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The prototype Diels-Alder (DA) reaction between butadiene and ethene (system 1) and the DA reaction involving 1-methoxy-butadiene and cyano-ethylene (system 2) are investigated with an explicit-time-dependent Density Functional Theory approach. Bond orders and atomic net charges obtained in the dynamics at the transition state geometry and along the reaction coordinate toward reactants are used to provide a picture of the process in terms of VB/Lewis resonance structures that contribute to a resonance hybrid. The entire dynamics can be divided into different domains (reactant-like, product-like, and transition state domains) where different Lewis resonance structures contribute with different weights. The relative importance of these three domains varies along the reaction coordinate. In addition to the usual reactant-like and product-like covalent Lewis structures, ionic Lewis structures have non-negligible weights. In system 2, the electron-donor OCH3 on the diene and the electron-acceptor CN on the dienophile make more important the contributions of ionic Lewis structures that stabilize the transition state and determine the decrease of the reaction barrier with respect to system 1.

The carbon nanotube (CNT)-confined chloride exchange SN2 reaction for methyl chloride has been examined using either a full quantum mechanical (QM) DFT approach based on the M06-2X functional or a hybrid approach where a (6,6) CNT is satisfactorily described by the molecular mechanics (MM) UFF force field and the substrate by the M06-2X functional (M06-2X/UFF approach). We found that inside the CNT the reaction is disfavored with respect to the gas phase, the intrinsic reaction barrier Ea (difference between the preliminary complex I and transition state TS) being 17.9 kcal mol–1 (13.2 kcal mol–1 in the gas phase). The augmented barrier, with respect to the gas phase, can be ascribed to a complex interplay between Cl···π and C–H···π interactions (i.e., interactions of the two Cl atoms and the C–H bonds of the substrate with the carbon electron cloud of the tube wall). While the Cl···π interactions behave like a molecular glue which sticks the two Cl atoms to the tube wall and remain approximately constant in I and TS, the importance of the stabilizing C–H···π interactions is significantly lower in TS with a consequent increase of the barrier. The barrier increases with the increase of the tube length to reach the asymptotic value of 19.9 kcal mol–1 for tube length larger than 24.4 Å. This value is the minimum length of a (6,6) CNT model system that can emulate the CNT-confined SN2 reaction and provides useful suggestions to build reliable model systems for other SN2 reactions and, in general, different chemical processes. Furthermore, the activation barrier Ea is strongly affected by the tube radius. Because of the reduced volume inside the tube causing a strong structural distortion in TS, Ea is very large for small tube radii (34.4 kcal mol–1 in the (4,4) case). When the volume increases enough (tube (5,5)) to avoid the distortion, the barrier suddenly decreases and remains approximately constant (about 20 kcal mol–1) for tubes in the range (5,5) to (8,8). The activation barrier grows for a (9.9) tube, and the value again remains approximately constant (about 22 kcal mol–1) for larger tubes.

LAC (hydroxylactone (1R,5S)-1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one) is one of the most interesting products of the pyrolysis of cellulose and represents a useful chiral building block in organic synthesis. A computational investigation at the DFT level on the mechanism of formation of LAC shows that this species can be obtained following two reaction paths, path A and path B, starting from a well-known pyrolysis product (ascopyrone P). A series of internal rearrangements involving in all cases a proton transfer leads directly to LAC (path B). An alternative path (path A) can be also followed. From this path, via a "gate" connecting the two reaction channels, it is possible to reach path B and form LAC. In both cases, the rate-determining step of the process is the initial keto-enol isomerization. We found that water, which is present in the reaction mixture, "catalyzes" the reaction by assisting the proton transfers present in all the steps of the process. In particular, water lowers the barrier of the rate-determining step that becomes 40.9 kcal mol-1 (79.4 kcal mol-1 in the absence of water). The corresponding computed rate constant is 4.3×10 s-1 at 500 °C, a value which is consistent with the presence of LAC in the absence of metal catalysts. The results of this study on the non-catalyzed process underpin the important role played by water in the formation of pyrolysis products of cellulose where proton transfer is a key mechanistic step.

Full geometry optimization has been performed within the semiempirical QCFF/PI model for the 1812 fullerene structural isomers of C{sub 60} formed by 12 pentagons and 20 hexagons. All are local minima on the potential energy hypersurface. Correlations of total energy with many structural motifs yield highly scattered diagrams, but some exhibit linear trends. Penalty and merit functions can be assigned to certain motifs: inclusion of a fused pentagon pair entails an average penalty of 111 kJ mol{sup -1}; a generic hexagon triple costs 23 kJ mol{sup -1}; a triple (open or fused) comprising a pentagon between two hexagonal neighbors gives a stabilization of 19 kJ mol{sup -1}. These results can be understood in terms of the curved nature of fullerene molecules: pentagons should be isolated to avoid sharp local curvature, hexagon triples are costly because they enforce local planarity and hence imply high curvature in another part of the fullerene surface, but hexagon-pentagon-hexagon triples allow the surface to distribute steric strain by warping. The best linear fit is found for H, the second moment of the hexagon-neighbor-index signature, which fits the total energies with a standard deviation of only 53 kJ mol{sup -1} and must be minimized for stability; thismore » index too can be interpreted in terms of curvature. 26 refs., 5 figs.« less

