The enantioselective hydrogenation of ring-substituted acetophenones that possess no functional group in the α-position to the keto group represents the latest extension of the application range of the Pt–cinchona system. The influence of the type of solvent, pressure, temperature, and modifier/substrate/Pt molar ratios was investigated in the hydrogenation of 3,5-di(trifluoromethyl)acetophenone. Modification of a 5 wt% Pt/Al₂O₃ catalyst by cinchonidine (CD) afforded the corresponding (S)-1-phenylethanol (69.5% ee). Working in strongly polar solvents, addition of trifluoroacetic acid in a weakly polar solvent, and replacing CD by its ether derivatives resulted in the inversion of enantioselectivity. Addition of CD or any of its derivatives always led to a lower reaction rate, contrary to the generally observed rate acceleration in the hydrogenation of α-functionalized activated ketones over the same catalyst system. Another fundamental difference to the hydrogenation of α-functionalized activated ketones is that both the quinuclidine N and the OH functions of CD influence the stereochemical outcome of the reaction, as clarified by using O- and N-substituted derivatives of CD. Ab initio calculations confirmed these remarkable mechanistic differences. Inversion of enantioselectivity in the presence of strongly polar and acidic solvents is attributed to special interactions with the OH function of CD, and to the formation of a CD–acid ion pair, respectively. A possible explanation for the moderate ee's in the hydrogenation of ring-substituted acetophenones is that a reaction pathway without involvement of the OH function of CD is also feasible. This competing pathway is even faster and provides low ee to the opposite enantiomer.

The comparison between experimental and calculated VCD spectra allowed the unequivocal assignment of the absolute configuration of heptahelicene C₃₀H₁₈ as P(+).

The adsorption of several ketones interesting for the enantioselective hydrogenation on cinchona-modified platinum has been modeled using relativistically corrected density functional theory. Two metal clusters, containing 19 and 31 Pt atoms, respectively, have been used to model a Pt(111) surface. The two adsorption modes η¹ and η² have been described, and their importance for the mechanism of hydrogenation has been pointed out. The effect of an ester group in α position and of α-fluorination of a ketone on its adsorption has been studied, and an explanation for the reactivity enhancement due to the ketone substitution has been proposed.

Epoxidation of cyclohex-2-en-1-ol and cyclooct-2-en-1-ol on titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) as oxidant was studied by in situ attenuated total reflection (ATR) Fourier transform infrared spectroscopy. Probing of the catalytic liquid–solid interface revealed different adsorption behaviors for the two allylic alcohols on the aerogel. Cyclohexenol was found to adsorb stronger and less reversible on the catalyst surface and Ti sites than cyclooctenol. The spectroscopic measurements under working conditions support the previously proposed hydroxy-assisted mechanism for the formation of cyclohexenol oxide and the silanol-assisted mechanism for cyclooctenol epoxidation. The evidence of the former is traced to the occurrence of a framework vibration upon adsorption of cyclohexenol, whereas the latter is supported by large negative bands of the silanol groups at 3700 and 980 cm^−1 in the case of cyclooctenol epoxidation.

The enantioselective hydrogenation of several isatine derivatives over cinchonidine modified Pt/Al₂O₃ was investigated. A maximum enantiomeric excess (e.e.) of 45% was found for (R)-5,7-dimethylisatin. The enantiomeric excess was limited by racemization catalyzed by the basic cinchonidine in solution, leading to low enantiomeric excess at high cinchonidine concentration. The modifier in solution also catalyzed the formation of the corresponding isatide. High cinchonidine concentration favored isatide formation, whereas low cinchonidine concentration and high hydrogen pressure favored alcohol formation. The isatide, formed from the isatin reactant and the alcohol, underwent disproportionation. Though both hydrogenation and isatide formation are fast reactions, isatide formation was considerably faster at least at the beginning of the reaction. Substitution of the isatin reactant had relatively little effect on enantiomeric excess but affected considerably the rate of racemization.

