The asymmetric hydrogenation of cyclohexane-1,2-dione over cinchonidine-modified platinum was investigated. Despite the fact that the first hydrogenation step is close to nonenantioselective, a high enantiomeric excess is obtained for the (R)-α-hydroxyketone due to kinetic resolution. In the second hydrogenation step one out of the four reactions of the network is substantially accelerated with respect to the others and with respect to the reaction in the absence of modifier, leading to an enantiomeric excess of (1R,2R)-trans-cyclohexane-1,2-diol of over 80%. Comparison with recently reported asymmetric hydrogenation of α-hydroxyethers indicates striking similarities, which hint at similar reactant–modifier interaction in both cases. The importance of cis versus trans conformation of the reactant for the reactant–modifier interaction emerges from a comparison of suggested reaction intermediates for cyclohexane-1,2-dione and butane-2,3-dione hydrogenation, respectively.

A method to selectively probe the different adsorption of enantiomers at chiral solid−liquid interfaces is presented, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The weak spectral changes upon adsorption of enantiomers at a chiral interface are followed in time, while periodically changing the absolute configuration of the admitted chiral molecule. A subsequent digital phase-sensitive data analysis reveals spectral differences arising due to the different diastereomeric interactions of the two enantiomers with the chiral interface. The main advantage of the method compared to conventional difference spectroscopy is the enhanced signal-to-noise ratio. The method is selective for differences in diastereomeric interactions of the enantiomers. Its potential is demonstrated by studying the adsorption of ethyl lactate on a chiral stationary phase, which is amylose tris[(S)-α-methylbenzylcarbamate] coated onto silica gel. d-Ethyl lactate interacts stronger with the chiral stationary phase. In particular the spectral shifts reveal a stronger N−H···O=C hydrogen bonding interaction between amide group of the chiral stationary phase and the ester group of the ethyl lactate. The spectra also indicate that one of the three (S)-α-methylbenzylcarbamate side chains of the amylose derivative is predominantly involved in the interaction with the ethyl lactate. Furthermore, the experimental observations indicate that more than one interaction mode is populated at room temperature and that interaction with the ethyl lactate may induce a conformational change of the amide group of the chiral stationary phase.

The epoxidation of cyclohexene over titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) was investigated by in situ attenuated total reflection (ATR) infrared spectroscopy. In order to distinguish between relevant and spectator species and to increase sensitivity, ATR was combined with modulation spectroscopy. In the latter technique the catalyst system is periodically perturbed by a forced concentration change. The interaction of cyclohexene with the aerogels is found to be weak. In contrast, TBHP adsorbs strongly on the catalysts on two different sites. Modulation experiments reveal that TBHP adsorbed on Si-OH groups is a spectator, whereas the one adsorbed on the Ti-sites is involved in the catalytic cycle. The latter species is stronger adsorbed and the associated signals increase with the Ti content of the catalyst. Adsorption of the TBHP on the Ti sites results in a strong shift of the C-O stretching vibration and changes in the Ti-O-Si modes of the catalyst. The study furthermore reveals vastly different pore diffusion for cyclohexene and TBHP due to their different interaction with the polar catalyst surface. In the modulation experiments the reaction product appears retarded with respect to the admittance of the reactants, which indicates that pore diffusion and kinetics of adsorption and desorption are important factors for the catalysis. Methylation of the aerogel has a beneficial effect on the catalysis, which can be traced to the different pore size distribution and polarity with respect to the unmodified catalyst. When the flow-through ATR cell is slowly heated, a change in the framework vibrations of the catalyst occurs simultaneously with the onset of reaction, indicating reaction induced structural changes.

Prominent nonlinear effects in enantioselectivity were observed with a transient technique when ethyl pyruvate was hydrogenated over Pt/Al₂O₃ in the presence of two cinchona alkaloids, which alone afford the opposite enantiomers of ethyl lactate in excess. The changes in reaction rate and ee, detected after injection of the second alkaloid, varied strongly with type and amount of the alkaloid, and with the order of their addition to the reaction mixture. For example, under ambient conditions in acetic acid cinchonidine (CD) afforded 90% ee to (R)-ethyl lactate and addition of equimolar amount of quinidine (QD) reduced the ee to (R)-ethyl lactate only to 88%, though QD alone provided 94% ee to (S)-lactate in a slightly faster reaction. The stronger adsorption of CD on Pt in the presence of hydrogen and acetic acid was proved by UV–vis spectroscopy. The different adsorption strengths result in an enrichment of CD on the Pt surface and also in a crucial difference in the dominant adsorption geometries. CD is assumed to adsorb preferentially via the quinoline rings laying approximately parallel to the Pt surface. In this position it can interact with ethyl pyruvate during hydrogen uptake and control the enantioselectivity. The weaker adsorbing QD adopts mainly a position with the quinoline plane being tilted relative to the Pt surface and these species are not involved in the enantioselective reaction. Competing hydrogenation of the alkaloid, and steric and electronic interactions among the adsorbed species, can also influence the alkaloid efficiency and the product distribution. Hydrogenation of the quinoline rings at low alkaloid concentration resulted in unprecedented swings in the enantiomeric excess.

The enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone (1a), 3,6-dimethyl-4-hydroxy-2-pyrone (2a), 4-methoxy-6-methyl-2-pyrone (3a), and 4,6-dimethyl-2-pyrone (4a) was studied over a 5 wt% Pd/TiO₂ catalyst. Various cinchona alkaloids and their O- and N-methyl derivatives were applied as chiral modifiers. The catalytic experiments combined with FTIR, NMR, and NOESY-NMR spectroscopic analysis and ab initio calculations revealed an interesting feature of the reactions: the ee is determined by competing reactant–modifier interactions. These interactions may involve the OH function and the quinuclidine N of the alkaloid modifier. When the reactant possesses an acidic OH group (1a and 2a), the reaction via the energetically most stable bidentate complex controls the enantioselectivity. Protic or basic solvents diminish the ee in these reactions by stabilizing a single-bonded (acid–base type) interaction. Different mechanisms are proposed for the hydrogenation of the nonacidic pyrones 3a and 4a. These models can well interpret the catalytic results but require further confirmation. Besides, the studies provided the first experimental evidence for an intrinsic rate acceleration coupled with the enantiodifferentiating process over chirally modified Pd.

Alternative exposure of Pd thin films and Pd/TiO₂ catalysts to dissolved hydrogen and oxygen leads to significant changes in the reflectivity of infrared radiation as observed in attenuated total reflection spectroscopy. The reflectivity decreases and the absorbance increases upon changing from oxygen- to hydrogen-saturated solvent. Reflectivity calculations based on the Drude model for the Pd thin film show that a slight change in the concentration of the free electrons of the metal could be at the origin of the observed effect. Alternatively, the reversible formation of a surface oxide layer can lead to a similar observation. The reflectivity changes can be used to follow the changes of the metal catalyst, similar to potential measurements, however without the need to work in conducting media. They can be correlated with the observation of adsorbed species and the formation of reaction products. The potential of the method for in situ studies of catalytic solid−liquid interfaces is demonstrated for the oxidation of 2-propanol and ethanol. Upon changing from reducing to oxidizing conditions, the observation of reaction products is slightly offset with respect to the observed reflectivity change in both cases, whereas the frequency of the CO vibration shifts at the same time as the reflectivity increases.

A new type of high pressure spectroscopy view-cell for investigation of multiphase reactions is presented. It allows visual observation of the reaction mixture at conditions up to 200 °C and 200 bar. Measurements of the reactor cell’s upper part by transmission spectroscopy with variable path length and of the cell’s bottom part by attenuated total reflection (ATR) spectroscopy can be performed quasi-simultaneously. By coating the internal reflection element with a catalyst film, in situ investigations of heterogeneous catalysts can be performed. The potential of this new experimental setup is demonstrated using examples of heterogeneous and homogeneous catalytic reactions. For the heterogeneously catalyzed hydrogenation of ethyl pyruvate over Pt/Al₂O₃ in “supercritical” ethane the reaction progress could be monitored by spectroscopic investigation of the fluid phase. Quantitative evaluation of the spectra combined with digital imaging of the reaction mixture allowed simultaneous determination of phase behavior and reaction kinetics. ATR-IR spectra of the catalyst film could be measured at the same time. In the homogeneously catalyzed formylation of morpholine with “supercritical” carbon dioxide and hydrogen, not only number and nature, but also the composition of the different phases could be determined. The catalyst was found to be confined to the liquid phase. Although the aim of these preliminary studies was to test the functionality of the new cell, already significant new insight on the investigated catalytic reactions could be gained.

The interaction of 2-methoxypropene, a vinyl ether, with heterogeneous acid catalysts containing sulfonic acid groups covalently bound to SiO₂ (Deloxan ASP, Degussa) and sulfuric acid adsorbed on TiO₂-modified amorphous SiO₂ (Degussa), respectively, was investigated by in situ attenuated total reflection infrared spectroscopy. Rapid hydrolysis is observed, which does not, however, require the acid sites. The resulting acetone is adsorbed predominantly on SiOH groups. Promoted by the acid sites a further transformation is observed on the catalysts. Based on the time behavior of the ATR signals of acetone and the product the further reaction likely involves the condensation of 2-methoxypropene and acetone. During the buildup of the reaction product hydronium ions disappear from the catalyst surface. Upon desorption of the reaction product the hydronium ions become prominent again on the catalyst containing adsorbed sulfuric acid. This behavior is less pronounced on the catalyst, which contains sulfonic acid groups. The two investigated catalysts contain vastly different relative concentrations of Brønsted and Lewis acid sites, which can explain the difference in the relative concentration of intermediate and product at the interface in the observed consecutive reaction.

The combination of ATR−IR and modulation spectroscopy allowed for the study of the interaction of ketopantolactone with Pt/Al₂O₃ films chirally modified by cinchonidine under hydrogenation conditions. The spectra reveal a significant influence of ketopantolactone on the adsorption of the modifier and indicate a N−H−O hydrogen bond between modifier and reactant. The latter was corroborated by a comparative study with N-methyl cinchonidine chloride modified Pt/Al₂O₃.

A new ATR-IR cell was designed, and its performance was characterized by modulation excitation spectroscopy (MES). The new cell allows concentration modulation at relatively high frequency without unnecessary phase delay in the response. The response delay due to convection and diffusion was studied at different flow rates and modulation frequencies by experiments and simulations. The diffusion behavior of a small relatively fast-diffusing molecule, acetonitrile, was compared with that of a large slow-diffusing molecule, hemoglobin, in water. Experimentally, significant differences in their diffusion behavior were observed. The flow and diffusion behavior of the probe molecules was described using two different models, the diffusion layer model and the convection−diffusion model, and the theoretical results were compared with the experiments. The diffusion layer model allows estimating an effective diffusion layer thickness near the surface of the internal reflection element. However, the simulated response is significantly different from the experimental one. On the other hand, the convection−diffusion model describes the flow and diffusion behavior of the solute molecules with high accuracy. This work forms the basis for the investigation of chemical and physical kinetics such as surface reaction and diffusion by MES. It also suggests criteria for appropriate experimental conditions in ATR-IR MES experiments.

This contribution gives an overview of our recent effort to probe catalytic solid-liquid interfaces in situ and to investigate recognition processes at chiral surfaces. Attenuated total reflection infrared spectroscopy in a dedicated low volume flow-through cell is used to investigate the working catalytic interface. The latter technique is combined with modulation spectroscopy, which relies on the perturbation of the system under investigation by a periodically varying external parameter. A digital phase-sensitive detection results in high quality spectra. The method furthermore yields kinetic information and helps disentangle complex spectra. The described tool is therefore ideally suited for the investigation of complex systems. Applications in the fields of heterogeneous catalysis and recognition at chiral solid-liquid interfaces are presented. Our aim is a better molecular level understanding of these processes and, based on this knowledge, a rational design of better catalyst materials.

