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.