The behavior of ethyl pyruvate during adsorption on vapor deposited alumina-supported platinum films and on a commercial 5 wt % Pt/Al₂O₃ catalyst has been studied in the absence and presence of coadsorbed cinchonidine, which is usually applied as a chiral modifier in the platinum-catalyzed enantioselective hydrogenation of α-ketoesters. The in situ ATR−IR measurements, performed at room temperature using hydrogen-saturated CH₂Cl₂ as solvent, revealed that upon adsorption on the platinum some of the ethyl pyruvate decomposes leading to strongly adsorbed CO and other fragmentation products. The CO originating from decomposition of ethyl pyruvate reached approximately 14% of the amount of adsorbable CO on the free platinum surface and is proposed to be adsorbed preferentially on energetically favored sites such as edges and corners. The presence of cinchonidine (10^-4 M) lead to a drastic decrease of the decomposition rate of ethyl pyruvate by a factor of about 60 under the conditions used. Competitive adsorption experiments of CO and cinchonidine in the presence of hydrogen indicated that cinchonidine can displace the adsorbed CO, confirming the strong anchoring of cinchonidine on the platinum surface, which is a prerequisite for its action as a chiral modifier. The findings of the adsorption studies provide a plausible explanation for the earlier made observation that the sequence of admission of α-ketoester, chiral modifier, and hydrogen affects the catalytic performance of platinum-catalyzed enantioselective hydrogenation. The decomposition is likely to occur also with other α-ketoesters and may have a bearing on the initial transient period, typically observed during hydrogenation of such compounds on cinchona-modified platinum catalysts.

The adsorption of cinchonidine on platinum has been calculated with relativistically corrected density-functional theory, by first studying the interaction of the 1(S)-(4-quinolinyl)ethanol with a platinum cluster of 31 metal atoms, and by successive addition and separate optimization of the quinuclidine moiety. The conformations of the alkaloid on the surface were analyzed and their possible interactions with a surface chemisorbed methylpyruvate and acetophenone are discussed. A chiral space that is able to selectively accommodate surface enantiomers and to promote their rapid hydrogenation in a ligand-accelerated fashion has been determined. The role of the O-alkylation of the alkaloid in the modulation of enantioselectivity has been rationalized within the new interaction model.

FTIR and NMR spectroscopy and ab initio calculations were applied to understand the nature of enantioselection in the hydrogenation of the heteroaromatic ring in furan- and benzofurancarboxylic acids over cinchonidine-modified Pd. Most probably, cinchonidine adsorbs on Pd, via its quinoline moiety, approximately parallel to the surface, and the protonated quinuclidine N atom and the OH function of the alkaloid form a cyclic complex with the deprotonated acid dimer (2:1 acid:cinchonidine). The acid dimer adsorbs via the electron-rich furan ring and the carboxylate groups close to parallel to the Pd surface; the furan O atom points toward the OH function of cinchonidine. In this position, hydrogen uptake from the Pd surface results in the (S)-enantiomer as the major product. Another cyclic complex (1:1) involving cinchonidine and only one acid molecule is also feasible in solution, but this rigid structure is thermodynamically less favored, and it may be difficult to fulfill the geometric constraints imposed by adsorption on the metal surface.

A method to selectively probe the different adsorption of enantiomers at chiral solid−liquid interfaces is applied, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The spectral changes on the surface are followed while the absolute configuration of the adsorbate is changed periodically. Demodulated spectra are calculated by performing a subsequent digital phase-sensitive data analysis. The method is sensitive solely to the difference of the interaction of the two enantiomers with the chiral surface, and the small spectral changes are amplified by the phase-sensitive data analysis. Its potential is demonstrated by investigating an already well-studied system in liquid chromatography, namely, the enantiomer separation of N-3,5-dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu) using tert-butylcarbamoyl quinine (tBuCQN) as the chiral selector immobilized on the surface of porous silica particles. The performed experiments and density functional theory calculations confirm an interaction model that was proposed earlier based on solution NMR and XRD in the solid state. It emerges that the ionic interaction is the strongest one, but the main reason for the potential for enantioseparation of the chiral stationary phase (CSP) is the distinct formation of a hydrogen bond of the (S)-enantiomer with the chiral selector. This H-bond is established between the amide N−H of DNB-(S)-Leu with the carbamate C=O of the CSP. The (R)-enantiomer instead shows no specific hydrogen bonds. Only the unspecific ionic bonding between the protonated quinine part of the tBuCQN and the carboxylate of the DNB-(R)-Leu (holds also for DNB-(S)-Leu) is observed.

In situ attenuated total reflection infrared spectroscopy in a flow-through cell combined with online UV−vis spectroscopy was used to investigate the oxidation of 2-propanol over Pd/Al₂O₃ catalyst. The state of the catalyst was driven fast between reduced and oxidized by admitting alternately dissolved hydrogen and oxygen, and the response of the catalytic solid−liquid interface was followed in time. Besides the oxidation product acetone and the water that forms, when hydrogen and oxygen are simultaneously adsorbed on the catalyst surface, an additional species was observed with a characteristic band at ~1065 cm^-1. On the basis of the transient character of the adsorbate and density functional theory calculations, we assign this species to adsorbed 2-propoxide. Its observation indicates that the second dehydrogenation step is rate limiting in an oxidative dehydrogenation mechanism. The results furthermore show that adsorbed hydrogen and oxygen limit the dissociative adsorption of 2-propanol and that 2-propoxide can be hydrogenated back to the reactant in the presence of adsorbed hydrogen.