An attempt has been made to study the reaction between a uranium atom and a nitrogen molecule theoretically using multiconfigurational wave functions. The C_2v part of the reaction surface has been computed for several electronic states of various spin multiplicities. The system proceeds from a neutral uranium atom in its (5f)³(6d)(7s)², ⁵L ground state to the linear molecule NUN, which has a ¹Σ⁺_g ground state and uranium in a formal U(VI) oxidation state. The effect of spin–orbit coupling has been estimated at crucial points along the reaction. These preliminary results shows that the system proceeds from a quintet state for U + N₂, via a triplet transition state to the final closed shell molecule. An eventual energy barrier for the insertion reaction is caused by the spin–orbit coupling energy.

Results are presented from a theoretical study of the lower electronic states of the CUO molecule. Multiconfigurational wave functions have been used with dynamic correlation added using second order perturbation theory. Extended basis sets have been used, which for uranium were contracted including scalar relativistic effects. Spin–orbit interaction has been included using the state-interaction approach. The results predict that the ground state of linear CUO is Φ₂ with the closed shell Σ+0 state 0.5 eV higher in energy. This is in agreement with matrix isolation spectroscopy, which predicts Φ₂ as the ground state when the matrix contains noble gas atoms heavier than Ne. In an Ne matrix, the experiments indicate, however, that CUO is in the Σ+0 state. The change of ground state due to the change of the matrix surrounding CUO cannot be explained by the results obtained in this work and remains a mystery.

One of the prototype compounds for metal−metal multiple bonding, the Re₂Cl₈^2- ion, has been studied theoretically using multiconfigurational quantum chemical methods. The molecular structure of the ground state has been determined. It is shown that the effective bond order of the Re−Re bond is close to three, due to the weakness of, in particular, the δ bond. The electronic spectrum has been calculated with the inclusion of spin−orbit coupling. Observed spectral features have been reproduced with good accuracy, and a number of new assignments are suggested.

Quantum chemical calculations suggest that group 4 tetra-azides M(N₃)₄, where M = Ti, Zr, Hf, and Th, are stable species. They present a unique structural feature; namely, the M−N−N−N fragments are linear. These species are energetically more stable than the corresponding isomers with general formula η⁵-N₅ −M−η⁷-N₇, and the Th species, Th(N₃)₄, is the most stable of all. Possible mixed nitride azides NMN₃ were also investigated.

Quantum chemical calculations suggest that a series of molecules with the general formula MAu₄ are stable, where M = U, Th, and a group-4 atom. They correspond to Au in the formal valence state −1 and indicate that gold can act as a ligand similar to the halogen series. Of the MAu₄ species studied, UAu₄, the first predicted mixed gold uranium compound, has a short M−Au bond distance, 2.71 Å, which would locate Au between Br and I from the bond length point of view in the U-tetrahalide series. Energetically, the U−Au bond is weaker than the corresponding U−Br and U−I bonds.

Quantum chemical calculations suggest that inverse sandwich compounds with the general formula MN₇ M', where M is an alkali metal (K,Rb,Cs), N₇ is a ten-π-electron ring, and M' is an alkaline-earth metal (Ca,Sr,Ba), are local C_7v minima. Among these systems, the CsN₇Ba molecule is the stablest of all and presents a barrier of 35 kcal/mol to dissociation towards CsNBa and three N₂ molecules. Substantial 5d character is found in the bonding. Possible ways of making these high-energy compounds are discussed.

Here we report a study of the optical luminescent properties for a variety of vacuum-ultraviolet (VUV)-irradiated cosmic ice analogs and the complex organic residues produced. Detailed results are presented for the irradiated, mixed molecular ice: H₂O : CH₃OH : NH₃ : CO (100 : 50 : 1 : 1), a realistic representation for an interstellar/precometary ice that reproduces all the salient infrared spectral features associated with interstellar ices. The irradiated ices and the room-temperature residues resulting from this energetic processing have remarkable photoluminescent properties in the visible (520-570 nm). The luminescence dependence on temperature, thermal cycling, and VUV exposure is described. It is suggested that this type of luminescent behavior might be applicable to solar system and interstellar observations and processes for various astronomical objects with an ice heritage. Some examples include grain temperature determination and vaporization rates, nebula radiation balance, albedo values, color analysis, and biomarker identification.

Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4−300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)₂[Fe₄S₄(SR)₄], where RS^- = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4‘), 2,4,6-triphenylbenzylthiolate (5, 5‘), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat⁺ = ⁺NEt₄ (1, 2, 3, 4‘, 5‘, 6), ⁺PPh₄ (4, 5). The newly synthesized 2^2-, 3^2-, 5^2-, and 6^2- complexes are, as 1^2- and 4^2-, excellent models of the reduced HiPIPs:  they exhibit the [Fe₄S₄]^3+/2+ redox couple, because of the presence of bulky ligands which stabilize the [Fe₄S₄]³⁺ oxidized core. Moreover, the presence of SCH₂ groups in 4^2-, 5^2-, and 6^2-, as in the [Fe₄S₄] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported:  it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D_2d-distorted geometry of the [Fe₄S₄]²⁺ core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261−397 cm^-1. A firm determination of the double exchange parameters B or, equivalently, the transfer integrals β = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin ⁹/₂. Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4‘, 5‘, and 6 but also of (NEt₄)₂[Fe₄S₄(SCH₂C₆H₅)₄] and the isomorph (NEt₄)₂[Fe₄S₄(S-t-Bu)₄] and (NEt₄)₂[Fe₄Se₄(S-t-Bu)₄] compounds reveal two absorption bands in the near infrared region, at 705−760 nm and 1270−1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe₂X₂]⁺ (X = S, Se) units. The |B| and |β| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699−787 and 3497−3937 cm^-1. Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534−6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |β| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe₄S₄]²⁺ clusters, especially in the Fe−Fe distances, cannot probably lead to drastic changes in the direct Fe−Fe interactions (parameter β) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261−397 cm^-1 range spanned by J. The cluster's geometry, hence the efficiency of the Fe−μ₃-S−Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.

We report the characterization of multifunctional rigid-rod β-barrel ion channels with either internal aspartates or arginine–histidine dyads by planar bilayer conductance experiments. Barrels with internal aspartates form cation selective, large, unstable and ohmic barrel-stave (rather than toroidal) pores; addition of magnesium cations nearly deletes cation selectivity and increases single-channel stability. Barrels with internal arginine–histidine dyads form cation selective (P_K⁺/P_Cl^–= 2.1), small and ohmic ion channels with superb stability (single-channel lifetime > 20 seconds). Addition of "protons" results in inversion of anion/cation selectivity (P_Cl^–/P_K⁺= 3.8); addition of an anionic guest (HPTS) results in the blockage of anion selective but not cation selective channels. These results suggest that specific, internal counterion immobilization, here magnesium (but not sodium or potassium) cations by internal aspartates and inorganic phosphates by internal arginines (but not histidines), provides access to synthetic multifunctional pores with attractive properties.

Electron paramagnetic resonance (EPR) imaging was applied to investigate further the in vitro degradation process of poly(ortho esters) containing 30 mol % lactic acid units in the polymer backbone (POE₇₀LA₃₀) and developed for controlled drug delivery. The objective of this study was the direct and continuous determination of pH values inside the degrading POE₇₀LA₃₀. pH-sensitive nitroxide spin probes 4-amino-2,2,5,5-tetramethyl-3-imidazoline-1-yloxy, 2,2,3,4,5,5-hexamethylimidazolidine-1-yloxy, and 2,2,4,5,5-pentamethyl-3-imidazoline-1-yloxy were calibrated in buffer solutions in order to cover a pH range between 1.0 and 8.0. Nitroxide spin probes were incorporated in POE₇₀LA₃₀, and polymer samples were incubated in 0.1 M phosphate buffer (pH 7.4) at 37 °C. At selected times, polymer samples were removed for the determination of pH values inside the eroding POE₇₀LA₃₀ by EPR at a frequency of 9.4 GHz. EPR imaging showed that the in vitro degradation of POE₇₀LA₃₀ followed a two-phase process:  in the first week of incubation, diffusion of water, and in consequence polymer degradation, were limited to the surface of the hydrophobic POE₇₀LA₃₀ where pH values between 6.0 and 7.4 were measured. After 1 week of incubation, water diffused into the core of the sample, allowing the determination of pH values inside the eroding POE₇₀LA₃₀ until complete erosion. Results indicated the formation of a pH gradient, with the most acidic environment inside the eroding sample where the lowest pH value of 3.8 was measured and higher pH at the surface. It was also possible to observe a polymer erosion front moving down within the polymer matrix in the course of time. The pH value of 3.8 measured inside the degrading POE₇₀LA₃₀ remained constant until polymer samples disintegrated at day 23, where no EPR signal was detectable. In conclusion, EPR imaging allows the noninvasive spatially resolved observation of pH changes within POE₇₀LA₃₀, and results confirmed that the in vitro erosion mechanism of POE₇₀LA₃₀ was neither bulk erosion nor pure surface erosion.

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.

Free ion formation in acetonitrile is examined through transient photoconductivity for a set of ketones excited at different wavelengths. According to the photophysical parameters of the ketones and the incident photon energy, two mechanisms can be operative: triplet–triplet annihilation (bimolecular process) and/or photoionization (monomolecular biphotonic process). By using a tunable laser, excited state mediated photoionization was studied. From the threshold energy (E_thr) required for this process to occur, ionization potentials in solution (I_S) were deduced and compared to the corresponding values in gas phase (I_G). A simple energetic model enables the determination of the oxidation potential (E_ox) of the ketones that are compared to the corresponding values obtained through electrochemical measurements.

The use and the number of sulfonylurea herbicides have increased since the early 1980s. A good understanding of their degradation is of ecological importance, since environmental pollutants can be issued from them. It is claimed that microbial degradation and chemical hydrolysis present the main degradation pathways but photodegradation cannot be neglected. Time-dependent density functional theory has been used to help in the elucidation of the photochemical behavior of sulfonylureas.

Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO (“dissected NICS”, LMO-NICS) method. Applications to (4n + 2) π-electron systems, ranging from [n] annulenes to D_nh S₃, S₅, and N₆H₆²⁺ rings as well as to D_2h cyclobutadiene, show the extent to which their diatropic character results from the σ framework and from the π orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the σ orbitals, but is found only for “electron-rich” π systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size.

As shown by detailed nucleus-independent chemical shift (NICS) analyses of the contributions of each molecular orbital, the very recently reported gas-phase all-metal Al₄Li₃^- anion and its relatives (Kuznetsov, A.E.; Birch, K.A.; Boldyrev, A.I.; Li, X.; Zhai, A.I.; Wang, L.S. Science 2003, 300, 622) are aromatic rather than antiaromatic. The paratropic (antiaromatic) four-π-electron contribution is overcome by the predominating diatropic effects of σ aromaticity. However, true antiaromatic all-metal clusters, such as Sn₆^2- (Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 297), do exist.

A tetragonal La²⁺ center (symmetry C_4v) was identified in single crystals of BaFCl and SrFCl doped with lanthanum with the aid of electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR). This center forms a donor-acceptor couple with initially present F(F^-) centers. Switching takes place by illumination of appropriate wavelength. The kinetics of the process was monitored by EPR as La²⁺ and the unswitched F center are paramagnetic. The results of our experimental investigation of this kinetics are presented. A foregoing spectroscopic characterization of the La²⁺ center allowed one to identify a d-d (the B₁-E) transition, a charge-transfer band (for BaFCl at 10940cm^-1 and at 17890cm^-1, respectively) and to obtain a value of 710cm^-1 for the spin-orbit coupling constant in the ground state. In order to narrow the choice of possible acceptor-donor partners a detailed EPR/optical search was further done to identify a number of lattice defects and oxygen centers—in addition to a La-oxygen molecular structure.

Copper(II) complexes of the pentadentate bispidine ligands exist in two isomeric forms (see structure) with bond-length differences up to 0.5 Å. The stabilization of either isomer may be achieved by a variation of the substituent at N7.