C 76 has 19151 possible fullerene structural isomers of which up to 55 are compatible with the experimental 19-line 13 C NMR spectrum, but only two have isolated pentagons. Calculations of the relative energies of all 55 candidates within the semi-empirical QCFF/PI model pick out the isolated-pentagon D 2 isomer as the structure of lowest energy for C 76 , confirming previous calculations and providing support for the isolated-pentagon rule. Energy differences between isomers of C 76 are interpreted in the model as torsional strains induced by pentagon adjacencies.

Two epimeric series of foldamers characterized by the presence of a repeating alpha-epsilon-dipeptide unit have been prepared and characterized by 1H NMR and ECD spectroscopies together with X-ray diffraction. The first series contains L-Ala and D-4-carboxy-5-methyl-oxazolidin-2-one (D-Oxd). The other series contains L-Ala and L-Oxd. The L,D series of oligomers forms ordered beta-turn foldamers, characterized by a 311 pattern. The L,L series is not ordered. Simulations show that an ordered L,L trimer lies more than 2 kcal/mol higher than the more stable non folded extended conformations. Cu2+ forms complexes with both series, but is not able to order the L,L series. Analysis of the EPR spectra shows that the L,D foldamers bear two types of complexation sites that are assigned as a nitrogen donor of the triazole ring and a carboxylate ligand. The L-Ala-D-Oxd-Tri-CO motif may be introduced in any peptide sequence requiring the presence of a stable β-turn conformations, like in the study of protein-protein interactions.

Fumaric and maleic amides are the photoactive units of an important and widely investigated class of photocontrollable rotaxanes as they trigger ring shuttling via a cis−trans photoisomerization. Here, ultrafast decay and photoinduced isomerization in isolated fumaramide and solvated nitrogen-substituted fumaramides (that are employed as threads in those rotaxanes) have been investigated by means of CASPT2//CASSCF computational and time-resolved spectroscopic techniques, respectively. A complex multistate network of competitive deactivation channels, involving both internal conversion and intersystem crossing (ISC) processes, has been detected and characterized that accounts for the picosecond decay and photochemical/photophysical properties observed in the singlet as well as triplet (photosensitized) photochemistry of fumaramides threads. Interestingly, singlet photochemistry appears to follow a non-Kasha rule model, where nonequilibrium dynamical factors control the outcome of the photochemical process: accessible high energy portions of extended crossing seams turn out to drive the deactivation process and ground-state recovery. Concurrently, extended singlet/triplet degenerate regions of twisted molecular structures with significant spin−orbit-coupling values account for ultrafast (picosecond time scale) ISC processes that lead to higher photoisomerization efficiencies. This model discloses the principles behind the intrinsic photochemical reactivity of fumaramide and its control.

Classical molecular dynamics (MD) simulations combined with docking calculations, potential of mean force estimates with the umbrella sampling method, and molecular mechanic/Poisson-Boltzmann surface area (MM-PBSA) energy calculations reveal that C-60 may block K+ channels with two mechanisms: a low affinity blockage from the extracellular side, and an open-channel block from the intracellular side. The presence of a low affinity binding-site at the extracellular entrance of the channel is in agreement with the experimental results showing a fast and reversible block without use-dependence, from the extracellular compartment. Our simulation protocol suggests the existence of another binding site for C-60 located in the channel cavity at the intracellular entrance of the selectivity filter. The escape barrier from this binding site is similar to 21 kcal/mol making the corresponding kinetic rate of the order of minutes. The analysis of the change in solvent accessible surface area upon C-60 binding shows that binding at this site is governed purely by shape complementarity, and that the molecular determinants of binding are conserved in the entire family of K+ channels. The presence of this high-affinity binding site conserved among different K+ channels may have serious implications for the toxicity of carbon nanomaterials.

We present a novel method to calculate energies of nanotube caps with different levels of accuracy and a comprehensive study of its application to the IPR caps of the (10,0) carbon nanotube. The two most stable caps for (10,0) have 42 atoms, an energy of 8.7 eV, and correspond to sections of the third most abundant fullerene, C84. These caps are isoenergetic with a chemically unstable cap with 40 carbon atoms related to a C80 isomer that is also chemically unstable. Energies for the other caps are between 9.3 and 10 eV. A method to calculate cap energetics with fullerenes with an error less than 3% is also presented.