Modification of a metal surface by a strongly adsorbed chiral organic molecule has proven to be an interesting strategy for heterogeneous chiral catalysis. Platinum chirally modified by cinchona alkaloids, successfully applied for the enantioselective hydrogenation of α-ketoesters, is probably the most prominent catalyst based on this concept. Despite considerable research efforts toward understanding of this complex catalytic system, the proposed mechanistic models are still debated. Here we discuss how enantiodifferentiation can be induced on a catalytically active surface and validate the models proposed for the platinum−cinchona system in the light of the existing molecular knowledge.

Cumulated triple bonds: A new class of molecules, the isoelectronic series of NUIr, (depicted) has been postulated and theoretically studied. The bond between the actinide and the 5d-metal atom is very short and is shown to correspond to triple bonding. The N — U bond is also a triple bond.

The fragmentation behaviour of the ion MeP(O)OMe⁺ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH₂O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)₂⁺ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH₂O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable.

In the present paper we report the results of a multiconfigurational computational study on potential-energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S₂(ππ*) and S₁(nÏ€*) states. We have found that there is a singlet state, S₃ at the trans geometry, on the basis of the doubly excited configuration n²Ï€*², that has a deep minimum at about 90° of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S₂(ππ*) and for the wavelength-dependence of azobenzene photochemistry. We have characterized the S₁(nÏ€*) state by calculating its vibrational frequencies, which are found to correspond to the recently observed transient Raman spectrum. We have also computed the potential-energy curve for the triplet T₁(nÏ€*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.

In this paper, we identify the most efficient decay and isomerization route of the S₁, T₁, and S₀ states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S₁ potential energy surface and for the S₀/S₁ conical intersections (CIs) that are closer to the minimum energy path on the S₁. We found only one TS, at 60° of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S₁ minimum, were found on the torsion pathway for CNNC angles in the range 95−90°. The lowest CI along the inversion path was found ca. 25 kcal/mol higher than the S₁ minimum and was character₁ state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S₁ minimum of the E isomer. We have calculated the spin−orbit couplings between S₀ and T₁ at several geometries along the CNNC torsion coordinate. These spin−orbit couplings were about 20−30 cm^-1 for all the geometries considered. Since the potential energy curves of S₀ and T₁ cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T₁ lifetime is very short (~10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S₀−T₁−S₀ crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.

The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO⁺. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin–orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction.

Quantum chemical calculations suggest that a series of molecules with the general formula MAu₆ are stable, where M is a a group 6 atom, Cr, Mo, W, respectively. These species have a structure analogous to the corresponding MH₆ compounds, while they differ from the MX₆, where X is a halogen. The further reaction MAu₆ + 3Au₂→MAu₁₂ is strongly exothermic.

A new method is presented, which makes it possible to partition molecular properties like multipole moments and polarizabilities, into atomic and interatomic contributions. The method requires a subdivision of the atomic basis set into occupied and virtual basis functions for each atom in the molecular system. The localization procedure is organized into a series of orthogonalizations of the original basis set, which will have as a final result a localized orthonormal basis set. The new localization procedure is demonstrated to be stable with various basis sets, and to provide physically meaningful localized properties. Transferability of the methyl properties for the alkane series and of the carbon and hydrogen properties for the benzene, naphtalene, and anthracene series is demonstrated.

Quantum chemical calculations predict the existence of new molecular species with general formula MH₁₂, where M is a group 6 atom. The previous MH_n species had n values up to 9. The new systems with n = 12 would be a new record for metal hydrides.

The absorption spectrum of ozone was recorded at low temperatures (down to −135 °C) by high resolution Fourier transform spectrometry and intra cavity laser absorption spectroscopy (ICLAS) near 10,400 cm^−1. A preliminary analysis of the rotational structure of the absorption spectra of ¹⁶O₃ and ¹⁸O₃ shows that this spectral region corresponds to a superposition of two different electronic transitions, one with a very broad rotational structure, showing for the first time the asymmetric stretching frequency mode ν₃ of the electronic state ³A₂, the other formed by a completely diffuse band, probably the 2₀¹ band of a new transition due to the triplet electronic state ³B₂. Predissociation effects induce large broadening of the rotational lines for the transition centered at 10,473 cm^−1 identified as the 3₀² band of the ³A₂ â†� X¹A₁ electronic transition. The rotational structure cannot be analyzed directly but instead the band contour method was used to confirm the symmetry of the transition and to estimate the spectroscopic constants for the ¹⁶O isotopomer. The origin of the band is at 10,473±3 cm^−1 and the value of the ¹⁶O₃(³A₂) antisymmetric stretching frequency mode is equal to 460±2 cm^−1. We believe that the diffuse band is due to the ³B₂ state and is located at about 10,363±3 cm^−1 for ¹⁶O₃ and 10,354±3 cm^−1 for ¹⁸O₃. The isotopic rules confirm the different results obtained for ¹⁸O₃ and ¹⁶O₃.