The 13²,17³-cyclopheophorbide a enol (CPP) is shown to convert mainly to a ~1:1 mixture of (13²R/S) chlorophyllones a (Chlone), when chromatographed over silica gel or alumina supports. 15¹-hydroxychlorophyllonelactone a and some other chlorophyll a related compounds are also tentatively identified as minor transformation products of CPP. This raises the possibility that the chlorophyllones reported in recent sediments may be analytical artifacts from CPP. However, data for the surface sediments from Lake Motte as well as literature data for other contemporary sediments show that, (i) they are not artifacts, (ii) considering that CPP is the intermediate compound in the formation of chlorophyllones from chlorophyll a, the hydroxylation of CPP in the sedimentary environment involves an enzymatic process leading preferentially to 13²S chlorophyllone a.

In this paper we discuss on the quantum efficiency in spin crossover compounds. Spin crossover solids are text-book examples of photo switchable materials that present a thermal spin transition from the diamagnetic low-spin state, thermodynamically stable at low temperatures, to the paramagnetic high-spin state becoming the thermodynamically stable state at elevated temperature. By irradiating them with an appropriate wavelength, they can pass from the stable low spin state to the metastable high spin state at temperatures below the thermal transition temperature. For the compound [Fe(pic)₃]Cl₂·EtOH, the question regarding the quantum efficiency of the photo-conversion process that is the number of molecules converted by one single photon and its possible dependency on irradiation intensity gave rise to a controversy. The experimental results presented in this paper demonstrate that the quantum efficiency of the photo-conversion at 11 K is on the order of unity, with no noticeable dependency of the quantum efficiency on light intensity. It does, however, depend to a small extent on the fraction of complexes already converted to the high-spin state.

The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed.

Electronic energy transfer (EET) rate constants between a naphthalene donor and anthracene acceptor in [ZnL^4a](ClO₄)₂ and [ZnL^4b](ClO₄)₂ were determined by time-resolved fluorescence where L^4a and L^4b are the trans and cis isomers of 6-((anthracen-9-yl-methyl)amino)-6,13-dimethyl-13-((naphthalen-1-yl-methyl)amino)-1,4,8,11-tetraazacyclotetradecane, respectively. These isomers differ in the relative disposition of the appended chromophores with respect to the macrocyclic plane. The trans isomer has an energy transfer rate constant (k_EET) of 8.7 × 10⁸ s^-1, whereas that of the cis isomer is significantly faster (2.3 × 10⁹ s^-1). Molecular modeling was used to determine the likely distribution of conformations in CH₃CN solution for these complexes in an attempt to identify any distance or orientation dependency that may account for the differing rate constants observed. The calculated conformational distributions together with analysis by ¹H NMR for the [ZnL^4a]²⁺ trans complex in the common trans-III N-based isomer gave a calculated Förster rate constant close to that observed experimentally. For the [ZnL^4b]²⁺ cis complex, the experimentally determined rate constant may be attributed to a combination of trans-III and trans-I N-based isomeric forms of the complex in solution.

Two new, “user-friendly” derivatives of triptycene containing AsH₂ and SiH₃ fragments were synthesized. Both solids are crystalline, air-stable compounds characterized by elevated melting points and resistance toward moisture. The highly reactive As−H and Si−H bonds are protected by the presence of the surrounding phenylene hydrogen atoms, which ensure a remarkable kinetic stabilization of these primary hydrides. After X-ray irradiation of a single crystal of triptycenesilane, a persistent silyl radical was trapped and characterized.

The reduction products of two diphosphaalkenes (1 and 2) and a bis(diphosphene) (3) containing sterically encumbered ligands and corresponding to the general formulas Ar−X==Y−Ar‘−Y==X−Ar, have been investigated by EPR spectroscopy. Due to steric constraints in these molecules, at least one of the dihedral angles between the CXYC plane and either the Ar plane or the Ar‘ plane is largely nonzero and, hence, discourages conformations that are optimal for maximal conjugation of P==X (or P==Y) and aromatic π systems. Comparison of the experimental hyperfine couplings with those calculated by DFT on model systems containing no cumbersome substituents bound to the aromatic rings shows that addition of an electron to the nonplanar neutral systems causes the X==Y−Ar‘−Y==X moiety to become planar. In contrast to 1 and 2, 3 can be reduced to relatively stable dianion. Surprisingly the two-electron reduction product of 3 is paramagnetic. Interpretation of its EPR spectra, in the light of DFT calculations on model dianions, shows that in [3]^2- the plane of the Ar‘ ring is perpendicular to the CXYC planes. Due to interplay between steric and electronic preferences, the Ar−X==Y−Ar‘−Y==X−Ar array for 3 is therefore dependent upon its redox state and acts as a “molecular switch”.

The kinetic energy functional T_s[ρ] in a reference system of non-interacting electrons is a key quantity in density functional theory. Approximating it as an explicit functional of the electron density ρ is the object of continuous interest since the earliest days of quantum mechanics (Thomas–Fermi electron gas theory). A simple proof of the exact inequality T_s[ρ_A + ρ_B] − T_s[ρ_A] − T_s[ρ_B] ≥ 0 valid for a special class of spin-compensated pairs of electron densities ρ_A and ρ_B (v^AB-representable pairs) is provided. The derived relation is discussed to rationalize some of the results of the past attempts to approximate T_s[ρ]. It is also discussed as a tool for deriving approximations to the functional T_s[ρ] and/or the bi-functional T^nad_s[ρ_A, ρ_B] = T_s[ρ_A + ρ_B] − T_s[ρ_A] − T_s[ρ_B].

The electronic structure of La₂CuO₄ has been investigated using first-principles cluster calculations and the chemical shieldings at the copper nucleus have been determined with several state-of-the-art quantum chemical methods. We have also calculated the copper shieldings for CuCl, which is often used as a reference substance for copper nuclear magnetic resonance shifts measurements, and found an appreciable paramagnetic contribution in agreement with precise measurements. The calculated chemical shift at the copper nucleus in La₂CuO₄ for an applied field parallel to the CuO₂ planes is generally smaller than, but still in reasonable agreement with, the values derived from experiment with the assumption that the spin susceptibility vanishes at zero temperature. For the field perpendicular to the planes, the quantum chemical result is substantially smaller than the experimental data but in accord with a perturbation theoretical estimate. Inconsistencies in previous representations and interpretations of the copper magnetic shift data are pointed out and corrected.

The combination of one contact and three pseudo-contact contributions to the NMR hyperfine paramagnetic shift of each proton in the triple-stranded helicates [Ln₃(L1)₃]⁹⁺(Ln = Ce–Yb except Pm, Gd) produce intractable ¹H NMR spectra whose assignment is limited by the large electronic contribution to the nuclear relaxation processes. The detailed analysis of the NMR spectra for the diamagnetic complexes [Ln₃(L1)₃]⁹⁺(Ln = La, Lu, Y) shows that the triple-helical structure found in the solid state is maintained in solution. Extension of the classical one-nucleus crystal-field dependent model-free method for paramagnetic D₃-symmetrical homotrimetallic lanthanide complexes possessing two different metallic sites (i.e. two second-rank crystal-field parameters: B20^central and B20^terminal) allows (i) the complete interpretation of the paramagnetic signals for Ln = Ce–Yb and (ii) the detection of a concomitant abrupt change of the contact terms F_i and of the pseudo-contact terms S_i=B20^centralG1i+B20^terminal(G2i+G3i) occurring near the middle of the lanthanide series. The derivation and application of a novel three-nuclei crystal-field independent method eventually demonstrates that the helicates [Ln₃(L1)₃]⁹⁺ adopt a single D₃-symmetrical structure along the complete lanthanide series in solution, which ascribes the discontinuity observed for S_i to a concomitant decrease of the two crystal-field parameters. Comparison with structural models is limited by the extreme sensitivity of the structural factors C_ikl and D_ikl to minor geometrical variations affecting the wrapping of the ligand strands, but calculations of the geometrical factors Gmi(m= 1–3) for [Ln₃(L1)₃]⁹⁺ in solution confirm the formation of a regular triple-helical structure. Generalization of this novel three-nuclei method for addressing the solution structure of rhombic lanthanide complexes is discussed.

A new synthetic approach, reacting alkaline earth metal iodides with butyllithium, lithium hydroxide, and/or lithium butoxide under salt elimination, is presented, giving access to some interesting clusters of calcium, strontium, and barium, partially in combination with lithium. The so far largest calcium cluster Li[{Ca₇(μ₃-OH)₈I₆(thf)₁₂}₂(μ₂-I)]·3THF, 4, and the new strontium compound [Sr₃I₃(OH)₂(thf)₉]I, 5, are shown to feature common building blocks of OH-capped M₃ triangles. On the basis of mainly electrostatic interactions, these clusters are not volatile. By introducing LiO^tBu, the two clusters [IM(O^tBu)₄{Li(thf)}₄(OH)] (6, M = Sr; 7, M = Ba) are prepared, 7 exhibiting volatility as an important physical property, which makes it a potential precursor for chemical vapor deposition. The structural relationship between 4, 5, 6, and 7 and their respective starting materials is shown, and possible reaction mechanisms are proposed. Exhibiting surprising and new structural motifs, the bonding modes of these clusters are investigated by the electron localization function as well as by ab initio calculations.

Density functional theory generalized gradient approximation calculations, which were tested in our previous detailed study of [RhCl(PF₃)₂]₂ (Seuret et al., 2003, Phys. Chem. chem. Phys., 5, 268-274), were applied for a series of homologous organometallic compounds of the [RhXL₂]₂ (X = Cl, Br, or I; L = CO, PH₃, or PF₃) type. Various properties of the studied compounds were obtained. Optimized geometries of [RhCl(PH₃)₂]₂ and [RhCl(CO)₂]₂ are in very good agreement with available experimental data. Geometries of other compounds as well as other properties (thermochemistry of selected fragmentation channels, barriers to structural changes, frontier orbitals) which are not available experimentally were predicted. All the considered compounds are not planar. Enforcing planarity of the central [RhX]₂ moiety requires only a small energetic cost ranging from 2.2 to 3.9 kcal mol^-1. The analysis of frontier orbitals indicates that the metals provide the most favourable site for the electrophilic attack in all considered compounds. The analysis of the shape of the lowest unoccupied molecular orbitals indicates that the halogens and ligands provide the most favourable site for the nucleophilic attack for [RhCl(CO)₂]₂ or [RhCl(PF₃)₂]. For [RhBr(PF₃)₂]₂, [RhI(PF₃)₂]₂ and [RhCl(PH₃)₂]₂, the nucleophilic attack on the halogen is less probable. Except for [RhCl(CO)₂]₂, the least energetically expensive decomposition channel involves initial separation of ligands. For [RhCl(CO)₂]₂, its decomposition into the RhCl(CO)₂ fragments was found to be the least energetically expensive fragmentation reaction which is probably one of the reasons for the known catalytic activity of this compound.

The hydrolysis of terminal ^tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 − 3H]^3- reacts with Ln(ClO₄)₃ to give the poorly soluble and stable podates [Ln(L13 − 3H)] (log(β₁₁₀) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H₂O)₇ (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in [Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] ( Φ^tot_Eu = 1.8 × 10^-3) and [Tb(L13 − 3H)] ( Φ^tot_Eu = 8.9 × 10^-3) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (Φ^Eu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet (¹ππ*) and triplet (³ππ*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the “carboxylate effect” in lanthanide complexes are evaluated for programming predetermined properties in functional devices.

The charge recombination dynamics of geminate ion pairs formed by electron transfer quenching of cyanoanthracene derivatives by bromo- and iodo-anisole in acetonitrile was investigated using various ultrafast spectroscopic techniques. Without a heavy atom, the only charge recombination pathway is that leading to the neutral ground state. With heavy atom substituted anisoles, charge recombination to the local triplet state of the excited precursor is observed. Time constants for triplet charge recombination ranging from 400 ps to less than 10 ps, depending on the heavy atom and on the energy gap between the ion pair and the triplet state, have been measured. This heavy atom effect was observed with ion pairs formed upon electron-transfer quenching with driving force going from −0.15 to −0.6 eV, suggesting that these intermediates are in fact exciplexes. A new scheme for producing free ions with a high yield using this effect and a secondary electron donor is also demonstrated.

A [4](hetero)helicenium cation was resolved using the hexacoordinated phosphorus-containing binphat anion (see picture: N, blue; O, red; C, gray). Its absolute configuration was determined by vibrational circular dichroism spectroscopy. The barrier of interconversion of its enantiomers is higher than that of [6]helicene.