Molecules that are virtually insoluble in certain solvents may be uploaded to "hostile" phases by dendrimers. Prime examples of this phenomenon are Eosin Y, EY, and Rose Bengal, RB, that are not soluble in CH2Cl2 where they can, however, be solvated through the interaction with a fourth generation dendrimer of polypropylene amine, POPAM-4D. The two dyes share the same carbon framework and differ for the pattern of halogenation, and yet their cosolvation varies over a factor of 4: six Eosin Y and similar to 25 Rose Bengals are solvated by the macromolecule. Leveraging on a previous report where molecular dynamics simulations of 12 EY@ POPAM-4D in CH2Cl2 showed a reduction to the experimental limit of 6, we now perform similar calculations with an excess, i.e., 40, of RB@POPAM-4D. The simulations quantitatively reproduce the cosolvation effect. They also provide a microscopic understanding of its origin and of motions-interactions of the macromolecule and both of its guests.

The electrocyclic valence isomerization interconverting bismethano[60]fullerenes 1 and 2 with 1,2,3,6-(type I) and 1,2,3,4-addition patterns (type II) into the corresponding cluster-opened bismethano-homo[60]fullerenes was investigated computationally with respect to the energy profile and to the retention of molecular orbital symmetry. Whereas for 1 the closed form is unstable and spontaneously isomerizes to the cluster-open adduct, the energy difference between the open and closed isomers of 2 is comparatively small. The B3LYP/6-31G* energy of open 2 is only 1.2 kcal mol - 1 higher than that of the closed form. Analysis of the frontier orbital symmetries of 1, 2, and the [5,6]-bridged methano-homo[60]fullerene 3 showed that during these electrocylic valence isomerizations the orbital symmetry is retained and, as a consequence, a Woodward-Hoffmann-type selection principle is valid.

Structures and energetics of several types of all-carbon peapods made by C60s encapsulated inside single-wall carbon nanotubes are calculated. The interactions between the tubes and the molecules determine a minimal diameter for exothermic encapsulation of 11.74 A and a radial deformation of the C60 cage that is a linear function of the tube diameter. Many relative orientations of the “peas” have similar energies and can be thermally equilibrated, although preferential ways of interaction between fullerenes do exist. In particular, for a triplet of embedded fullerenes, the lowest energy arrangement is characterized by a facing describable as pentagon-hexagon-pentagon. The activation energy for the translation of isolated C60 molecules inside the tubes is very low and an isolated C60 can diffuse freely inside the pod: The major factor controlling the dynamics of molecular transport in the peapods is therefore the C60−C60 interaction.

Mapping of the potential energy surface of the torsions of the phenyl groups of C70Ph8 and C70Ph10 by molecular mechanics reveals the existence of two conformers for each molecule. Energetically, both pairs are quasi- degenerate and therefore they could be experimentally detectable despite the low activation barriers for the inter-conversion (2.4 kcal mol-1 for C70Ph8 and 3.9 kcal mol-1 for C70Ph10, respectively). In an effort to pin down the existence of the conformers, the laser induced luminescence spectra of C70Ph8 and C70Ph10 were recorded in several solid solutions. The spectral features for C70Ph8 are rather independent of the nature of the environment and closely resemble those observed before in liquid solution with two main bands separated by more than 2000 cm-1. For C70Ph10, two new low-energy bands appear which were previously undetected in the liquid phase studies and which depend on the matrix. It is proposed that the doublet bands are fingerprints of the presence of the conformers.

Catenanes can undergo rotation of one ring through the cavity of the other. Since macroscopic and molecular properties must clearly vary with the relative positions and orientations of the interlocked components, a complete understanding of the way that the rings rotate is of considerable importance. Here we show that low-dimensional quantum-mechanical modeling can yield rate constants and barriers similar to those obtained by temperature-dependent nuclear magnetic resonance experiments. Data from both non-hydrogen bond disrupting (e.g. CDCl3) and hydrogen bond disrupting (e. g. [D6]DMSO) solvents are well reproduced demonstrating the validity of the model. The successful simulation of the rates of circumrotations by entirely harmonic transition state theory originates from the description of the anharmonic levels of the systems through an effective harmonic frequency, not very different from twice the zero point energy. The nature of the model makes it extendable, in principle, to the calculation of properties dependent upon circumrotational activity.