Excitation energy transfer processes play an important role in many areas of physics, chemistry and biology. The three-dimensional oxalate networks of composition [MIII(bpy)3][MIMIII(ox)3]ClO4 (bpy=2,2-bipyridine, ox=oxalate, MI=alkali ion) allow for a variety of combinations of different transition metal ions. The combination with chromium(III) on both the tris-bipyridine as well as the tris-oxalate site constitutes a model system in which it is possible to differentiate unambiguously between energy transfer from [Cr(ox)3]3– to [Cr(bpy)3]3+ due to dipole-dipole interaction on the one hand and exchange interaction on the other hand. Furthermore it is possible to just as unambiguously differentiate between the common temperature dependent phonon-assisted energy migration within the 2E state of [Cr(ox)3]3–, and a unique resonant process.

The compound of stoichiometry Mn(II)₃[Mn(III)(CN)₆]₂·zH₂O (z = 12−16) (1) forms air-stable, transparent red crystals. Low-temperature single crystal optical spectroscopy and single crystal X-ray diffraction provide compelling evidence for N-bonded high-spin manganese(II), and C-bonded low-spin manganese(III) ions arranged in a disordered, face-centered cubic lattice analogous to that of Prussian Blue. X-ray and neutron diffraction show structured diffuse scattering indicative of partially correlated (rather than random) substitutions of [Mn(III)(CN)₆] ions by (H₂O)₆ clusters. Magnetic susceptibility measurements and elastic neutron scattering experiments indicate a ferrimagnetic structure below the critical temperature T_c = 35.5 K.

The lessons learned from p-octiphenyl β-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and “invisible,â€� i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod β-barrel 1 is inert and large (d ≈ 12 Ã…). Multifunctionality of rigid-rod β-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, K_D = 190 μM, n = 4.9) and spherical bilayers (poly-l-glutamate, K_D ≤ 105 nM, n = 1.0; adenosine triphosphate, K_D = 240 μM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, K_M = 0.6 μM). The thermodynamic instability of rigid-rod β-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained k_cat = 0.03 min^-1 with the k_cat ≈ 0.18 min^-1 for stable analogues gave a global K_D ≈ 39 μM³ for supramolecular catalyst 1 with a monomer/barrel ratio ≈ 20 under experimental conditions. The demonstrated “invisibilityâ€� of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod β-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported.

The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G ^* _et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.

A complex sedimentary sample from the Monterey Formation (CA, USA) has been submitted to GC–MS analysis followed by mass spectral deconvolution using Automated Mass Spectral Deconvolution and Identification System (AMDIS). Adjusting the parameters of the software allowed for the extraction of the spectrum of an unusual steroidal hydrocarbon coeluting with the major compound of the chromatogram. Following a careful interpretation of the “extracted� mass spectrum, the structure of the unknown has been postulated to be the 4,14-dimethylcholestane (DMC). Possible origins of this rare steroid are briefly discussed. Thus, application of AMDIS appears to be particularly suitable for the GC–MS analysis of natural complex mixtures characterized by a high number of analytes present in low amounts.

Polycyclic aromatic hydrocarbons (PAHs) including isomeric pairs were separated in capillary electrokinetic chromatography using a cationic surfactant cetylpyridinium bromide (CPBr) as additive. With addition of 2 mM CPBr into the running electrolyte, dynamic coating occurs in the capillary and EOF is reversed. Changes of electroosmotic and electrophoretic mobilities with increasing CPBr concentration were investigated. Under optimum separation conditions, running electrolyte contains 50% MeCN, 20 mM acetate, and 40 mM CPBr at pH = 4.0. Using high concentration of organic solvent, aggregation of surfactants into micelles is prevented. Significant retentions indicate solvophobic, n- and π-electron interactions between CPBr monomers and PAHs.