The electron transfer (ET) quenching dynamics of excited perylene (Pe), cyanoperylene (PeCN), methanolperylene (PeOH), and methylperylene (PeMe) in N,N-dimethylaniline (DMA) has been investigated using ultrafast fluorescence up-conversion. Measurements of the rotational dynamics of PeCN and PeMe in nonpolar and polar inert solvents using optically heterodyned polarization spectroscopy are also presented. The fluorescence decay in DMA is strongly nonexponential and about 10 times faster with PeCN than with the other electron acceptors. The quenching dynamics has been analyzed with a model distinguishing three types of donor molecules surrounding the acceptor:  those with optimal orientation for ET and those requiring orientational or translational diffusion prior to ET. According to this model, which can account for the whole fluorescence decay, the faster quenching dynamics of PeCN is not due to a larger ET rate constant, but to a larger number of donor molecules, typically three to four, with an optimal orientation. This is explained by the effect of dipole−dipole interaction between PeCN and the donor molecules, which favors mutual orientations with a large electronic coupling. With the other acceptors, this interaction is either not present or does not lead to ET active geometries. The occurrence of this interaction is substantiated by the rotational dynamics measurements.

Cyclic voltammetry of Mes*P==C(NMe₂)₂ (1) and Mes*P==C(CH₃)NMe₂ (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp₂Fe]PF₆ leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental ³¹P and ¹³C isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*P^•−(C(NMe₂)₂)⁺. Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe₂)₂ moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P−^•C(NMe₂)₂. No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the P−C double bond (P^-−C⁺), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal π electron distribution (P⁺−C^-):  no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1^•+PF₆^-. As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorus−carbon bond caused by the presence of the amino groups.

We report on the first stage of our theoretical study of the quino[2,3,4-kl]acridinium,1,13-dimethoxy-5,9-dipropyl-cation. This molecule, involved in the synthesis of novel triazaangulenium dyes of high chemical stability, is a chiral [4]-helicenium. The structure and the IR spectrum of the quino[2,3,4-kl]acridinium,1,13-dimethoxy-5,9-dimethyl-cation derived from theoretical calculations which use various density functional theory methods, are compared with the geometry derived from X-ray diffraction measurements and the experimental IR spectrum. Our study shows that the chosen variant of DFT methods (Becke88 for exchange, P86 for correlation, 3-21G** basis set) reproduces the experimental geometry within 0.004 Å and the IR frequencies within 15 cm⁻¹.

Ultrafast time-resolved fluorescence measurements have been carried out to investigate vibrational relaxation dynamics of perylene derivatives in solution. The early results obtained with cyanoperylene in acetonitrile are presented and discussed.

The excitation of a charge transfer band by a laser pulse of finite duration and the ensuing charge recombination are calculated in the framework of the perturbation theory. The influence of the spectral characteristics of the laser pulse on the charge recombination dynamics is investigated for models including several nuclear modes that differ greatly in their timescales. It is shown that, in the area of applicability of the perturbation theory, the variation of the pulse carrier frequency inside the absorption band can significantly change the effective charge recombination rate constant.

Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (~20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of ~10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4^•-. Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 °C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4^•- is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (π* orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure.

X-irradiation of single crystals of Tp–GeH₃ (Tp: triptycene) led to the trapping of the radical Tp–^√GeH₂. The angular variations of the resulting EPR spectra were recorded at 300 and 77 K. The drastic temperature dependence of the spectra was caused by both a strong anisotropy of the g-tensor and a rotation of the ^√GeH₂ moiety around the C–Ge bond. The determination of the EPR tensors as well as the analysis of this motion required to take the presence of disorder in the crystal into account. In accordance with DFT calculations, Tp–^√GeH₂ is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the C–GeH₂ bond is blocked at 90 K and is almost free above 110 K. The experimental barrier, obtained after simulation of the EPR spectra as a function of the rotational correlation time, is equal to 1.3 kcal mol⁻¹, which is slightly inferior to the barrier calculated by DFT (3.6 kcal mol⁻¹). Calculations performed on Tp–CH₃, Tp–GeH₃ and Tp–^√GeH₂ show that the rotation barrier ΔE_rot around the C–Ge bond drastically decreases by passing from the germane precursor to the germanyl radical and that ΔE_rot increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined.

Quinodimethanes are highly reactive toward dienophiles since Diels−Alder cycloaddition results in an aromatic product. Density functional-based ¹³C, ¹H NMR, NICS, and MO-NICS calculations indicate that the increase of aromatic character of the developing benzenoid ring along the reaction path is especially pronounced after the transition state is reached, even though the number of π orbitals decreases. The forming aliphatic ring exhibits large ring current effects during the reaction.

Several methods to address aromaticity in terms of nucleus-independent chemical shifts (NICS) are compared. These include NICS at the ring centre NICS(0), NICS 1 Å above the ring plane NICS(1), aromatic ring current shielding (ARCS), and dissected NICS, i.e. NICS calculated from selected π orbitals NICS_π, again in the ring plane and 1 Å above. The methods are tested on the basis of density-functional theory (DFT) and the individual gauge for local orbitals (IGLO) technique. Applications include simple organic rings (C₄H₄, C₄H₄²⁺, C₆H₆, C₅H₅^–, C₇H₇⁺) and transition metal carbonyl complexed molecules Fe(CO)₃C₄H₄ and Cr(CO)₃C₆H₆.

An approximate one-electron functional for the classical Coulomb energy J[ρ] is presented. The analytical form of the terms appearing in the functional is justified by the scaling relations of the exact form of the classical Coulomb energy, and, the coefficients in front of each term are determined by a least-squares fit of the exact values for rare-gas atoms. It is shown that the approximation, tested on a set of neutral atoms (2≤Z≤54), can predicts energies with accuracy and leads to a potential v_J(r)=δJ[ρ]/δρ(r) which is in qualitative agreement with the exact one.

A transient grating setup with evanescent wave probing has been developed to investigate ultrafast processes at liquid–liquid interfaces. In order to evaluate the selectivity of this method to the interface, the speed of sound in the low refractive index medium has been measured as a function of the penetration depth of the probe pulse. Our preliminary results indicate an increase of the speed of sound in methanol with decreasing the probe depth from 100 to 70 nm. However, no correlation was found in acetonitrile in the same range. Modifications of the experiment for improving the selectivity to the interface are proposed.

The Yb³⁺ paramagnetic center of the trigonal symmetry (“oxygen” paramagnetic center T₂) in CaF₂ and SrF₂ single crystals is studied by EPR and optical spectroscopy. The Stark level energies of the Yb³⁺ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated.

Experimental and theoretical techniques have been applied to study the decomposition of the [RhCl(PF₃)₂]₂ molecule which is known as a precursor in electron beam induced deposition (EBID) of Rh. Mass spectrometry (MS) has been carried out to study the electron ionisation and fragmentation of isolated molecules. Auger electron spectroscopy has been used to characterize the EBID deposit. The MS data indicate the presence of free phosphorus and rhodium ions. This is in agreement with the analysis of the composition of the EBID deposit containing: 60% Rh, 12–25% P, 2–13% Cl, no F, 3–20% O and N. Theoretical calculations (density functional theory) has been used to characterize the precursor molecule and to derive the enthalpies of several simple decomposition reactions. The calculated geometries are in a good agreement with the available X-ray crystallographic data. The [RhCl(PF₃)₂]₂ appears not to be rigid: the PF₃ groups can rotate with a relatively low barrier (0.6 kcal mol^–1) whereas the barrier for the butterfly-like motion of (RhCl)₂ moiety is only 3.5 kcal mol^–1. According to the theoretical results, the lowest energy pathway of the decomposition corresponds to a consecutive loss of PF₃ ligands, resulting in a (RhCl)₂ moiety (without phosphorus). The same conclusion is also valid for the ionised precursor. Experimental data combined with the theoretical results concerning the energetics of the considered various simple decomposition processes indicate that the electron induced dissociation of the precursor cannot be seen as a simple one-step decomposition process.

We analyze the performance of gradient-free local density approximation (LDA) and gradient-dependent generalized gradient approximation (GGA) functionals in a density functional theory variational calculations based on the total energy bifunctional (E[ρ₁,ρ₂]). These approximations are applied to the exchange-correlation energy and to the nonadditive component of the kinetic energy of the complex. Benchmark ab initio interaction energies taken from the literature for 25 intermolecular complexes for which the interaction energies fall into the 0.1–3.0 kcal/mol range are used as reference. At the GGA level, the interaction energies derived from E[ρ₁,ρ₂] are more accurate than the Kohn–Sham ones. LDA leads to very good interaction energies for such complexes where the ρ₁,ρ₂ overlap is very small (Ne-Ne, Ar-Ar, for instance) but it is not satisfactory for such cases where the overlap is larger. Introduction of gradient-dependent terms into the approximate part of E[ρ₁,ρ₂] improves significantly the overall accuracy of the interaction energies. Gradient-dependent functionals applied in E[ρ₁,ρ₂] lead to the average error and the average absolute error of the interaction energies amounting to 0.08 kcal/mol and 0.29 kcal/mol, respectively.

The enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone in the presence of acetic acid and trifluoroacetic acid has been studied on cinchonidine-modified Pd/TiO₂. Catalytic experiments and theoretical calculations indicate the formation of a cinchonidine–trifluoroacetic acid cyclic ion pair. We propose that this is the actual modifier, which interacts with 4-hydroxy-6-methyl-2-pyrone in the enantiodifferentiating step. The new mechanistic model is assumed to be valid also for other reactions over cinchona-modified Pt or Pd, in the presence of trifluoroacetic acid.

Cyclic cinchonidine ∶ acid complexes (1 ∶ 1 and 1 ∶ 2) of the chiral modifier cinchonidine (CD) and an alkenoic acid, tiglic acid, in dichloromethane solvent have been observed by FTIR spectroscopy. Both the OH and the quinuclidine N atom of CD are involved in the hydrogen bond with the acid molecule(s). Such dual-site modifier–reactant interactions play an important role in the enantioselective hydrogenation of alkenoic acids over CD-modified Pd catalysts. The stability of these 1 ∶ 1 and 1 ∶ 2 complexes has been probed by addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a stronger base than CD. DBU builds ion pairs with the acid (with 1 ∶ 1 and 1 ∶ 2 stoichiometry) and a hydrogen bond with the OH of CD. However, despite the large difference in basicity between CD and DBU, 1 ∶ 2 CD ∶ acid complexes can still be detected when more than 0.5 equivalent DBU was added with respect to the acid, at which ratio the enantiomeric excess (ee) drops dramatically. Hence, the molecular structure of CD favours formation of cyclic complexes via a dual-site interaction, which is not possible for DBU ∶ acid complexes, and stabilises 1 ∶ 2 CD ∶ acid species, which are proposed to be responsible for enantiodifferentiation.

The catalytic properties of mesoporous iron oxide–silica aerogels prepared by a sol–gel process combined with ensuing supercritical extraction with CO₂ was investigated in the selective oxidation (SCO) of ammonia and the selective reduction (SCR) of NO by ammonia. The main parameters changed in the aerogel preparation were the type of base used as gelation agent, the iron content, and the calcination temperature. The aerogels differed significantly in acidity and iron dispersion. Diffuse reflectance infrared Fourier transform spectroscopy studies of ammonia adsorption at different temperatures revealed that ammonia was bound to Brønsted- and Lewis-type sites, the latter being dominant at 300°C. A fraction of low coordinated Fe²⁺ sites were probed by NO adsorption measurements. Lewis-type sites were found to be associated with low-coordinated iron sites. Catalytic tests were performed in a continuous fixed-bed reactor in the temperature 210–550°C range and at ambient pressure. The catalytic activity of the aerogels in SCO correlated with the abundance of more strongly bound ammonia adsorbed on Lewis sites (low coordinated iron). High selectivity to nitrogen (97%) could be reached up to 500°C, whereas at higher temperature the formation of N₂O and NO became significant. The apparent activation energy of N₂ formation ranged from 69 to 94 kJ/mol, whereby catalysts with higher selectivity and activity showed lower activation energy. In SCR, selectivity to nitrogen was for all aerogels >98% at T<460°C, and activation energies varied from 38 to 53 kJ/mol. The catalytic activity for SCR did not correlate with the population density of Lewis sites. We propose that SCO predominantly occurs on Lewis sites consisting of highly dispersed iron atoms of low coordination, whereas in SCR these sites do not play an important role.