Several arylfullerenes, C70Ph2n (n = 1–5), are characterised. Their electrochemical properties, investigated by cyclic voltammetry, are the result of a competition between the electron-withdrawing effect exerted by the phenyl groups and the destabilization of redox orbitals deriving from the saturation of double bonds. C70Ph2 is easier to reduce than C70, a feature possibly shared with C70Ph4 that makes these derivatives of C70 amongst the best electron acceptor molecules so far discovered. In the following derivatives, destabilization prevails, and reductions become increasingly more cathodic along the series. NMR spectroscopy is used to investigate C70Ph2n (n = 1–3). Finally, calculation of the LUMO energies supports the earlier reduction of C70Ph2 and calculations also indicate the layering of the injected charges, in the multiply-reduced species, over concentric shells. p

Gaseous carbon begins to aggregate in the form of linear chains and rings. While chains have consistently been observed in a variety of conditions that led to their spectroscopic characterization, rings remain elusive, a fact that seemingly conflicts with their well-known higher stability (from 10 atoms onwards). The increase in temperature favors thermodynamically the chain population. Calculations, for C8, C10, and C12, of the temperature-dependent rate constants for chain−ring and ring−chain interconversion from ab initio B3LYP/6-311G* data shows that from ∼1000 K, that is, at temperatures just below the lower end of those used in the clusters production, the conversion is rapid in the experimental time-scale both in the singlet and the triplet multiplicities.

A Sum over Molecular Orbitals scheme with an ab initio 6-31G basis set is used to calculate the static second hyperpolarisabilities, γ , of the eight linear polyynes ranging from C 20 H 2 to C 160 H 2 . The values nearly parallel, with a negative offset, those obtained for the same number of carbons, N , for polyenes. In analogy with H(CH) 2 n H, γ / N vs. N saturates for HC 2 n H at N ∼50. Further comparison with the fullerenes is presented.

Structures and energies are calculated at the DFTB level for C 36 -based fullerenes, hydrides, oligomers and solids. The two fullerenes with minimal pentagon adjacencies are isoenergetic. The isomer implicated in recent experiments has C 6 v broken symmetry, a small HOMO–LUMO gap and can gain or lose up to six electrons. C 36 forms stronger inter-cage bonds than larger fullerenes. A favoured σ -bonding pattern rationalises a dimer with ten times the stabilisation of (C 60 ) 2 , a linear polymer, a ‘superbenzene’ oligomer, a ‘supergraphite’ layer and a hexagonal close-packed solid with a monomer stabilisation of 522 kJ mol −1 and a d -spacing (6.82 A) compatible with the experiments.

The inelastic neutron scattering (INS) spectrum of the original benzylic amide [2]catenane is recorded and simulated by a semiempirical quantum chemical procedure coupled with the most comprehensive approach available to date, the CLIMAX program. The successful simulation of the spectrum indicates that the modified neglect of differential overlap (MNDO) model can reproduce the intramolecular vibrations of a molecular system as large as a catenane (136 atoms). Because of the computational costs involved and some numerical instabilities, a less expensive approach is attempted which involves the molecular mechanics-based calculation of the INS response in terms of the most basic formulation for the scattering activity. The encouraging results obtained validate the less computationally intensive procedure and allow its extension to the calculation of the INS spectrum for a second, theoretical, co-conformer, which, although structurally and energetically reasonable, is not, in fact, found in the solid state. The second structure was produced by a Monte Carlo simulated annealing method run in the conformational space (a procedure that would have been prohibitively expensive at the semiempirical level) and is characterized by a higher degree of intramolecular hydrogen bonding than the x-ray structure. The two alternative structures yield different simulated spectra, only one of which, the authentic one, is compatible with the experimental data. Comparison of the two simulated and experimental spectra affords the identification of an inelastic neutron scattering spectral signature of the correct hydrogen bonding motif in the region slightly above 700 cm−1. The study illustrates that combinations of simulated INS data and experimental results can be successfully used to discriminate between different proposed structures or possible hydrogen bonding motifs in large functional molecular systems.

A sum over molecular orbitals scheme with a 6-31G basis set is tested to calculate the static second hyperpolarisabilities γ of polyenic chains with N double bonds (5⩽ N ⩽80). On scaling ab initio values by a factor that `substitutes' the HOMO–LUMO gap with the optical gap, the calculated γ s are in good agreement with experimental results reported on substituted polyenic chains up to roughly 70 double bonds. This agreement is taken to indicate that the present simplified procedure can behave as an effective hamiltonian which may find further application. Alternatively, one can postulate that the nature of the response in polyenic chains can be reduced to a function of the molecular orbitals only.

The lowest electronically excited states of the simplest benzylic amide [2]catenane (1) have been studied by a combination of experimental and theoretical techniques. Experimentally, steady-state and transient absorption spectra, emission spectra, and the optoacoustic response in solution have been recorded for the catenane and a model compound (N, N'-dibenzylisophthalic diamide (2)), as well as the electron energy loss spectrum of the catenane deposited on the Au(111) surface. The theoretical picture builds on the results of semiempirical quantum chemical calculations and shows that partial delocalisation of the electronic states over the many chromophores of the catenanes can occur.

