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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.

The inelastic neutron scattering (INS), spectra of the isophthaloyl [2]catenane and the sebacoyl [2]catenane are measured and compared with simulated spectra obtained by molecular dynamics (MD), based on a molecular mechanics model (MM), that has been extensively used before to treat this class of molecules. Both the isolated molecule approximation (IMA), and the crystal approach, implemented through boundary conditions, are used. Good agreement with the experiments is met for the spectra of the two catenanes for the location of the most prominent bands. The crystal calculations give better intensity distributions than the IMA. Further analysis of the simulations shows that the combination of INS spectroscopy and MD is well suited to assess the strength of intramolecular and intermolecular hydrogen bonds that are the crucial interactions that govern the dynamics of these systems.

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.

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.

Fluorescence excitation spectra covering the origin–band region of the two partially methyl-deuterated toluenes (α-deuterotoluene, C 6 H 5 CH 2 D and α-dideuterotoluene, C 6 H 5 CHD 2 ) are reported. The spectra, observed in a cold supersonic beam, show transitions between low methyl–rotor levels, and are analyzed in terms of vibrationally adiabatic torsional potentials with two-, four-, and sixfold periodicity. To interpret the resulting potential parameters, ab initio vibrational force field calculations are carried out at the four stationary structures of each isotopomer. The calculations yield accurate values for the adiabatic torsional barriers and qualitatively correct results for the vibrationally adiabatic potentials. Specifically, they show that the twofold potential term dominates in the partially methyl-deuterated isotopomers and that this component has the same magnitude but opposite sign in the mono- and dideuterocompounds. The potential parameters are interpreted in terms of nonbonded interactions between a CH bond of the methyl group and the phenyl ring; specifically, they represent the difference between the interactions of a methyl-CH and a methyl-CD bond with the ortho-CH bonds of the phenyl ring. Modeling of this interaction as a function of H–H separation in terms of a simple analytical expression shows that the repulsive potential is very soft and that individual H–H interactions are of the same order of magnitude as vibrational frequencies.

To describe tunneling of light atoms, a method is developed that takes account of the multidimensional nature of the process but remains tractable without becoming inaccurate. It combines the instanton formalism with ab initio potentials and force fields and makes effective use of a number of practical approximations suggested by the nature of the calculations. The tunneling potential is constructed from ab initio calculations that are fully optimized at stationary points. All other vibrations are represented by their harmonic force fields. Changes in the harmonic force fields between stationary points are expressed as couplings with the tunneling mode. The transfer rate is calculated for the instanton path, i.e., the path of least resistance, modulated by adjacent paths which define the damping required for nonoscillatory transfer. The multidimensional transfer integrals, involving all modes that change between the initial state and the transition state, are reduced to quasi‐one‐dimensional integrals by ...

Time‐resolved resonance Raman (RR) spectra of the benzyl radical and its perdeutero isotopomer are presented. The radicals are created by laser flash photolysis (λ=266 nm) of benzylchloride in solution. The spectra are excited at a wavelength of 315 nm, in resonance with the intense D3(2 2A2)←D0(1 2B2) transition. Twenty Raman bands, both polarized and depolarized, are observed. The RR spectra are analyzed through quantum chemical force field for π electrons (QCFF/PI) and ab initio calculations of equilibrium geometries, vibrational frequencies, Franck–Condon factors, and vibronic interactions. Polarized intense bands are assigned to totally symmetric a1 modes, and a number of depolarized bands to fundamentals of b1 modes. The observed activity of b1 modes suggests vibronic coupling, which is confirmed theoretically by calculations of vibronic interactions between the D3(2 2A2) and D5(4 2B2) states. The results from semiempirical and ab initio calculations are compared with experiment. The contributions o...

The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6‐31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is lar...

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...

The vibronic structure of the lowest lying intense polyenic electronic transition of cis‐ and trans‐hexatriene is investigated with the recently introduced ab initio configuration interaction singles (CIS) procedure using the 6‐31+G basis set. First, the molecular structures are optimized and the vibrational frequencies are calculated at the stationary points along the central CC torsional curve. Selected points along such a curve are also optimized to furnish a better understanding of the isomerization process in the 1B state. Then, the displacement parameters are obtained and used to simulate the Franck–Condon activity in the 1A–1B transitions of the two isomers of hexatriene. A simple scaling procedure of the displacement parameters is proposed to improve the agreement between theory and experiment. The model calculations of the Franck–Condon structure of both the harmonic and the anharmonic degrees of freedom simulate very well the absorption spectra of both isomers and explain the origin of the diffe...