The products of the reaction of the most energetic form of hydrogen, gas phase H atoms, with ethylene, acetylene and ethane adsorbed on a Ni(1 1 1) surface at 60 K are probed. Adsorbed ethylidyne (CCH₃) is identified by high resolution electron energy loss spectroscopy to be the major product (30% yield) in all three cases. Adsorbed acetylene is a minor product (3% yield) and arises as a consequence of a dynamic equilibrium between CCH₃ and C₂H₂ in the presence of gas phase H atoms. The observation of the same product for the reaction of H atoms with all three hydrocarbons implies that CCH₃ is the most stable C₂ species in the presence of coadsorbed hydrogen. The rates of CCH₃ production are measured as a function of the time of exposure of H atoms to each hydrocarbon. A simple kinetic model treating each reaction as a pseudo-first order reaction in the hydrocarbon coverage is fit to these data. A mechanism for the formation of CCH₃ via a CHCH₂ intermediate common to all three reactants is proposed to describe this model. The observed instability of the CH₂CH₃ species relative to C₂H₄ plays a role in the formulation of this mechanism as does the observed stability of CHCH₂ species in the presence of coadsorbed hydrogen. The CH₂CH₃ and the CHCH₂ species are produced by the translational activation of ethane and the dissociative ionization of ethane and ethylene, respectively. In addition, the binding energy and the vibrational spectrum of ethane adsorbed on Ni(1 1 1) are determined and exceptionally high resolution vibrational spectra of adsorbed ethylene and acetylene are presented.

Iron oxide aerogels were synthesized from tetramethoxysilicon(IV) (TMOS) or tetraethoxysilicon(IV) (TEOS) and iron nitrate using an acid-catalyzed solution–sol–gel method combined with subsequent extraction of the alcoholic solvent with supercritical CO₂. The main parameters varied in the sol–gel synthesis were: the type of N-base used as the gelation agent (N,N-diethylaniline, trihexylamine, ammonium carbonate, ammonia), the concentration of the iron precursor, and the water content. The silicon precursor was prehydrolyzed to improve its reactivity. After calcination at 600 °C, the structural and chemical properties of the aerogels containing 0–20wt% nominal Fe₂O₃ were characterized by means of nitrogen adsorption, X-ray diffraction (XRD), transmission and scanning electron microscopy, temperature programmed reduction, X-ray photoelectron spectroscopy (XPS), UV-Vis, DRIFT and EPR spectroscopy. XRD and electron microscopy indicated that all aerogels were amorphous, irrespective of the sol–gel conditions used. The aerogels were predominantly mesoporous, with pore size maxima ranging between 20–50 nm, but also exhibited some microporosity. For the 10 wt% iron oxide samples, the specific pore volumes ranged from 0.7 to 2 cm³ g⁻¹ and BET-surface areas from 150 to 636 m² g⁻¹, depending on conditions. With increasing iron content, the BET surface area decreased from 740 to 340 m² g⁻¹, accompanied by increasing microporosity. XPS revealed significant silicon enrichment on the surface. Spectroscopic investigations (UV-Vis, EPR) uncovered different iron-containing species, ranging from tetrahedrally coordinated iron atoms incorporated in the silica matrix to iron oxide nanoclusters. Formation of isolated iron atoms was favored with low iron content samples. The N-base used to force gelation had a significant effect on the morphology and population density of Fe(OH)Si in the aerogels.

The influence of acetic acid (AcOH) and trifluoroacetic acid (TFA) on the hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been investigated by using Pt/Al₂O₃ modified by cinchonidine and O-methylcinchonidine. We have shown that the sometimes dramatic changes in enantioselectivity and rate cannot simply be interpreted by protonation of the alkaloid modifier. We propose a new three-step reaction pathway, involving interaction of the carboxylic acid with the reactant and the chiral modifier. The mechanism is supported by IR spectroscopic identification of cyclic TFA–modifier ion pairs. This new approach can rationalise the poorly understood role of acids in the enantioselective hydrogenation of activated ketones over cinchona-modified platinum metals.

Manganese oxide–silica mixed oxide aerogels with different morphological and chemical properties were prepared using the sol–gel method and ensuing extraction of the solvent with supercritical CO₂. Two types of manganese precursor, varying hydrolysis conditions of the silica and manganese precursors, influence of base addition for gelation, and calcination temperatures were investigated. Base addition had a strong effect on textural properties, producing high-surface-area, mesoporous aerogels, whereas these properties were only marginally affected by kind of manganese precursor used. The presence of different manganese oxide species was evidenced by X-ray diffraction, Raman and diffuse reflectance infrared Fourier transform spectroscopy, and temperature-programmed reduction. Mn⁴⁺, Mn³⁺, and Mn²⁺ oxide species were found after calcination at 600°C in air. Sol–gel processing with manganese(II) nitrate resulted in highly dispersed mixed oxides. Basic gelation of these sols strongly influenced the state of the manganese, leading to crystallites of hausmannite and to amorphous Mn⁵O⁸ in the calcined samples. Aerogels derived from the less reactive Mn(III) (acac)₃ did not contain any manganese oxide crystallites when prepared under the same basic conditions. The catalytic performance of the aerogels in the selective oxidation of ammonia strongly depended on the state of the manganese. Samples containing crystalline Mn₃O₄ were more active than amorphous aerogels with dispersed manganese oxide species and afforded high selectivity to N₂O. The presence of amorphous Mn₅O₈ further increased the activity and the selectivity to nitrous oxide, reaching 74% at 360°C. Nitrogen formation was found to be related to the amount of strongly Lewis-bound ammonia. The amorphous aerogels showing more Lewis-bound ammonia produced mainly nitrogen below 480°C, affording a selectivity of 78% at 360°C.

The origin of the rate acceleration in enantioselective hydrogenation of α-functionalised ketones over cinchona alkaloid modified platinum has been studied using a combined experimental and theoretical approach, and the rate acceleration is traced to a lowering of the energy of the carbonyl π orbitals in the diastereomeric complex formed between reactant and modifier.

The relation between the electronic structure of α-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings.

Vibrational circular dichroism (VCD) spectra of the chiral modifiers cinchonidine, an alkaloid, and (R)-2-(pyrrolidin-1-yl)-1-(1-naphthyl)ethanol (PNE) were measured and simulated. For cinchonidine independent information from NMR investigations on the distribution of conformers was used to simulate VCD spectra from calculated spectra of the individual conformers. Agreement with experiment is reasonably good. For the structurally similar synthetic modifier PNE VCD spectra show that an open conformer predominates in solution. The difference between the most stable conformers of cinchonidine and PNE in solution is the intramolecular hydrogen bond found in the latter, which forms due to the enhanced flexibility of the pyrrolidinyl moiety in PNE as compared to the quinuclidine moiety in cinchonidine. The similar enantiodifferentiating power of cinchonidine and PNE as chiral modifiers in the heterogeneous enantioselective hydrogenation of ethyl pyruvate indicates that the rigidity of this part of the molecule is not a prerequisite for enantioselection. It is furthermore shown that binding of a non-chiral carboxylic acid to the alkaloid induces VCD in vibrations associated with the acid. Observation of this induced VCD allows probing of the chiral binding site.

Pd/Al₂O₃ model catalysts have been prepared by physical vapour deposition and characterised by means of XPS, STM, and in situ ATR–IR spectroscopy. Morphological changes in the Pd film induced by dissolved hydrogen leads to enhanced infrared absorption and could be followed with both STM measurements and IR spectroscopy. Adsorption of CO, pyridine, quinoline, 2-methylquinoline, and the chiral auxiliary cinchonidine has been studied in situ at 283 K in CH₂Cl₂ solvent. Two different species have been observed for cinchonidine on Pd. One is oriented with the quinoline moiety nearly parallel to the Pd surface, likely through the π-system, whereas in the second the σ-bonding through the N lone pair prevails and induces a tilting of the ring with respect to Pd. No indication of the presence of α-quinolyl species has been found, in contrast to adsorption on Pt/Al₂O₃ catalysts. Compared to adsorption on Pt, cinchonidine is more weakly bound on Pd under hydrogenation conditions. Also, the relative stability of the π- and N lone pair-bonded species is different for the two metals, with the π-bonded species being relatively more stable on Pt. Similarities and differences found in the adsorption of the chiral modifier on the two metals are discussed and traced mainly to the different d-orbital diffuseness of Pd and Pt.

The mechanism of alcohol oxidation was investigated using the conversion of cinnamyl alcohol (1) over Pd-based catalysts as a sensitive test reaction. Studies in a slurry reactor revealed that dehydrogenation and oxidative dehydrogenation of 1 follow the same reaction pathways independent of the presence or absence of oxygen and reaction conditions. Hydrogenation and hydrogenolysis side reactions indicated the presence of hydrogen on the metal surface during reactions. Catalyst deactivation in Ar is attributed to decarbonylation reactions and site blocking by CO. Introduction of molecular oxygen induced a dramatic enhancement of alcohol conversion rate by a factor of up to 285 due to oxidative removal of CO. Strong adsorption of CO on Pd/Al₂O₃ and its rapid removal by oxygen were corroborated by in situ ATR-IR spectroscopy. All these observations conform to a model according to which oxidation of 1 follows the classical dehydrogenation mechanism, and the key role of oxygen is the continuous oxidative removal of CO and other degradation products from the active sites. This oxidative cleaning of the metal surface allows a high rate of alcohol dehydrogenation even when the oxidation of the co-product hydrogen is slow and incomplete. It is likely that the observed effects are not limited to the oxidation of 1 on Pd, and regeneration of the active sites by oxygen generally plays an important role during aerobic oxidation of alcohols on platinum metals.

The potential of modulation excitation spectroscopy and phase-sensitive detection in combination with attenuated total reflection (ATR) for in situ infrared spectroscopy of catalytic solid−liquid interfaces is demonstrated. The method is based on the periodic variation of an external parameter such as reactant concentration. The periodically varying signals are subsequently demodulated using a phase-sensitive detection scheme. In this way, the small periodically varying signals are separated from the large static ones, yielding high quality difference spectra. Species, which have different response to the excitation, i.e., species with different kinetics, can easily be separated in the spectra. The method is applied to the enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone over a 5 wt % Pd/TiO₂ powder catalyst modified by cinchonidine. Upon modulation of the reactant concentration, the ATR spectra exhibit varying signals from dissolved reactant, product as well as from adsorbed species. Part of the signals are associated with carboxylates adsorbed on the TiO₂. The kinetics of these species are distinctly different from the one of the primary hydrogenation product. The carboxylates are formed from alcoholysis of the lactone, which is obtained by a second hydrogenation step. The enantiomeric excess was also measured phase sensitive. Its time dependence indicates a negative influence of the carboxylates on enantioselection.

The adsorption of carboxylic acids (formic, acetic, and pyruvic acid) from corresponding solutions in CH₂Cl₂ solvent on Al₂O₃ and TiO₂ thin films has been studied by attenuated total reflection infrared spectroscopy. The metal-oxide films were vapor-deposited on a Ge internal reflection element, which was mounted into a specially designed flow cell. The system allowed in situ monitoring of the processes occurring at the solid-liquid interface. The metal-oxide films were characterized by X-ray photoelectron spectroscopy, ellipsometry, and atomic force microscopy. Formic acid and acetic acid adsorbed predominantly as bridging species on alumina surfaces. Adsorbed free acids were not observed under a flow of neat solvent. Based on the position of the ν_AS(COO) and of the keto-group stretching vibration of the pyruvate ion, pyruvic acid is proposed to coordinate to the Al₂O₃ surface in a monodentate fashion, whereas, on TiO₂, a bidentate species is preferred. Comparison of the adsorption behavior on the vapor-deposited alumina film and on an α-Al₂O₃ layer deposited from a water suspension of the corresponding metal-oxide powder indicated that pyruvic acid adsorbs in a similar mode, irrespective of the metal-oxide deposition technique.

