorescence in the red. Supporting theoretical calculations using the recently developed time-dependent density-functional response theory formalism (TD–DFRT) allow a tentative assignment for the observed transitions. The astrochemical significance of the results is briefly discussed.

Nitrosyl metal complexes, such as the sodium nitroprusside, have attracted chemists' interest for more than 30 years. The existence of long-lived metastable states easily populated by irradiation are the principal reason for this interest. Those long-lived states are interesting either for technical applications or for fundamental research. In this work, we present a comparative density functional theory (DFT) study of the ground state of two different nitrosyl compounds:  sodium nitroprusside and cyclopentadienylnitrosylnickel(II).

The photochemical reactions of the nitroprusside and the CpNiNO complexes are explained on the basis of ΔSCF and time-dependent density functional theory (TD-DFT) calculations. Both similarities and differences in the photochemical processes are highlighted.

The photochemistry of the CpNiNO complex has been investigated using density functional theory. The whole potential energy curve along the NiNO angle coordinate is presented for the first time with both ground and metastable states, and transition states connecting the minima. The excited states of the GS, MS_I, and MS_II species have been calculated using time-dependent density functional theory. Furthermore, the structure of the excited states pertaining to the photochemistry of CpNiNO has been optimized. From these results it is shown that the backward GS � MS_II � MS_I reaction is more efficient than the forward GS → MS_II → MS_I scheme.

The density-functional approach based on the partition into subsystems was applied to study the benzene dimer. For several structures, the calculated interaction energy and intermolecular distance were compared with the previous theoretical results. A good agreement with high level ab initio correlated methods was found. For instance, the interaction energies obtained in this work and the CCSD(T) method agree within 0.1 - 0.6 kcal/mol depending on the structure of the dimer. The structure with the largest interaction energy is T-shaped, in agreement with CCSD(T) results. The T-shaped structure of benzene dimer was suggested by several experimental measurements. The calculated interaction energy of 2.09 kcal/mol agrees also well with experimental estimates based on the dissociation energy which ranges from 1.6±0.2 to 2.4±0.4 kcal/mol and the estimated zero-point vibration energy of 0.3 - 0.5 kcal/mol.

The CO molecule is frequently used as a probe in studies of zeolites where it adsorbs on metal cations. Compared with the free CO molecule, the stretching frequency of CO adsorbed in a zeolite is blue-shifted. The magnitude of the shift depends on the cation. The theoretical studies by Ferrari et al. [J. Chem. Phys., 105, 4129 (1996)] show that the isolated cation does not provide a good model of the zeolite because the calculated shifts are significantly overestimated. In this work, the effects of the interactions between theMe⁺CO (Me=Li, Na, or K) complex and the zeolite framework on the properties of CO adsorbed on the cation site are investigated. The properties of the investigated complexes are studied using the embedded molecule approach applying the orbital-free effective embedding potential derived within the subsystem formulation of density functional theory. In order to identify the major microsopic effects affecting the properties of the bound probe molecule, a hierarchy of cluster models is used to represent the zeolite framework. For the largest cluster model applied, the calculated frequency shifts agree within few cm^−1 with experimental data. 

Vertical excitations calculated for the CrO₄^2- , MnO₄^2-  , RuO₄, CrF₆, FeCp₂, RuCp₂ and CpNiNO species are compared to experimental spectra. The results obtained from the time-dependent density-functional theory−response theory (TD-DFRT) method are compared to both previously reported ΔSCF calculations and experiment. The results show that, in general, excited states of metal oxide and metallocene compounds are well described by TD-DFRT. However, serious difficulties are met with the CrF₆ system.

The predictive power of DFT, HF, and MP2 ²⁹Si NMR chemical shift calculations for silane molecules, including fluoro- and methylsilanes (Si_nH_2n+2 (n = 1, ..., 5), Si_nF_2n+2 (n = 1, ..., 3), and SiH_mX_4-m (X = F, CH₃)) is compared. A systematic accumulation of error proportional to the number of hydrogen neighbors to silicon sites is observed for DFT for all applied exchange-correlation functionals, whereas MP2 is not affected by this problem. A proposed empirical correction scheme for DFT provides excellent agreement with experiment with any exchange-correlation functional employed in this study.