Free-jet absorption millimeter-wave spectroscopy and gradient-corrected density-functional calculations are combined to study trans-1-(2-pyridyl)-2-(4-pyridyl)-ethylene, 2,4-DPE, a stilbenelike molecule. The interplay between experiments and computational theory backed by the large amount of data and modeling already available for stilbene allows us to establish the parameters of the two-dimensional potential of the pyridyl torsions which well reproduce the experimental defect of inertia (−1.295 uA2). The large difference between the defects of inertia of 2,4-DPE and stilbene (≈−15 uA2) and further computer simulations suggest that the latter may warrant a reinvestigation. The effect of introducing a weakly binding N⋯H interaction in the stilbene frame is discussed and a very simple model based on the ab initio calculations is found to rationalize the energetics and structural differences of the two conformers of 2,4-DPE.

The energetic costs of widening the classical fullerene definition to include carbon cages with octagonal as well as pentagonal and hexagonal faces are investigated theoretically. Relative energies of all 16 C40 and 620 C48 cages that can be assembled with one face octagonal, 14 pentagonal and all others hexagonal are calculated within two independent semi-empirical models and compared with the 295 C40 and 2664 C48 one-square, one-heptagon and classical-fullerene cages. All isomers are lo-cal minima on the potential surface, and many non-classical structures fall within the energy range spanned by the classical fullerenes. Penalties for introduction of a single non-classical face increase in the order heptagon < square < octagon, estimated for C48 as 58–123, 108–236, and 329–450 kJ mo−1, respectively, depending on model. The energy variation across the range of classical and non-classical structures is rationalised by extension of the isolated-pentagon rule: when 1

The structures of C119 and C129, the two lowest odd-numbered fullerenes, are investigated by semiempirical quantum chemical calculations. In the process, the use of a recently proposed parametrization for a tight-binding Extended-Huckel model is validated to study the relative stability of fullerenes. Three general routes for the generation of the structures of odd-numbered fullerenes are described and used to generate systems that can be loosely characterized as having either a four-membered ring or a patch of adjacent pentagons, or a set of multiple connections between the cages of the pristine fullerenes. The structures of the six isomers of C119 obtained in this manner are optimized and it is confirmed that the most stable isomer of C119 has C2 symmetry and forms three bonds between the C59 and the C60 moieties. By analogy with C119, 56 structures of C129 are generated and their geometries are optimized. As in the case of C119, it is found that the most stable isomer of C129 can be formed both from th...

The lowest energy vibro-electronic transition, S 0 -T 1 , of [1.1.1]propellane is studied by a combination of ab initio molecular orbital and density functional theory methods together with Franck-Condon model calculations. To avoid a possible bias in the simulation of the vibrational envelope, the accuracy of the ground state parameters is assessed through a comparison with experimental results. At odds with the behaviour of unsaturated carbon systems, where electron excitation usually promotes bond elongation and/or torsion, the T 1 state of [1.1.1]propellane undergoes cleavage of the central bond. The possible implications of this behaviour are discussed.

Time-resolved resonance Raman spectra are reported for the lowest excited triplet state of stilbene and three of its isotopomers. The spectra were obtained using a two-laser pump-and-probe arrangement under various experimental conditions. The spectrum of trans-stilbene after direct excitation in a glassy medium at low temperature (glycerol at 203 K) is compared with that of cis-stilbene under sensitized excitation in solution at room and low temperature. The dependence of resonance Raman spectra on excitation wavelength in both cases is investigated. The observed spectra and isotopic shifts are discussed and interpreted on the basis of quantum chemical molecular orbital calculations. Optimized geometries and vibrational frequencies in the T1 state are calculated by a semiempirical QCFF/PI Hamiltonian and by means of ROHF ab initio methods using the 6-31G basis set. TI - T, transition energies and moments are calculated using QCFF/PI and CNDO/S methods, and the triplet-triplet transition responsible for the observed T1 - T, absorption and resonance Raman spectra is identified as theT1 -Tlo transition. Corresponding resonance Raman intensities arecalculated by QCFF/PI. It is concluded that trans- and cis-stilbene adopt a common equilibrium geometry in the TI state, with the ethylenic C=C bond of the ground state being weakened to a bond with essentially single-bond character in the T1 state. The observed spectra in the glass are assigned to a planar geometry, implying that a relative minimum is found at a planar trans geometry (C2h point group) on the potential energy surface of the T1 state. A number of observed non-totally symmetric vibrational modes are tentatively assigned to combinations of either a,, or b, modes, deriving their intensities from relatively large frequency changes upon T1 - Tlo excitation.

The structures of the S 0 and T 1 states of E- and Z-hexatriene are optimized at several levels of ab initio theory. The corresponding force fields and harmonic frequencies are calculated for a number of isotopomers in both states. Excellent agreement between theory and experiment is obtained by applying the scaling parameters for S 0 to the T 1 force field. The time-resolved resonance Raman data available for the triplet state of E- and Z-hexatriene are reviewed in the light of the present ab initio results and the spectra are found to be due only to the E isotopomers.