The results of a study on the ground-state of monocarbonate, bicarbonate, and tricarbonate complexes of neptunyl using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to neptunium in the formal oxidation state (V) have been fully optimized at the CASPT2 level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. Some water molecules have been explicitly included in the calculation. This study is consistent with the hypothesis that the monocarbonate complex has a pentacoordinated structure with three water molecules in the first coordination shell and that the bicarbonate complex has a hexacoordinated structure, with two water molecules in the first coordination shell. The typical bond distances are in good agreement with experimental results. The tricarbonate complex was studied with explicit counterions, which resulted in somewhat longer Np−carbonate bond distances than experiment indicates.

Ab initio calculations at the B3LYP and MP2 levels suggest that a series of compounds with the general formula N₅MN₇ (M = Ti, Zr, Hf, Th) are locally stable. These compounds are thermodynamically at least as stable as the recently suggested ScN₇ molecule. N₅ThN₇ seems the most stable of all. It lies 21.5 kcal/mol below a transition state, corresponding to the opening of one N−N bond in the N₇ ring, and only 132 kcal/mol above Th + 6 N₂, or 22 kcal/(mol N₂).

The two-photon spectrum of the 2¹A_g ← 1¹A_g transition in trans-stilbene has been calculated at the complete active space self-consistent field (CASSCF) level of theory. Energies were obtained at the complete active space second-order perturbation (CASPT2) level of theory, while the geometries of both the initial and final states were optimized at the CASSCF level. The energy and the geometry optimizations were performed using an active space of 14 electrons in 14 active π orbitals. The vibrational frequencies of both states and the two-photon transition (TPT) cross-section were calculated with a smaller active space where the two lowest π orbitals were kept inactive. A newly implemented algorithm, in the quantum chemical package Molcas was used to determine the two-photon transition intensity. This method requires only the linear response of the CASSCF wavefunction. Furthermore, the vibronic structure of this TPT was studied. The Franck-Condon factors were obtained by calculating the overlap between the vibrational states involved, which were determined from the force fields of both the initial and final states, at the CASSCF level of theory. The results are in agreement with experiment.

Quantum chemical calculations of vibrational frequencies of Stilbene were followed by the computation of the potential energy surface for the two rotors related to the single bonds. By the flexible model approach applied to the computed surface we have confirmed previous assignment of mode 37 and determined frequency of the elusive mode 48. The same analysis was performed not only for the ground, but also for the excited electronic state. The shape of the potential energy surface in S₀ is in agreement with that of styrene and the barrier height obtained from the fitting in S₁ is increased with respect to S₀, as expected.

The existence of a series of triatomic molecules with the general formula MNM‘, where M is an alkaline metal (K, Rb, Cs), and M‘ is an alkaline earth metal (Ca, Sr, Ba), has been predicted by quantum chemical methods. Among these, the CsNBa molecule shows a feature not found before, the presence of a multiple bond between barium and nitrogen. As a consequence of this novel bonding situation, the molecule is linear. The same holds for all Ba triatomics, MNBa, independent of the nature of the alkali M atom, and for all Sr compounds, MNSr. The presence of a multiple bond makes CsNBa, and other related Ba and Sr molecules, particularly stable and appealing experimentally. The systems with the alkaline earth metal M‘ = Ca, on the other hand, turned out to be bent. Calculations have also been performed on the negative ions BaN^- and CaN^-, which form a well-defined entity in the MNM‘ systems (M‘ = Ba, Ca). The results show that the two ions have a different electronic structure in the ground state, which is one reason for the different properties of the MNM‘ systems and explains why the molecules containing the BaN^- moiety are linear, while those containing CaN^- are bent.

The results of a theoretical study of the ground state, 1¹A_g, and of the lowest ¹B_u states oftrans-stilbene are presented. The vertical and adiabatic excitation energies of the lowest ¹B_ustates have been computed using multiconfigurational SCF theory, followed by second-order perturbation theory. It is shown that the two lowest excited states are separated by a small energy gap in the Franck−Condon region. They are the 1¹B_u, characterized by the HOMO→LUMO single excitation substantially localized on the ethylenic moiety, and the 2¹B_u, formed by a combination of one electron excitations localized mainly on the benzene rings. The most intense transition is found to be the lowest in energy when the interaction between different states is included at the level of second-order perturbation theory. The vibronic structure of emission and absorption spectra of the two lowest ¹B_u states have been determined within the Franck−Condon approximation. The spectrum calculated for the 1¹B_ustate agrees with the experimental spectrum, while the low intensity band computed for the 2¹B_u state has no experimental counterpart. It is concluded that this band is buried in the strong 1¹B_u absorption and therefore not observed.

A novel ruthenium complex with a 6,7-dicyanosubstituted dppz ligand has been synthesised: its crystal structure and physico-chemical studies are reported.

Quantum-chemical calculations, at the self-consistent-charge density-functional-based non-orthogonal tight binding (SCC-DFTB) level, are used to provide the input for unimolecular reaction rate theory calculations to predict the temperatures at which rapid, i.e., microsecond timescale, equilibration between mono-cyclic and bi-cyclic carbon clusters can occur. The computational results are discussed in the form of a set of trends for their variation with the size of the cluster, the length of the carbon–carbon bond broken or formed, the vibrational frequencies, the energy differences and the rate constants. The temperatures used experimentally to prepare fullerenes and nanotubes are compatible with the rapid equilibration of rings and bi-cyclic rings, a factor that explains the lack of defects in these higher forms of carbon clusters and the general trend towards the formation of the most stable fullerene for a given nuclearity.

We use variable-cell first principles molecular dynamics as an optimization tool to investigate the structural and electronic properties of Mg-based anhydrous hydrotalcite-like compounds. The formation energy as a function of the ratio R between di- and trivalent cations shows a minimum at R ~ 3, in good agreement with experimental stability ranges for these materials. At the same value R ~ 3, a maximum is found in the calculated interlayer distance, suggesting a correlation between energetic stability and structure. The energies and character of the electronic states of hydrotalcites containing different interlayer anions and trivalent cations have been compared. The nature of the anions is found to have a major influence on the electronic properties. In particular, OH^- anions, rather than, e.g., Cl^-, lead to a significantly smaller HOMO−LUMO gap, with a LUMO spatially more localized in the interlayer region. These features are related to the observed differences in the catalytic properties of hydrotalcites containing OH^- vs Cl^- anions.

The absorption and photostimulated spectra of single crystals of the new substance Ba₁₂F₁₉Cl₅ doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm⁻¹ line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu²⁺ ions. This system may be of interest as an UV storage phosphor.

The physical and photophysical properties of three classic transition metal complexes, namely [Fe(bpy)₃]²⁺, [Ru(bpy)₃]²⁺, and [Co(bpy)₃]²⁺, can be tuned by doping them into a variety of inert crystalline host lattices. The underlying guest-host interactions are discussed in terms of a chemical pressure.

The application of statistical simulations to the estimation of transfer free energies of pharmacologically relevant organic molecules is reported. Large-scale molecular dynamics simulations have been carried out on a series of four solutes, viz. antipyrine, caffeine, ganciclovir, and α-d-glucose, at the water−dodecane interface as a model of a biological water−membrane interfacial system. Agreement with experimentally determined partition coefficients is remarkable, demonstrating that free energy calculations, when executed with appropriate protocols, have reached the maturity to predict thermodynamic quantities of interest to the pharmaceutical world. The computational effort that warrants accurate, converged free energies remains, however, in large measure, incompatible with the high-throughput exploration of large sets of pharmacologically active drugs sought by industrial settings. Compared to the cost-effective, fast estimation of simple partition coefficients, the present free energy calculations, nevertheless, offer a far more detailed information about the underlying energetics of the system when the solute is translocated across the water−dodecane interface, which can be valuable in the context of de novo drug design.

The adsorption of methanol on V₂O₅ and its mild oxidation to formaldehyde has been studied applying density functional theory. The model used throughout is a cluster model saturated by hydrogen atoms. It is shown that the adsorption of methanol is energetically favored if the cluster is partially reduced (i.e., protonated because of the dissociative adsorption of water). Methanol behaves as a soft base and adsorbs as a methoxonium cation. The proposed mechanism is based on two steps, the first being the dissociation of methanol to form a methoxy group on the surface. This dissociation occurs between the oxygen and the carbon atoms of methanol. Finally, for the second step, which corresponds to the desorption of formaldehyde, the calculations show that filling of the vanadyl oxygen vacancy created by formaldehyde desorption is crucial to cope with an energetically feasible reaction pathway.

The [Ru(bpy)₃][LiCr(ox)₃] system (bpy = 2,2‘-bipyridine, ox = oxalate) has two crystallographically non-equivalent [Cr(ox)₃]^3- sites. In steady-state resonant and nonresonant fluorescence line narrowing (FLN) experiments on the R₁ lines of the two non-equivalent [Cr(ox)₃]^3- chromophores, multiline spectra are observed at 1.6 K. Such multiline spectra are clear evidence for resonant energy transfer processes within the inhomogeneously broadened R₁ lines. In addition, time-resolved experiments show that also site-to-site energy transfer occurs, which turns out to be resonant, too, however with a non-negligible phonon-assisted contribution even at 1.5 K.

Efficient resonant energy transfer occurs within the R₁ line of the ⁴A₂ → ²E transition of the [Cr(ox)₃]^3- chromophore in mixed crystal [Rh(bpy)₃][NaAl_1-xCr_x(ox)₃]ClO₄ (x = 0.05−0.9, ox = oxalate, bpy = 2,2‘-bipyridine). This manifests itself in the form of multiline patterns in resonant fluorescence line narrowing (FLN) experiments at 1.5 K. The conditions for such a resonant process to occur are that the inhomogeneous line width of the R₁ line is larger than the zero-field splitting of the ground state, which, in turn, is larger than the homogeneous line width of the transition. The number of lines and their relative intensities depend critically upon the [Cr(ox)₃]^3- concentration and the excitation wavelength within the inhomogeneous distribution. The basic model for resonant energy transfer as presented by von Arx et al. (Phys. Rev B 1996, 54, 15800) is extended to include the effects of diluting the chromophores in an inert host lattice and of nonresonant R₂ excitation. In addition, Monte Carlo simulations serve to explain the temporal evolution of the multiline pattern following pulsed excitation.

[Fe(pic)₃]Cl₂·EtOH (pic = 2-picolylamine) is a spin-crossover compound that can be converted from the low-spin state to the high-spin state at temperatures below the thermal transition temperature by way of light irradiation in the visible part of the electromagnetic spectrum. For this compound, the question regarding the quantum efficiency of this photoconversion process and its possible dependence on irradiation intensity gave rise to some controversy. The experimental results presented in this paper demonstrate that the quantum efficiency of the photoconversion at 11 K is on the order of unity, with no noticeable dependence on irradiation intensity. It does, however, depend to some extent on the fraction of complexes already converted to the high-spin state.

Variable-temperature ¹H and ¹³C NMR measurements of the D₃-symmetrical triple-helical complexes [Ln(L1-2H)₃]^3- (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)₃]^3- (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P ↔ M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A₂⁰²> parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Geometries and ²⁹Si NMR chemical shifts are calculated for silanes Si_nH_2n+2, n=1,…,5, methylsilanes SiH_nMe_4−n, methoxysilanes SiH_n(OMe)_4−n, and methylmethoxysilanes SiMe_n(OMe)_4−n, n=0,…,4. Geometries and ²⁹Si NMR chemical shifts are in satisfying agreement with experiment within LCGTO-DFT at the DZVP/LDA level for geometries and IGLO-III/GGA (GGA=PW91,PBE) level for shielding constants, which is an improvement to B88PW86, P86PW86 and B3LYP results. If an auxiliary basis is applied to express the Coulomb potential, g-functions have to be included to reproduce SiOSi angles and ²⁹Si NMR chemical shifts correctly.