An I_a mechanism was assigned for water exchange on the hexaaquaions Rh(OH₂)₆³⁺ and Ir(OH₂)₆³⁺ on the basis of negative ΔV_‡ experimental values (−4.2 and −5.7 cm³ mol^-1, respectively). The use of ΔV_‡ as a mechanistic criterion was open to debate primarily because ΔV_‡ could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I_a) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH₂)₆³⁺ is 32.2 kJ mol^-1 in favor of the I_a mechanism (127.9 kJ mol^-1), as opposed to a D pathway; the value for the I_a mechanism being close to ΔH_‡ and ΔG_‡ experimental values (130.5 kJ mol^-1 and 129.9 kJ mol^-1 at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I_a pathway (ΔV_‡calc = −3.9 and −3.5 cm³ mol^-1, respectively, for Rh³⁺ and Ir³⁺), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: −1.8 for the D, and −0.9 cm³ mol^-1 for I_a mechanism. This finding clarifies the debate about the interpretation of ΔV_‡ and unequivocally confirms the occurrence of an I_a mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.

The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, Ln^III, to give the nine-co-ordinate triple-helical complexes [Ln(Li)₃]³⁺ (i=1–4) existing as mixtures of C₃-symmetrical facial and C₁-symmetrical meridional isomers. Although the β₁₃ formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D₃-symmetrical analogues [Ln(Li)₃]³⁺ (i=5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)₃]³⁺ is significantly larger. The fac-[Ln(Li)₃]³⁺↔mer-[Ln(Li)₃]³⁺ (i =1–4) isomerisation process in acetonitrile is slow enough for Ln=Lu^III to be quantified by ¹H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand -stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 13 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₃H₅N)₂(H₂O) ( 6, C₄₅H₅₄EuF₉N₁₀O₁₃S₃, monoclinic, P2₁/c, Z=4) and [Eu(L4)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₄H₄O)_1.5 ( 7, C₅₁H₆₆EuF₉N₈O_15.5S₃, triclinic, P, Z=2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.

Highly symmetric spirobi[dibenzazepinium] cation 3 reacts with P₄-t-Bu to form exclusively a ring-expanded tertiary amine; this unusual reactivity can be traced back to the geometry of the ylide.

The electron transfer quenching dynamics of excited perylene and cyanoperylene in various donating solvents has been investigated by using ultrafast fluorescence up-conversion and multiplex transient grating. The strongly nonexponential fluorescence decays have been analyzed by using the orientational model described in the first article of this series (J. Phys. Chem. A 2003, 107, 5375). It appears that the solvent dependence of the quenching dynamics is strongly connected to the number of surrounding donor molecules enabling ultrafast electron transfer. This number depends mainly on the driving force for electron transfer, on steric interactions, and on the occurrence of dipole−dipole interactions with the acceptor. The quenching product is an exciplex with a strong charge-transfer character. The complicated wavelength dependence of the fluorescence dynamics in the exciplex region, as well as the spectral dynamics observed in the transient grating data, is attributed to dipolar solvation, which leads to an increase of the charge-transfer character of the exciplex. The strong donor dependence of the exciplex lifetime is very similar to that reported earlier for the charge recombination time of geminate ion pairs in acetonitrile, and can be rationalized in terms of different intramolecular reorganization energies and electronic coupling constants.

EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P — P bond (3.479 Å), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P — P distance in neutral 1 is large (5.8 Å). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P — P single bond (P — P 2.389 Å). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C — Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P — P distance. The ability of the phosphinine radical anion to reversibly form weak P — P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P — P bond.

The packing preferences of dimers formed by nitrogen-containing planar polycyclic aromatic hydrocarbons ((C₃₀H₁₅N)₂ and (C₃₆H₁₅N)₂) were studied by means of theoretical calculations. Potential energy curves corresponding to various relative motions of the monomers (vertical displacement, rotating, slipping, and combinations of them) were derived. It was found that the monomers in such Ï€-stacked dimers are rather strongly held together (the interaction energy is about −9 kcal/mol) in an off-centered arrangement. It emerges as a general picture that the aligned structures are less stable than the ones where the nitrogen atoms, as the centers of the considered monomers, are not on top of each other but offset by 1.8−2.7 Ã…. Displacing the centers further results in a rapid reduction of the interaction energy. Within these relatively large relative motions (up to about 3 Ã…) of the monomers, however, no significant loss of stability of the dimers is noted. In the case of C₃₀H₁₅N, changing the orientation of the enantiotopic faces in the dimer formation leads to two nonequivalent minimum energy structures of similar energies but notably different geometries. The most stable structure of both dimers studied resembles that of two adjacent layers of graphite. We conclude, therefore, that the studied molecules could be considered as good building block candidates for the fabrication of columnar organic conductors.