The geometry of the ground and excited singlet and triplet 1B2 electronic states of cyclopentadiene is optimized by ab initio (configuration interaction singles) and semiempirical (quantum chemical force field) methods. The optimized structures provide displacement parameters of the totally symmetric modes for the singlet→singlet and singlet→triplet transitions. The calculated singlet→singlet absorption spectrum for cyclopentadiene and its fully deuterated isotopomer agree very well with the observed spectra. It is demonstrated that the difference of these spectra from those of a linear polyene (butadiene) results from the strong Franck–Condon activity of the lowest frequency a1 mode which describes CCC bending. It is shown that the properties of the triplet and singlet 1B2 states differ greatly as a result of their different nature; the former being mostly covalent, whereas the latter is mostly ionic. This leads to different Franck–Condon structures of the 1 1A1→1 1B2 and 1 1A1→1 3B2 transitions. The ion...

Controlling a molecular brake: Binding interactions between the thread and the macrocycle of a [2]rotaxane can be tuned in a quasi-continuous manner by adding hydrogen-bond-forming solvent molecules one at a time to an isolated [2]rotaxane molecule. Conformational changes that detach the thread from the macrocycle can be induced controllably, and the system resembles a molecular version of applying and releasing a brake.

The vibronic intensity borrowing activity of the lowest electronically excited singlet states of C60 has been obtained through quantum chemical calculations. The vibrational structure of the UV–visible spectra is found to be dominated by false origins. The calculated intensities of the false origins of the T1g state agree with the vibrational structure observed in the fluorescence spectrum. The same false origins are recognized to be responsible for the vibrational structure of the red edge portion of the absorption spectrum. Only two bands in the spectra are assigned as combination bands involving an ag or a Jahn–Teller active mode. Absorption bands that may be associated with false origins of the states T2g and Gg which are quasidegenerate with S1 are tentatively assigned.

The structure, the vibrational frequencies and the normal modes of C6−60 are calculated with an upgraded QCFF/PI semi-empirical quantum chemical method. We use a configuration interaction wavefunction expanded on 400 determinants (all the one- to six-electron excitations in the space spanned by the triply degenerate HOMO and the triply degenerate LUMO of C6−60 are included). The optimized structure of the isolated hexaanion belongs to the Ih point group. The two inequivalent bond lengths of the C60 frame are found to be similar. The frequency lowering observed upon doping is quantitatively ascribed to two mechanisms: the first is electronic in nature and is due to the bond order decrease caused by the addition of the extra six electrons to the antibonding orbitals of the neutral molecule; the second is of vibronic origin and is promoted by the mixing of electronic states caused by the vibrational motions. The normal modes of the hexaanion are projected onto the space of those of the neutral compound and the mixing caused by the additional charges is found to be negligible. The calculated frequencies are discussed in the light of recent infrared and Raman data of A6C60 (with A an alkali atom) and an inversion of the assignment of the 1ag (highest) and 2hg (second highest) frequencies is tentatively proposed. Analysis of the force field in internal coordinates finds that both the electronic and the vibronic mechanisms affect almost only the diagonal and the nearest neighbour stretch force constants.

The in-plane force field and the vibrational frequencies of benzene and octatetraene are calculated by DGauss, the local density functional theory (LDFT) program available from Cray Research. Default procedures are adopted for the energy functional, basis set (DZVP), integral precision and convergence requirements. The calculated vibrational frequencies are in much better agreement with the experiment than the raw ab initio data obtained at Hartree-Fock self consistent field (HF SCF) level with comparable basis sets. Comparison of the force constants obtained by LDFT and HF SCF theory shows that these two levels of theory are complementary and vibrational spectroscopy should be benefit from their combined use.

The authors report Pople-Pariser-Parr (PPP) calculations of the energies of the lowest electronically excited states of six C{sub 60} anions ranging from the monoanion to the hexaanion. Multiplicities up to septet are included. The results for the monoanion are compared with the existing absorption spectrum. The stability of various anions and the possibility of obtaining them by electrochemical reduction are discussed. In most of the species studied, the authors find a remarkable density of electronic levels close to the electronic ground state. This density is due to the existence, in neutral C{sub 60} of a triply degenerate lowest unoccupied molecular orbital (LUMO), together with a close-lying LUMO+1 of the same degeneracy. 26 refs., 2 figs., 6 tabs.

We report QCFF/PI quantum-chemical calculations of the molecular orbital energies, the structure, and the vibrational frequencies of a C84 isomer of D2 symmetry. Comparison with the results obtained by the same method for two C84 clusters of D6h and Td symmetry is given. The relative stability of the three isomers is D6h > Td > D2. The origin of this stability order is discussed. It is also found that the Raman and the infrared spectra of the three isomers should be rather different so that these spectra could be used to assign the molecular symmetry of this recently isolated novel form of carbon.