Calculations of ¹³C nuclear shieldings for low-energy isomers of C₃₆H_2x (x=2,3) suggest that it should be possible to use experimental¹³C shifts, when these become available, to distinguish the isomeric form of the underlying fullerene cage and, in the case of isomers based on the six-fold symmetrical cylindrical fullerene cage 36:15, the degree of polar hydrogenation.

Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.

Recent formal developments and applications of the 'freeze-and-conquer' strategy proposed by Wesolowski and Warshel in 1993 to study large systems at quantum mechanical level are reviewed. This universal approach based on density functional theory allows one to link, via the orbital-free embedding potential, two parts of a larger system described at different levels of accuracy leading thus to significant savings in computational costs. As a result, applicability of conventional methods of quantum chemistry can be extended to even larger systems. It is shown that the 'freeze-and-thaw' approach applying the first-principles based approximation to the orbital-free embedding potential recently developed in our group provides a powerful and universal technique to study such embedded molecules (or molecular complexes), which are not linked with their microscopic environment by covalent bonds.

Electrochemical and chemical reductions of Rh^(I) complexes of L_P4 (a macrocycle containing four phosphinine rings) and of L_P2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with ³¹P nuclei. An additional coupling (27 MHz) with ¹⁰³Rh is detected, in the liquid state, for the spectrum obtained with [L_P2S2Rh⁽⁰⁾]; moreover, resolved ³¹P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh^(I) complexes of model macrocycles L‘_P4 and L‘_P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L‘_P2S2Rh⁽⁰⁾], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (ρ_Rh =0.35 for [L‘_P2S2Rh⁽⁰⁾]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle.

Lipophilic linear semirigid side arms containing two or three successive phenyl rings separated by carboxylate spacers have been connected to the 5 or 6 positions of bent aromatic terdentate 2,6-bis(benzimidazol-2-yl)pyridine binding units to give extended V-shaped (L11) and I-shaped receptors (L12, L12b, and L13). The carboxylate spacers limit the flexibility of the side arms and provide crossed arrangements of the successive aromatic rings in the crystal structure of L12b (C₆₃H₆₁N₅O₁₀; triclinic, P_↑, Z = 2) in agreement with semiempirical calculations performed on optimized gas-phase geometries. Moreover, the carboxylate spacers in L11−L13 prevent efficient electronic delocalization between the connected aromatic rings and act as weak π acceptors producing a slight increase of the energy of the ¹ππ* and³ππ* levels centered on the terdentate binding unit. Intermolecular π-stacking interactions observed in the crystal of L12b are invoked to rationalize (i) the peculiar excimer emission ofL11 in the solid state and (ii) the rich and varied calamitic (I-shaped L12, L12b, and L13) and columnar (V-shaped L11) mesomorphism observed at high temperature. The Col_R mesophase detected for L11 demonstrates that V-shaped bent terdentate binding units are compatible with liquid-crystalline behavior. Complexation of L11 with lanthanide(III) produces I-shaped complexes [Ln(L11)(NO₃)₃] (Ln = La, Eu, Gd, Tb, and Lu) possessing a large axial anisometry as found in the crystal structure of [Lu(L11)(CF₃CO₂)₃(H₂O)] (LuC₈₁H₈₇N₅O₁₇F₉; triclinic, P_↑,Z = 2), which exists in the solid state as H-bonded dimers. No mesomorphism is detected for the complexes as a result of the large perpendicular expansion brought by the metallic coordination site, but the high energy of the ligand-centered ³ππ* prevents Eu(⁵D₀) → L11back transfer in the Eu(III) complex, which thus exhibits sizable red luminescence at room temperature, a crucial point for the design of luminescent materials.

The dynamics of charge recombination (CR) of ion pairs formed upon electron-transfer quenching of Zn tetraphenylporphine (ZnTPP) in the S₂ state has been investigated by fluorescence upconversion. These ion pairs have two possible CR pathways:  (A) a highly exergonic CR to the neutral ground state and (B) a moderately exergonic CR leading to the formation of ZnTPP in the S₁ state. Upon addition of quencher, the S₂ fluorescence decreases considerably, while the S₁ fluorescence is unaffected, indicating unambiguously that CR occurs via path B. A large fraction of the S₂ fluorescence quenching occurs in less than 100 fs. CR to the S₁ state of ZnTPP takes place with time constants around 400 fs.

The dynamics of charge recombination (CR) of excited donor−acceptor complexes composed of methoxy-substituted benzenes and pyromellitic dianhydride were investigated in four different solvents using both the multiplex transient grating and the transient absorption techniques. At constant driving force, the CR dynamics are substantially faster than those with methyl-substituted benzenes as donors. In acetonitrile (ACN), the CR time constant decreases from 3.5 ps with anisole down to 240 fs with tetramethoxybenzene. In valeronitrile, the CR is always slower than in ACN but is, in most cases, faster than diffusional solvation. The free energy, the solvent, and the temperature dependence of the CR dynamics can be qualitatively well reproduced using the hybrid model of Barbara and co-workers after incorporation of the contribution of inertial motion to solvation. The ability of this model to account for the absence of normal region at small driving force is also examined.

Polycrystalline LiBH₄ has been studied by Raman spectroscopy in the temperature interval 295–412 K and the frequency range 2700–130 cm⁻¹. The Raman active modes are consistent with the presence of a (BH₄)⁻ ion having a distorted tetrahedral configuration. As the temperature is increased the sudden disappearance of mode splitting points to the onset of a structural phase transition that leads to a higher local symmetry of the (BH₄)⁻ tetrahedron. The transition occurs at ~384 K, is of first-order and has a hysteresis of about 8 K. A strong and discontinuous broadening of bands remaining after the transition suggests the onset of large vibrational amplitudes of the (BH₄)⁻ tetrahedra about their trigonal axis.

The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent ³¹P nuclei. Additional couplings with two equivalent ¹³C are observed with Ar–P=¹³C=¹³C=P–Ar. The ³¹P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]^•– and [H–P=CH–CH=P–H]^•–. Although the optimized geometries of these two species are clearly different, neither of them leads to¹³C/³¹P hyperfine tensors in conflict with the experimental results. The absence of any ¹H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]^•– more probable. 

The paper shows the possibilities of the complementary use of the density matrix formalism for the simulation of the anisotropic EPR spectra and the DFT potential energy surface calculations to obtain a detailed picture of the motions of radical molecules. The combined approach is illustrated by a comparative EPR study of three phosphorus derivatives of barrelene. Three compounds were chosen as the model molecules for the observation of different temperature dependent dynamics of radical fragment. Each molecule based on the same barrelene skeleton has a different set of substituents which by influencing the local chemical environment are likely to modify the internal dynamics. The temperature dependent EPR spectra are simulated by means of the density matrix formalism and the geometry of radicals are calculated with DFT. The motion is described in terms of rotational barriers, DFT calculated energy profiles and hypothetical intramolecular distortions. These two approaches lead to a similar microscopic picture of the intramolecular radical motion.

The repartition of molecular hydrogen in space, and its depletion on solid particles in particular, is an important question of modern astrophysics. In this paper, we report a theoretical study of the physisorption of molecular hydrogen, H₂, on a major component of the interstellar dust known as polycyclic aromatic hydrocarbons (PAHs). Two different density functional theory approaches were used:  (i) the conventional Kohn−Sham theory and (ii) the subsystem-based approach (Kohn−Sham equations with constrained electron density, KSCED) developed in our group. The approximate exchange-correlation energy functional applied in all calculations and the nonadditive kinetic-energy functional needed in KSCED have a generalized gradient approximation form and were chosen on the basis of our previous studies. The results of both approaches show similar trends:  weak dependence of the calculated interaction energies on the size of the PAH and negligible effect of the complexation of two PAH molecules on the adsorption of molecular hydrogen. The KSCED interaction energy calculated for the largest considered PAH (ovalene), amounting to 1.27 kcal/mol, is in excellent agreement with experimental estimates ranging from 1.1 to 1.2 kcal/mol, whereas the one derived from supermolecular Kohn−Sham calculations is underestimated by more than 50%. This result is in line with our previous studies, which showed that the generalized gradient approximation applied within the KSCED framework leads to interaction energies of weakly bound complexes that are superior to the corresponding results of supermolecular Kohn−Sham calculations.

An approximate kinetic-energy functional of the generalized gradient approximation form was derived following the "conjointness conjecture" of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange-energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed-shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic-energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form.

The approximate nonempirical kinetic-energy functional proposed by Tal and Bader is analyzed for polyatomic systems. The performance of this functional and the functionals derived from the gradient expansion approximation truncated to zeroth, second, and fourth order is investigated for a testing set of 68 neutral and charged molecules. It is shown that the Tal–Bader functional, despite the simplicity of the idea behind its construction, leads to significantly better total kinetic energies than the gradient expansion approximation functionals. The local behavior of the kinetic-energy density derived from the Tal–Bader functional is also discussed.

A Comment on the Letter by Thorsten Klüner et al., Phys. Rev. Lett. 86, 5954 (2001). The authors of the Letter offer a Reply.

An approach in which the total energy of interacting subsystems is expressed as a bifunctional depending explicitly on two functions: electron densities of the two molecules forming a complex (ρ₁ and ρ₂) was used to determine the equilibrium geometry and the binding energy of several weak intermolecular complexes involving carbazole and such atoms or molecules as Ne, Ar, CH₄, CO, and N₂. For these complexes, the experimental dissociation energies fall within the range from 0.48 to 2.06 kcal/mol. Since the effect of the intermolecular vibrations on the dissociation energy is rather small, the experimental measurements provide an excellent reference set. The obtained interaction energies are in a good agreement with experiment and are superior to the ones derived from conventional Kohn–Sham calculations. A detailed analysis of relative contribution of the terms which are expressed using approximate functionals (i.e., exchange-correlationE_xc[ρ₁+ρ₂] and nonadditive kinetic energy T_s^nad[ρ₁,ρ₂] = T_s[ρ₁+ρ₂]−T_s[ρ₁]−T_s[ρ₂]) is made. The nonvariational version of the applied formalism is also discussed.

Characterization of ternary and quaternary metal hydrides by Raman spectroscopy appears to be rather scarce due primarily to the decomposition of the metal hydrides by the energy of the laser excitation source. We report the results of some recent room temperature Raman measurements collected with a 2–10 mW 488 nm laser source for M₂RuH₆ where M=Ca, Sr and Eu. The assignments from this study are combined with existing vibrational data for other metal hydrides.

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), ³Φ_u, Ω = 2 ground state with a U−O bond distance of 1.77 Å. 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 Å) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Å 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) Å) 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) Å) 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.

Ternary V₂O₅–WO₃/TiO₂ catalysts were prepared by sequential or simultaneous grafting steps of vanadia and tungsta onto titania and were compared with similarly prepared binary catalysts. Different grafting sequences including alternating grafting of vanadia and tungsta were compared.

During the grafting process the metal oxide precursor reacted with hydroxyl groups of the other grafted metal oxide. The interaction of vanadia with tungsta species resulted in a different reduction behavior of both metal oxides compared to the binary catalysts as indicated by temperature-programmed reduction with hydrogen. The strength of interaction of the grafted species depended on preparation sequence and metal loading.

At low coverage (‹monolayer) catalyst properties were found to depend strongly on loading, but relatively little on the grafting mode, as indicated by vibrational spectroscopy. Laser Raman experiments at different laser power revealed reversible effects due to temperature induced structural changes of surface vanadia species. For all catalysts, even at a loading of more than one and a half monolayers, no evidence of crystalline vanadia or tungsta could be found. After calcination of WO₃/TiO₂ catalyst at 1023 K instead of 573 K and subsequent grafting with vanadia, new species with hydroxyl groups showing a vibrational frequency below 3600 cm⁻¹ were formed. The increase of the calcination temperature had no significant influence on the reduction of vanadia by hydrogen.