The light-induced high-spin → low-spin relaxation for the Fe(II) spin-crossover compounds [Fe(btpa)](PF₆)₂ and [Fe(b(bdpa))](PF₆)₂ in solution, where btpa is the potentially octadentate ligand N,N,N‘,N‘-tetrakis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine and b(bdpa) is the analogous hexadentate ligand N,N‘-bis(benzyl)-N,N‘-bis(2-pyridylmethyl)-6,6‘-bis(aminomethyl)-2,2‘-bipyridine, respectively, has been studied by temperature-dependent laser flash photolysis. [Fe(b(bdpa))](PF₆)₂ shows single-exponential ⁵T₂ → ¹A₁ relaxation kinetics, whereas [Fe(btpa)](PF₆)₂ exhibits solvent-independent biphasic relaxation kinetics. The fast process of [Fe(btpa)](PF₆)₂ with a rate constant, k_HL, of 2.5 × 10⁷ s^-1 at 295 K and an activation energy, E_a, of 1294(26) cm^-1 in methanol can be assigned to the ⁵T₂ → ¹A₁relaxation as well. The slow process with a k_HL(295 K) of 3.7 × 10⁵ s^-1 and a E_a of 2297(32) cm^-1 in methanol - which is the slowest light-induced relaxation process observed so far for an Fe(II) spin-crossover complex in solution - is assigned to a coupling of the ⁵T₂ → ¹A₁relaxation process to a geometrical rearrangement within the pendent pyridyl arms.

The rate constants of back-electron-transfer (BET) reaction within geminate ion pairs generated upon static ET quenching of cyano-substituted anthracenes by aromatic amines and methoxy-substituted benzenes (MSB) at high concentration in acetonitrile have been measured directly using ultrafast multiplex transient grating spectroscopy. The free energy of BET, ΔG_BET, was varied between −3.0 and −0.6 eV, a range corresponding, in principle, to the inverted, barrierless, and normal regimes. When plotted vs ΔG_BET, the measured rate constants, k_BET, exhibit a large scattering. Good fits of the semiclassical expression for nonadiabatic ET are obtained if the rate constants are sorted according to the electron donor. The resulting electronic coupling matrix elements V are larger and the solvent reorganization energies smaller than those reported for BET within solvent-separated ion pairs, suggesting that BET takes place between ions in contact. However, in the low exergonicity region, the observed BET rate constants are slower than those reported for contact ion pairs formed by charge-transfer excitation. The dynamics of BET within radical pairs generated upon ET quenching of the N-methylacridinium cation has also been investigated, and the role of the electrostatic interaction within geminate ion pairs is discussed.

An investigation of the charge recombination (CR) dynamics of geminate ion pairs formed upon electron transfer quenching of azulene, benz[a]azulene, and xanthione in the second singlet excited state by several electron donors, using ultrafast time resolved spectroscopy and photoconductivity is reported. The ion pairs have two possible CR pathways:  (i) a highly exergonic CR to the neutral ground state or (ii) a moderately exergonic CR leading to the formation of the neutral acceptor in the first singlet excited state. This investigation shows strong evidence of the predominance of the second pathway. CR in ion pairs formed with the azulenes is faster by a factor of more than 50 than in ion pairs having a similar energy but with the first CR pathway only. The electron transfer quenching of xanthione in the second singlet excited state by several weak donors does not lead to a significant reduction of the triplet yield of this molecule. The relevance of these results to explain the absence of the inverted region in highly exergonic bimolecular charge separation reactions is discussed.

The excited-state dynamics of the radical cations of perylene (PE^•+), tetracene (TE^•+), and thianthrene (TH^•+), as well as the radical anions of anthraquinone (AQ^•-) and tetracenequinone (TQ^•-), formed by γ irradiation in low-temperature matrices (PE^•+, TH^•+, AQ^•-, and TQ^•-) or by oxidation in sulfuric acid (PE^•+, TE^•+, and TH^•+) have been investigated using ultrafast pump−probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE^•+ is substantially longer in low-temperature matrices than in H₂SO₄, where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE^•+*. The data suggest that both PE^•+* and TE^•+* decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ^•-*, TQ^•-* exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state.

Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1]^•- whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental ³¹P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding σ P−P orbital. Clearly, a one-electron bond (2.763 Ã…) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Ã… apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1]^2- which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1]^2-, the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P−P bond whose length (2.305(2) Ã…) is only slightly longer than a usual P−P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P−P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3]^•- Na⁺; however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)₂]^2-[{Na(THF)₂}₂]²⁺ a P−P bond is formed (2.286(2) Ã…) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon−sodium−phosphorus bridges.

Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The ³¹P and ¹³C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]√^− and [HP---CH=Cp]√ have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]√^− is readily protonated and that the EPR spectra are due to the phosphaallylic radical.

Stable for at least one week below -30°C: crystals of 1, the first highly persistent diphosphanyl radical, have been isolated and characterized. This phosphorus-centered radical exhibits hyperfine coupling whose anisotropy is considerably larger than that for well-established nitrogen radicals (hydrazyls, nitroxides). This feature is of potential interest for studies of fast molecular movements. Mes*=2,4,6-tBu₃C₆H₂.