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Franck—Condon activity of the totally symmetric modes of cyclopentadiene and cyclopentadiene- d 6 in the 1A 1 → 2A 1 and 1A 1 → 1B 2 transitions is investigated with the help of the QCFF/PI method. It is shown that a symmetric carbon ring bending is responsible for the difference between the vibronic structures of the spectra of cyclopentadiene and butadiene. It is suggested that this difference is a common feature in spectra of cyclic polyenes. Frequencies of non-totally symmetric modes in the ground and two lowest excited states of cyclopentadiene and its fully deuterated isotopomer are calculated and discussed.

The T 1 and T 2 geometrical structures of planar s-trans and s-cis butadiene are optimized, and the four corresponding force fields are calculated at the multi-configuration self-consistent field level of theory using the 4-31G basis set (MCSCF/4-31G). The results are found to be very similar for the two conformers: while both T 1 states are calculated to be planar, the T 2 force fields show an imaginary frequency which corresponds to the torsional mode about the single bond. The calculations are discussed in the light of existing electron impact energy loss spectra. Also their implications for both the photophysics and a possible Raman excitation profile of the torsional mode are examined.

Thiophene chains in the vacuum or in weakly perturbing environments adopt the energetically most stable all-trans conformation. On metal surfaces, such as gold, the interaction with their image potential may modify the picture, as is now being shown by a number of experiments. We use a model that was parametrized to accurately reproduce the energy of adsorption/desorption of a number of organic molecules on gold to investigate the stability of the 20 rotamers of sexithiophene on this metal. We find that electrostatic interactions modify the stabilities of the individual molecules. Adsorption of dimers tends to stabilize the deposition of the all-trans co-conformer. We also find that the mobility of the rotamers on the surface is not directly connected to the interaction energy.

The interaction of isolated aromatic nitrogen atoms with water is explored within free jets by using rotational spectroscopy. To the existing data on diazines, we add the case of the 1:1 complex of 1,3,5-triazine and water (where water donates a proton to one of the nitrogen heterocyclic atoms to form a planar adduct). An electrostatic model based on distributed multipoles accurately reproduces the structures of the four azine-water complexes and allows us to understand the forces that stabilize these structures. The applied intermolecular potential allows us to estimate the changes in the thermodynamic functions of the complexes-compared to the separated constituents-and evaluate the temperature at which the complexes are stable under standard conditions.

Geometry of the S0 and S1 states of s‐trans butadiene is optimized and the corresponding force fields calculated using a multiconfiguration self‐consistent field (MCSCF) method in the complete active space formulation. Three stable conformations of the S1(2Ag) state are shown to be nonplanar (S2, C2, and C1 symmetry), quasidegenerate and separated by a barrier along torsional coordinates of ca. 1600 cm−1. The planar C2h conformation forms a transition state between the S2 and C2 configurations. It is shown by model calculations that the out‐of‐plane deformation of the S1 state increases dramatically its rate of internal conversion to the ground state. Thus, although the energy gap for the transition is larger in butadiene than that in octatetraene, the internal conversion S1uS0 is estimated to be 3–5 orders of magnitude faster in butadiene. Fast internal conversion in this molecule explains its lack of fluorescence.

The QCFF/PI+CI method and the CNDO/S+CI method in the floating atomic orbital basis set are used to study vibronic structure of the 1 2B2↔1 2A2, 2 2B2 transition in benzyl‐h7 and benzyl‐d7 radicals, arising from the activity of a1 and b1 in‐plane modes. It is shown that the 6a10 line has its intensity almost entirely due to the borrowing from the 1 2A2→1 2A2 transition via the 1 2A2–1 2A2 coupling. We demonstrate tha the 1 2A2 and 2 2B2 states are strongly mixed by ν8b and ν6b modes. The unperturbed 1 2A2 state lies ∼850 cm−1 below the unperturbed 2 2B2 state. The coupling depresses strongly the position of the 6b10 level mixing it markedly with the level arising from C–CH2 rock (ν18b in other assignments) fundamental. Three‐mode vibronic analysis of the absorption and emission spectrum produces vibronic structure that agrees well with the observed spectra. The adiabatic frequency of the ν14 (b1) mode in the 1 2B2 state is shown to be depressed markedly by vibronically induced mode mixing with ν8b mode du...