We report an ab initio study of the geometry and force field of the S0, S1, and S2 electronic states of norbornadiene at the Hartree–Fock/6–31G* and configuration interaction singles/6–31+G levels of theory. The calculated Franck–Condon structures of the S0→S1 and S0→S2 transitions agree well with the structures observed in the optical absorption and energy loss spectra. It is argued that the 47 000 cm−1 absorption band which undergoes changes in the vibronic structure upon going from the gas to the solid phase owes its different activity to the freezing of the angle between the two ‘‘wings’’ of the cyclohexadiene part of the molecule. The complete neglect of differential overlap/spectroscopic parametrization method is used to locate valence states of the title molecule and to calculate vibronic coupling parameters to predict vibronic activity of the nontotally symmetric modes in the spectra. It is demonstrated that the highest frequency C=C stretching mode of the b2 symmetry is the most effective in brin...

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 absorption and single vibronic level fluorescence spectra of jet cooled 9,10‐dihydrophenanthrene possesses long progressions of a low‐frequency vibration identified as the out‐of‐plane nontotally symmetric CH2–CH2 bridge deformation. The intensity distribution in the observed progressions indicate that the molecule is nonplanar in the ground and the first two excited singlet electronic states described as the plus and minus combinations of the Lb states of the two perturbed benzene rings. The barrier height between the two equivalent nonplanar configurations, with the planar configuration being the transition state, is 2640 cm−1 in S0 and 1430 cm−1 in S1. These values are much larger than those in 9,10‐dihydroanthracene. The observed intensity distributions are reproduced well by model calculations with double minimum potentials whose parameters are in good agreement with those obtained by geometry optimization and force field and vibrational frequency calculations by the quantum chemical force field ...

Raman spectra of benzene and benzene‐d6 vapor obtained with excitation in the range from 208 to 188 nm are presented and discussed in terms of vibronic coupling of the resonant B1u state and the nearby E1u electronic state. The Raman spectra show strong activity in the binary overtones and combinations of the e2g modes ν8, ν9, and ν6 with the ν8 activity being dominant. (The Wilson numbering scheme for the modes of vibration is used throughout.) These bands, plus a progression in ν1 alone or in combination transitions involving two quanta of e2g modes, constitute the major intensity bands in the spectra. A simple first‐order vibronic coupling model can be constructed which accounts adequately for most of the intensity in the Raman spectra observed in resonance with the B1u state, but this model results in a calculated absorption spectrum that is qualitatively different from that observed. The nature of the discrepancy indicates a significant Duschinsky mode rotation in the B1u state relative to the ground...

The inversion potentials in the ground and the lowest excited singlet states of 9,10‐dihydroanthracene were deduced from the fit of the absorption and fluorescence spectra of jet‐cooled molecules to model potentials. The barrier height in the ground state (615 cm−1 ) is substantially larger than in the excited state (80 cm−1 ). Correspondingly, the folding angle in the ground state is smaller (144.6° vs 164.1°). Comparison of the calculated line intensities with the experimental absorption spectrum indicates that the transition dipole moment is dependent on the nuclear coordinate of the inversion mode. The measured quantum yield of the single vibronic level fluorescence is essentially independent of the quantum number and the symmetry of the inversion level excited.

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...

The adsorption of semifluorinated alkanethiols on Au/mica was studied by scanning tunneling microscopy (STM). The adlayer structure produced is based on a p(2 x 2) structure though lines of molecules displayed extensive kinks and bends. In addition, a considerable variation in the contrast of molecular features is found. Molecular modeling calculations confirm that, for the fluorinated thiols, inequivalently adsorbed molecules within a p(2 x 2) registry are present, an aspect that endows the local structure of the adlayer with a higher flexibility in comparison to nonfluorinated thiols, where one adsorption site is strongly favored in a (radical 3 x radical 3) R30 degrees structure. Simulated STM imaging on the optimized systems successfully recovered the effects on the molecular feature contrast induced by the flexibility of the fluorinated thiol adlayer.

A theoretical analysis is reported of the ground state‐to‐triplet absorption spectrum of ethylene, observed as an electron‐loss spectrum. The analysis is based on ab initio MO calculations of the vibrational force fields in the ground state and in the two stationary configurations (planar and staggered) of the triplet state. The calculations use the gamess program with the 6‐31++G** basis set. The transition turns the double bond into a single bond and thus increases the CC distance. This affects modes ν2 and ν3 , corresponding to mixed CC stretching and CH scissoring, and leads to a long ν3 progression together with some ν2 structure. The transition also excites torsional levels (mode ν4 ), preferentially those just above and below the torsional barrier. This relatively narrow torsional spectrum is superimposed on each of the CC‐stretch peaks. The systematic change in this structure observed along the ν3 progression is due to anharmonic ν3 ν4 coupling. Because of this coupling, a meaningful assignment of...