Ligand field splitting energies of lanthanides Ln³⁺ (Ln = from Ce to Yb) in octahedral environment are calculated using the Hohenberg–Kohn theorems based orbital-free embedding formalism. The lanthanide cation is described at orbital level whereas its environment is represented by means of an additional term in the Kohn–Sham-like one-electron equations expressed as an explicit functional of two electron densities: that of the cation and that of the ligands. The calculated splitting energies, which are in good agreement with the ones derived from experiment, are attributed to two main factors: (i) polarization of the electron density of the ligands, and; (ii) ion–ligand Pauli repulsion.

The theoretically calculated dimerization-induced shifts of the lowest excitation energies in two model systems, adenine−thymine and guanine−cytosine base pairs, are analyzed. The applied formalism is based on first principles and allows one to study the influence of the microscopic environment of a given molecule on its ground- [Wesolowski, T. A.; Warshel, A. J. Phys. Chem. 1993, 97, 8050] and excited-state [Casida, M. E.; Wesolowski, T. A. Int. J. Quantum Chem. 2004, 96, 577] properties. The assessment of the relative importance of such effects as (a) Coulomb interactions, (b) orbital interactions, (c) electronic polarization of the environment, and (d) electron density overlap effects is straightforward in this formalism. In the applied formalism, electron density overlap effects can be further decomposed into the exchange−correlation component which provides a small attractive contribution and the repulsive kinetic energy-dependent component. It is shown that the shifts can be attributed to the electrostatic interactions and the repulsive overlap-dependent term in the embedding potential. The electronic polarization of the environment plays a significant role (up to 30% of the total shift) only in transitions involving the orbitals localized on hydrogen bond donor groups. For all analyzed shifts, the contribution of the intermolecular orbital interactions is negligible. The analysis of this work provides strong evidence supporting the use of the widely applied embedding-molecule strategy in computational studies of chromophores in a condensed phase even in such cases where only one end of the hydrogen bond is included in the quantum mechanical part.

A model of nonequilibrium charge recombination from an excited adiabatic state of a donor-acceptor complex induced by the nonadiabatic interaction operator is considered. The decay of the excited state population prepared by a short laser pulse is shown to be highly nonexponential. The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes is explored. The charge recombination rate constant is found to decrease with increasing excitation frequency. The variation of the excitation pulse carrier frequency within the charge transfer absorption band of the complex can alter the effective charge recombination rate by up to a factor 2. The magnitude of this spectral effect decreases strongly with increasing electronic coupling.

The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon.

The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series of compounds and over a large temperature range. It could thus be firmly established that intersystem crossing in spin-crossover compounds is a tunnelling process, with a limiting low-temperature lifetime below 50 K and a thermally activated region above 100 K. This review begins with an elucidation of the mechanism of the light-induced spin transition, followed by an in depth discussion of the chemical and physical factors, including cooperative effects, governing the lifetimes of the light-induced metastable states.

A comparison of density functionals is made for the calculation of energy and geometry differences for the high- [⁵T_2g: (t_2g)⁴(e_g)²] and low- [¹A_1g: (t_2g)⁶(e_g)⁰] spin states of the hexaquoferrous cation [Fe(H₂O)₆]²⁺. Since very little experimental results are available (except for crystal structures involving the cation in its high-spin state), the primary comparison is with our own complete active-space self-consistent field (CASSCF), second-order perturbation theory-corrected complete active-space self-consistent field (CASPT2), and spectroscopy-oriented configuration interaction (SORCI) calculations. We find that generalized gradient approximations (GGAs) and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used (i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules). In contrast, CASPT2 calculations of the low-spin–high-spin energy difference ΔE_LH = E_LS−E_HS appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes (⁵D→¹I excitation of Fe²⁺) are overestimated by about 3000 cm^−1. An empirical shift of the molecular ΔE_LH based upon atomic calculations provides a best estimate of 12 000–13 000 cm^−1. Our unshifted SORCI result is 13 300 cm^−1, consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000–4000 cm^−1 while the B3LYP functional underestimates it by only about 1000 cm^−1. The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular ΔE_LH within the context of density functional theory (DFT) calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations.

In the three-dimensional oxalate network structures of composition [Co_xM^II_1-x(bpy)₃][M^ICr(ox)₃], the spin state of the [Co_x(bpy)₃]²⁺ complex can be tuned by means of chemical pressure. With M^I=Na it is a classic high-spin complex. Substitution of Na by Li stabilises the complex and it becomes a spin-crossover complex. Dilution with M^II=Fe reinforces this effect, and M^II=Zn reverses it.

Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)₄Cl₂] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)₃]²⁺ and mer-[Ru(Li)₃]²⁺ (i=2, 3; ΔGΘ_{isomerisation}=-2.7 kJ mol^-1). In more polar solvents (acetonitrile, methanol), the free energy of the facial ↔ meridional isomerisation process favours mer-[Ru(Li)₃]²⁺, which is the only isomer observed in solution at the equilibrium (ΔGΘ_{isomerisation}≤-11.4 kJ mol^-1). Since the latter process takes several days for [Ru(L2)₃]²⁺, fac-[Ru(L2)₃]²⁺ and mer-[Ru(L2)₃]²⁺ have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)₃]²⁺ provides only mer-[Ru(L3)₃](ClO₄)₂ after chromatography (RuC₆₀H₅₁N₉O₈Cl₂, monoclinic, P2₁/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial↔meridional isomerisation is reversed (ΔGΘ_{isomerisation}≥11.4 kJ mol^-1), and the Ru — N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)₃]⁵⁺ within hours ([RuLu(L1)₃](CF₃SO₃)_4.5Cl_0.5(CH₃OH)_2.5: RuLuC₁₀₆H₁₀₉Cl_0.5N₂₁O₁₉S_4.5F_13.5, triclinic, P, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar Ï€-acceptor properties to, but stronger Ï€-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand π→π^* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)₃]²⁺ (i=2, 3) chromophores. The concomitant short lifetime of the ³MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)₃]⁵⁺.

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.

The fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate.

An investigation of the ultrafast excited state dynamics of triporphyrin and hexaporphyrin arrays consisting of covalently linked Zn tetraphenylporphine (ZnTPP) and free base tetraphenylporphine (FbTPP) units is reported. The interchromophoric distance in the hexamers is of the order of 13 Ã…, while it varies from 26 to 70 Ã… in the trimers. These arrays exhibit several features that differ substantially from those of the monomeric units:  a broadening of the Soret band, a shortening of the S₂ lifetime of the ZnTPP chromophores, and additional ultrafast decay components of the S₁ fluorescence. In the hexaporphyrin arrays, most of these features are attributed to the presence of excitonic states that result from the strong coupling between the B_x,y transition dipoles. The time constants for S₁ energy transfer between ZnTPP chromophores as well as between ZnTPP and FbTPP moieties, deduced from anisotropic and isotropic time-resolved fluorescence measurements, were found to be of the order of a few tens of picoseconds. Moreover, back energy transfer from the FbTPP to ZnTPP units is also observed. At high to moderate excitation intensity, S₁−S₁ annihilation becomes an important decay mechanism of the excited state population of the hexaporphyrins. In the triporphyrins, the differences relative to the monomer are ascribed to the interaction with the phenylacetylene linkers, which lifts the degeneracy of the S₂ states. S₂ and S₁ energy transfer were found to take place in the triporphyrin with the shortest linker only. In the other triporphyrins, an efficient energy transfer from the linker to the porphyrin units was observed.

By means of ¹H-NOESY- and Raman-spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N — Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone-pair anomeric stabilization should be revised to 1.5-1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)-comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five-membered-ring sultams do not seem to be particularly well-stabilized by the S — C σ* orbital in the N-substituted pseudo-axial conformation, as opposed to an idealized anti-periplanar situation for the six-membered-ring analogues. In this latter case, the other anti-periplanar C — C σ* and C(1') — H/C(2') σ*orbitals are as important, if not more, when compared to the S — C σ* participation. In the pseudo-equatorial conformation, γ-sultams particularly benefit from the N lone-pair hyperconjugation with the anti-periplanar S — O₁ σ* and C(2) — H/C or C(1') — H/C σ* orbitals. This is also the case for δ-sultams when the steric requirement of the N-substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N-atom is prone to sp² hybridization or/and conformational changes (i.e., 12c). In that case also, the mode of stereoelectronic stabilization differs from γ- to δ-sultams.

Alkali borohydrides MBH₄ and their deuterides have been investigated by X-ray and neutron powder diffraction (M=K,Rb,Cs) and by infrared and Raman spectroscopy (M=Na,K,Rb,Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m  symmetry in which the [BH₄]^− complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P4₂/n mc symmetry in which the [BH₄]^− complexes are ordered such as in the isotypic sodium congener. The B---H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the ¹⁰B to ¹¹B splitting due to the presence of natural boron in the samples.