Dynamic EPR spectroscopy has been used to investigate the inversion of 1,3-dioxolan-2-yl and [2-D]-1,3-dioxolan-2-yl radicals. The former shows no detectable line broadening down to 93 K, which implies "instant" inversion on the EPR time scale, but the latter shows a temperature-dependent EPR spectrum from which the inversion rate constants have been deduced. These rate constants give rise to a curved Arrhenius plot which, together with the large primary isotope effect, indicates that the inversion proceeds by quantum-mechanical tunneling. To investigate the transition quantitatively, an ab initio UHF-MO calculation has been carried out, yielding the molecular geometry and the normal-mode frequencies in the equilibrium and transition state, as well as an inversion barrier height of ca. 2500 cm -1. The inversion is found to be dominated by two vibrational degrees of freedom, namely, a ring-bending mode in addition to the CH α-bending mode. The potential of the latter mode is approximated by a modified quartic double-minimum potential. The former mode is treated semiclassically: it modifies the quartic potential via an anharmonic cross-term yielding a series of quartic potentials, one for each state of the ring-bending mode, weighted by the Boltzmann population of the state. The observed rate constants can be fitted accurately to the CH α-level splittings generated by this model. The resulting effective one-dimensional inversion barrier compares favorably with the corresponding barrier deduced quantum-chemically.

We use molecular dynamics to calculate the vibrational density of states of several endohedral mono-metallofullerenes and compare the results with the experimental values. The vibrational patterns depend weakly on the charge transfer from the metal to the fullerene cage, with the largest variations of approximately 2 cm(-1) observed upon switching off completely the Coulomb interactions, and more strongly on the cage isomer, with variations of up to approximately 13 cm(-1). Analysis of the metal motions inside the cage shows that they can be chaotic. The origin of such chaotic behavior is discussed.

Adsorption of organics on a silver surface is simulated. An Embedded Atom Model is used for the metal, a standard force field for the organics, and a combination of the charge equilibration model and the Morse potential for their electrostatic and nonbonding interactions. The only adjustable parameters of this approach appear in the Morse potential. They are tuned to reproduce experimental and high level quantum chemical data. The adsorption energies of 13 molecules on the Ag(111) surface are obtained with an average error of less than 1 kcal mol(-1). The model should be transferable to molecules with the same chemical groups used in regressing the potential parameters when physisorption or weak chemisorption, i.e., no bond breaking, occur, and also to other Ag surfaces. When used to simulate perylene tetracarboxylic acid dianhydride (PTCDA) on Ag(111), correct geometry of mono- and multilayers are observed in molecular dynamics simulations at room temperature.

Here we report theoretical studies on the ruthenium-catalyzed reduction of acetophenone (and 2-hexanone) with the intent of understanding the relative roles of catalyst and substrate along the reaction path. Overall ten reaction pathways are examined. The first eight are for acetophenone: they arise from the presence of two catalysts, with the more enantioselective one labeled 1, and the poorer one labeled 2, multiplied by the two configurations that the metal center of the catalysts can assume, multiplied by the two approaches, Re- and Si-side, of the substrate to the catalyst. Two pathways are examined for 2-hexanone and entail the two approaches to the ketone of the more effective catalyst. Density functional theory calculations provide structures of the minima and transition states, which subsequently have been assessed with the "continuous chirality measure" model developed by Avnir and collaborators. The picture that emerges is that the asymmetric induction is due to the interplay between the organometallic system and the organic substrate; This is effective only for catalyst 1, which can interact effectively with acetophenone along only one in four of the reaction pathways, but not for 2 for which two out of four pathways are opened. For the hydrogenation of 2-hexanone, the same catalyst 1 cannot produce enantiomeric excesses because the conformation of the substrate in the transition state induced by the catalyst has a relative low chirality.

Free-energy perturbation calculations are used to evaluate the free energy of cavity formation in n-octanol. A detailed theoretical analysis of the procedure is given and some limiting value phenomena are discussed. The data become subject to a three-parameter fit and a revised formulation of the popular approach due to Pierotti of calculating cavitation free energies is given. Pierotti’s approach is based on the equation derived from scaled particle theory (SPT) by Reiss et al. [(2000) J. Chem. Phys. 31:369–380]. The revision of Pierotti’s approach has the important advantage of being completely independent of the solvent hard-sphere radius, an empirical parameter in the standard procedure, which is hard to define in a uniformly valid way.

Control over the interchromophore separation, their angular relationship, and the spatial overlap of their electronic clouds in several ZnP-C(60) dyads (ZnP=zinc porphyrin) is used to modulate the rates of intramolecular electron transfer. For the first time, a detailed analysis of the charge transfer absorption and emission spectra, time-dependent spectroscopic measurements, and molecular dynamics simulations prove quantitatively that the same two moieties can produce widely different electron-transfer regimes. This investigation also shows that the combination of ZnP and C(60) consistently produces charge recombination in the inverted Marcus region, with reorganization energies that are remarkably low, regardless of the solvent polarity. The time constants of electron transfer range from the mus to the ps regime, the electronic couplings from a few tens to several hundreds of cm(-1), and the reorganization energies remain below 0.54 eV and can be as low as 0.16 eV.