The conformational behaviour of several α-ketoesters was investigated using solution FTIR in combination with ab initio calculations. The α-ketoesters show marked differences in the O=C–C=O torsional potential energy surface depending on the substituent at the α-keto group. In general the torsional potential is characterised by broad minima corresponding to s-cis and s-trans conformations and low interconversion barriers. The s-trans conformation is more stable but the fraction of s-cis is considerable at room temperature and increases with solvent polarity due to the higher dipole moment of the latter. Hydrogen bonding with alcoholic solvents also leads to a stabilisation of the s-cis conformer. The interaction of ethyl pyruvate with R₃N⁺–H is much stronger when ethyl pyruvate adopts an s-cis conformation due to strong ion–dipole interaction. This type of interaction between ethyl pyruvate and protonated cinchonidine is considered to be crucial for the enantio-differentiation in the heterogeneous enantioselective hydrogenation of ethyl pyruvate over cinchonidine modified platinum in acidic media.

IR and NMR experiments revealed that the enantioselective hydrogenation of ethyl pyruvate in nonacidic solvents is complicated by the simultaneously occurring self-condensation (aldol reaction) of the reactant. Both enantioselective reactions are catalyzed by the chiral base cinchona alkaloid, but the hydrogenation is faster by several orders of magnitude than the aldol reaction. Catalytic experiments proved that the aldol products are not spectator species. The enol form of the major aldol product protonates the quinuclidine N of cinchonidine and enhances the enantiomeric excess of the hydrogenation reaction. The significance of this observation with respect to kinetic and mechanistic studies is discussed.

The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π* and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation over cinchona‐alkaloid‐modified Pt.

The adsorption of ethyl pyruvate on Pt(111) at low temperature was investigated by XP and UP spectroscopy. The assignment of the photoelectron spectra was assisted by calculation of correlated ionization potentials. Comparison of the XP and UP spectra of the condensed and chemisorbed layer indicates a strong ethyl pyruvate adsorption bond in the latter. Upon chemisorption, the HOMO of ethyl pyruvate, which is a lone-pair orbital delocalized over both C=O groups, is stabilized by about 0.7 eV with respect to the other orbitals, which is characteristic for a lone-pair bonding mechanism. The same bonding mechanism was found for coverages far below saturation. The XP spectra further indicate that the ketone C=O is more strongly involved in the chemisorption bond than the carboxyl C=O of ethyl pyruvate. The packing density of the saturated chemisorbed ethyl pyruvate layer, as determined by XPS, is high. This points toward an upright or tilted orientation of ethyl pyruvate in this layer, in line with the observed bonding mechanism.

Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the enantioselective hydrogenation over platinum catalysts have been calculated using ab initio methods. For s-trans-methyl pyruvate it was found that the complex yielding (R)-methyl lactate upon hydrogenation was more stable than the corresponding pro-(S) complex. The calculated energy difference of 1.8 kcal/mol corresponds to an enantiomeric excess of 92%, in good agreement with experiment. For the analogous complexes of s-cis-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of pro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded complexes with cinchonidine are considerably more stable than the corresponding s-trans complexes. However, the predicted low enantiomeric excess for the s-cis conformer is in contrast with experiment. Possible reasons for this behaviour are discussed.

We present a theoretical analysis of the temperature dependence of the vanadyl pyrophosphate VO₂P₂O₇ ³¹P NMR spectra. Four distinct phosphorus sites responsible for four signals are identified in the crystal structure. The magnetic states of the crystal are described by two alternative models: the spin ladder and the dimer chain. Within both models, finite clusters with and without periodic conditions are considered. The fit of the experimental NMR data allows us to define combinations of hyperfine coupling parameters which are found to be similar in both spin models.

The total rate constant k₁ has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction: (1) CH₃ + C₂H₃ ^→ Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C₂H₄ and CH₄, respectively. The kinetic studies were performed by monitoring the decay of C₂H₃ with methyl in excess, 6 < [CH₃]₀/ [C₂H₃]₀ < 21. The overall rate coefficient was determined to be k₁(298 K) = (1.02 ± 0.53) × 10^-10cm³ molecule^-1 s^-1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C₂H₃ + H and C₂H₃ + C₂H₃. The present result for k₁ at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH₃ + C₂H₅ and with a value for k₁ estimated by the geometric mean rule employing values for k(CH₃ + CH₃) and k(C₂H₃ + C₂H₃). Qualitative product studies at T = 298 K and 200 K indicated formation of C₃H₆, C₂H₂, and C₃H₅ as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH₃ + C₂H₃ reaction. We also observed the secondary C₄H₈ product of the subsequent reaction of C₃H₅ with excess CH₃; this observation provides convincing evidence for the combination-decomposition channel yielding C₃H₅ + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.

Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt₆](BF₆)₂.

Luminescence and energy transfer in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] (x ≈ 0.01, y = 0.006 − 0.22; bpy = 2,2‘-bipyridine, ox = C₂O₄^2-) and [Zn_1-x-yRu_xOs_y(bpy)₃][NaAl(ox)₃] (x ≈ 0.01, y = 0.012) are presented and discussed. Surprisingly, the luminescence of the isolated luminophores [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn(bpy)₃][NaAl(ox)₃] is hardly quenched at room temperature. Steady-state luminescence spectra and decay curves show that energy transfer occurs between [Ru(bpy)₃]²⁺ and [Cr(ox)₃]^3- and between [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] and [Zn_1-x-yRu_xOs_y(bpy)₃] [NaAl(ox)₃], respectively. For a quantitative investigation of the energy transfer, a shell type model is developed, using a Monte Carlo procedure and the structural parameters of the systems. A good description of the experimental data is obtained assuming electric dipole−electric dipole interaction between donors and acceptors, with a critical distance R_c for [Ru(bpy)₃]²⁺ to [Cr(ox)₃]^3- energy transfer of 15 Å and for [Ru(bpy)₃]²⁺ to [Os(bpy)₃]²⁺ energy transfer of 33 Å. These values are in good agreement with those derived using the Förster−Dexter theory.

The crystal chemistry of the Sm³⁺ to Sm²⁺ reduction in tetraborate lattices was investigated. In crystalline SrB₄O₇ in air it is mainly Sm²⁺ that is incorporated from a melt or glass containing predominantly Sm³⁺. For the process in air, a reduction and pick-up mechanism is assumed to take place at the crystal/nutrient interface. Stabilization of Sm²⁺ in SrB₄O₇ at high temperature and in an oxidizing atmosphere seems to be a particular property of the system, because no Sm²⁺ inclusion could be observed along the series MB₄O₇ (M = Ca, Ba, Cd, Pb), if similar reaction conditions were applied. So far, there is only one other oxide lattice (BaB₈O₁₃) known where at high temperatures significant amounts of Sm²⁺ are obtained for reactions in the air.Single crystals of SrB₄O₇ : Sm²⁺ were grown by the Czochralski method (k_eff for Sm is 0.5). Optical hole burning experiments for the transition ⁵D₁–⁷F₀ were performed at 80 K. A hole with a width of 0.21 cm^–1 and a depth of 5.25% was formed for the first time for Sm²⁺ in a borate crystal excited by the beam of a single frequency dye laser. A rather small inhomogeneous linewidth of 0.28 cm^–1 allowed the burning of a single hole only.

We have investigated the luminescence of CaF₂ thin films doped with very low concentrations of Sm²⁺ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The Sm²⁺ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain Sm³⁺ by spectral analysis. The formation of Sm³⁺ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.

Crystals of ordered Ba₆EuF₁₂Cl₂ were found to form during high temperature flux growth. The structure was refined in the hexagonal space group P 6 to R^F(R^F_W) = 0.024(0.024) for 326 reflections and 46 parameters. Lattice parameters are a = b = 1059.27(8) pm and c = 416.36(2) pm; Z = 1. The structure is isotypic to Ba₇F₁₂Cl₂. No solid solution of Ba/Eu was observed, the Eu²⁺ ions are located in the channels formed by 3 + 6 fluorine ions, occupying only one of the three metal sites of the Ba₇F₁₂Cl₂ structure.

We recently discovered a new compound with composition Ba₇F₁₂Cl₂. It was possible to show that the variation of the synthesis conditions makes it possible to obtain a disordered and an ordered modification with different lattice parameters and space groups (P6₃/m [176] and P6  [174]). For Pb₇F₁₂Cl₂ an ordered modification is reported in the literature. In this paper we present the synthesis and structural characterization from X-ray diffraction data of the disordered modification of Pb₇F₁₂Cl₂. Single crystals were grown from a flux and the structure was refined in the hexagonal space group P6₃/m to R(R_w)=0.043(0.038) for 284 reflections and 26 parameters. Lattice parameters are a=b=1021.90(8) pm and c=361.93(6) pm with Z=1. Propeller-type arrangements with chlorine as axis and fluorines as blades are observed. The ordered modification of Pb₇F₁₂Cl₂ was prepared by a new hydrothermal synthesis. Differences between both modifications are found in the lattice constants and atomic occupation parameters for the atom type Pb2 and the connected fluorine ions.

Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle:  (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO₂)]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994) Protein Sci. 3, 1058−1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3−4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes.

In this comment, it is pointed out that the generalized gradient approximation (GGA) functionals considered by Milet et al. [ J. Chem. Phys. 111, 7727 (1999)] share the same exchange part (B88) which violates significantly the Lieb–Oxford bound. Violation of this exact condition was shown to result in significant errors of the exchange energy in the case of weakly overlapping electron densities [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997); Zhang et al., J. Chem. Phys. 107, 7921 (1997)]. Numerical examples are given to illustrate that such exchange functionals which better satisfy the Lieb–Oxford bound lead to better interaction energies also for the complexes studied by Milet et al.

The use of hybrid ab initio QM/MM methods in studies of metalloenzymes and related systems presents a major challenge to computational chemists. Methods that include the metal ion in the quantum mechanical region should also include the ligands of the metal in this region. Such a treatment, however, should be very demanding if one is interested in performing the configurational averaging needed for proper calculations of activation free energies. In the present work we examine the ability of the frozen DFT (FDFT) and the constrained DFT (CDFT) approaches to be used in ab initio studies of metal-catalyzed reactions, while allowing for an effective QM (rather than a QM/MM) treatment of the reacting complex. These approaches allow one to treat the entire enzyme by ab initio DFT methods, while confining the SCF calculations to a relatively small subsystem and keeping the electron density of the rest of the system frozen (or constrained). It is found that the FDFT and CDFT models can reproduce the trend obtained by a full DFT calculation of a proton transfer between two water molecules in a (Im)₃Zn²⁺(H₂O)₂ system. This and related test cases indicate that our approximated models should be capable of providing a reliable representation of the energetics of metalloenzymes. The reasons for the special efficiency of the FDFT approach are clarified, and the strategies that can be used in FDFT studies of metalloenzymes are outlined.

The mechanism of the protonation of ferrocene, the simplest model for the electrophilic attack on a metallocene, has been studied extensively. However, neither experiment nor computation have reached agreement on the details of the mechanism. The different model calculations applied [Hartree–Fock, Möller–Plesset, and density functional theory (HF, MP2, and DFT) with different functionals] come to contradicting conclusions. As a complement to our previous work, we report the results obtained for neutral and protonated ferrocene using the coupled-cluster method [CCSD(T)] with polarized double- and triple-zeta basis sets. These calculations show that the metal-protonated and the agostic forms represent minima on the potential energy surface, whereas the ring-protonated form is higher in energy with no minimum structure identified. With regard to the reaction, these results indicate an exo reaction path. The CCSD(T) results are in good agreement with the predictions made by the DFT calculations, whereas the results obtained from the Hartree–Fock and MP2 computations appear to be incorrect.

Incorporation of [Co(bpy)₃]²⁺ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)₃][LiCr(ox)₃] produces chemical pressure that destabilises the normal high-spin ground state ⁴T₁ to such an extent that the [Co(bpy)₃]²⁺ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the ²E low-spin and the ⁴T₁ high-spin states.

The dynamics of charge recombination within geminate ion pairs formed by electron transfer (ET) quenching of excited aromatic hydrocarbons by aliphatic and aromatic amines was investigated using picosecond transient grating spectroscopy. With aliphatic donors, the rate constant of back ET, k_BET, shows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between k_BET and the exergonicity of the process was observed. This effect is ascribed to a decrease of the electronic coupling matrix element, V, which is affected by both the distance between the N atom of the donor and the aromatic plane of the acceptor and by the delocalization of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, k_BETis substantially slower than with the unhindered amines. This is also explained in terms of a smaller value of V because of charge delocalization.