The role of one-electron functions known as orbitals in various theoretical methods used to describe molecules and complex materials at a quantum mechanical level is outlined in a historical perspective. A hierarchy consisting of three types of general formalism, ordered according to the importance of orbital-dependent expressions in the total energy, is presented. Two such formalisms, less known to the general chemistry community, are discussed in detail together with their recent applications in modelling complex systems: a) the orbital-free formulation of density functional theory, which does not use orbitals at all and which can be seen as the modern realization of the original ideas of Thomas and Fermi, and b) the density partitioning based formalism, in which the orbitals are used only for smaller parts of a larger system (subsystems). The emphasis is placed on the second type of formalism, a topic of strong interest of our Geneva group.

The uranyl salophene complex and its co-complexes with several anions (H₂PO₄^-, HSO₄^-, NO₂^-, OH^-, Cl^-, F^-) in the gas phase are investigated theoretically. Equilibrium geometries of relevant species and complexation-induced structural changes are discussed. The ¹³C NMR chemical shifts calculated at the gas-phase optimized geometry agree very well with experimental liquid-phase results. The optimized geometry agrees also very well with available crystallographic data. This indicates that the gas-phase structures derived from theoretical calculations can be considered representative also for the condensed phase. For all anions, except H₂PO₄^-, the calculated gas-phase binding energies correlate well with experimental Gibbs free energies of complexation. The possible role of the solvent in the case of H₂PO₄^- complexation is discussed.

Temperature-dependent emission spectra of Sm²⁺-doped SrMgF₄ have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong ⁵D₀→⁷F₀ transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm^−1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF₄ which will require new experiments.

The C₂-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)₂H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF₆)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF₆] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF₆] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein C_α-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature ¹H, ³¹P, and ¹⁷O NMR spectroscopy, variable-pressure ³¹P and¹⁷O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol^-1 (12.2 kcal mol^-1) for the inversion of the pseudo-chirality at the ruthenium center.

The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔG_CS= −2.2 eV), but charge recombination is weakly exergonic (ΔG_CR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).

The components of nucleus-independent chemical shift (NICS) tensors for D_nhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICS_zz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICS_zz, which is dominated by contributions from the π system, characterizes NICS well.

Entirely unlike the aromatic closo B_nH_n^2- borane dianions, isoelectronic Si₆^2- and Si₁₂^2- are antiaromatic. Their O_h and I_h symmetries are responsible, as the other deltahedral silicon dianion clusters do not exhibit this behavior. These high symmetries prevent mixing among the degenerate lone pair and skeletal orbitals, leading to paratropic behavior.

Ba₆Mg₁₁F₃₄, a new compound of the pseudobinary BaF₂–MgF₂ system, has been synthesized by solid state techniques from stoichiometric amounts of BaF₂ and MgF₂ and its crystal structure determined by single crystal X-ray diffraction (space group P1 , a=7.5084(6), b=9.9192(8), c=10.0354(8) Å, α=81.563(2), β=72.402(2), γ=71.198(1)°, 3899 structure factors, 233 parameters, R(F²>2σ(F²))=0.018, wR(F² all) = 0.046). It is isotypic with the copper(II) analogue, Ba₆Cu₁₁F₃₄. The main features of the structure are a network of [MgF₆] octahedra and three different [BaF_x] polyhedra with x=12, 11+1 and 13. Ba₆Mg_11−xFe_xF₃₄ and Ba₆Mg_11−xMn_xF₃₄ solid solutions were prepared and their composition determined by single crystal structure analyses. The luminescence properties of Ba₆Mg₁₁F₃₄ doped with Eu²⁺ were studied using fluorescence spectroscopy. The observed luminescence was pale blue with a maximum at 465 nm.

BaFCl single crystals doped with Sm³⁺ ions were studied by using the EPR method. Several types of paramagnetic Sm³⁺ centres were found. The parameters of the corresponding spin Hamiltonians were determined. Structural models and ground states of the observed centres are proposed.

The Kohn-Sham equations with constrained electron density (KSCED) embedding formalism of Wesolowski and coworkers was originally developed and is good for the case of two weakly interacting molecular regions with weakly overlapping densities, such as might be expected in describing solvation. A generalization is given here for the case of three molecular regions with strongly overlapping densities with the idea that this generalized theory can offer a better description of embedding in the context of situations that might be encountered in, for example, chemisorption on surfaces or active sites in enzymes. This three-partition generalization includes the original two-partition formalism as a special case. Time-dependent response theory equations are then developed for the two- and three-partition theories for application to the problem of the calculation of polarizabilities and other response properties, including excitation spectra, of embedded molecules or molecular structures.

Raman spectra of the alkali borohydride series MBH₄ (M=Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M=Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH₄]^− anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH₄, the data suggest two energy barriers of reorientation at ~5 and ~ 60 kJ/mol, respectively.