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Extremely solvent-enhanced absorption and fluorescence spectra of carbazole were investigated by performing a generalized multi-set damped Franck–Condon spectral simulation. Experimental absorption and fluorescence spectra of carbazole in the gas phase were first well reproduced by performing an un-damped Franck–Condon simulation, but a one-set scaling damped Franck–Condon simulation severely underestimated the intensities of the peaks of experimental absorption and fluorescence spectra of carbazole in n-hexane. Then, a multi-set scaling damped Franck–Condon simulation was proposed and carried out for simulating the extremely solvent-enhanced absorbance and fluorescence, and here, the simulated spectra agreed well with the experimental ones. Five (four) representative solvent-enhanced normal modes corresponding to the combination of ring stretching and ring breathing vibrational motions were determined to be responsible for enhanced absorbance (fluorescence) in n-hexane solution. Furthermore, different scalings were applied to the ground and first-excited states, resulting in different enhancement of absorbance and fluorescence, and this analysis revealed atoms in the carbazole interacting with n-hexane solvent molecules and, hence, leading to different normal-mode vibrational vector patterns in the ground and first-excited states, respectively. Basically, the same conclusion was drawn from a simulation with HF-CIS and the three functionals (TD)B3LYP, (TD)B3LYP-35, and (TD)BHandHLYP. The present multi-set scaling damped Franck–Condon simulation scheme was demonstrated to successfully interpret extremely solvent-enhanced absorbance and fluorescence of carbazole in n-hexane-solvent. [ABSTRACT FROM AUTHOR]

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH3OH/TiO2(110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO2 substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity. In addition, the SFG spectra have obvious changes while the methanol coverage increases, especially for the OH vibration peaks. Our theoretical spectra agree reasonably well with experimental measurements at 1 ML coverage, and an interesting peak which is absent in the theoretical spectra is tentatively assigned to some CH3 stretch vibration of methanol adsorbed on the oxygen vacancy of TiO2. [ABSTRACT FROM AUTHOR]

Multi-reference configuration interaction with single and double excitation method has been utilized to calculate the potential energy surfaces of the five low-lying electronic states 1A1, 1A2, 3A2, 1B2, and 3B2 of carbon dioxide molecule. Topology of intersections among these five states has been fully analyzed and is associated with double-well potential energy structure for every electronic state. The analytical potential energy surfaces based on the reproducing kernel Hilbert space method have been utilized for illustrating topology of surface crossings. Double surface seam lines between 1A1 and 3B2 states have been found inside which the 3B2 state is always lower in potential energy than the 1A1 state, and thus it leads to an angle bias collision dynamics. Several conical/surface intersections among these five low-lying states have been found to enrich dissociation pathways, and predissociation can even prefer bent-geometry channels. Especially, the dissociation of O(3P) + CO can take place through the intersection between 3B2 and 1B2 states, and the intersection between 3A2 and 1B2 states. [ABSTRACT FROM AUTHOR]

Partially deuterated protonated water dimers, H2O·H+·D2O, H2O·D+·HDO, and HDO·H+·HDO, as important intermediates of isotopic labeled reaction of H3O+ + D2O, undergo direct dissociation and indirect dissociation, i.e., isomerization before the dissociation. With Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations, we have computed their dissociation and isomerization rate constants separately under the harmonic and anharmonic oscillator models. On the basis of the dissociation and isomerization rate constants, branching ratios of two primary products, [HD2O+]/[H2DO+], are predicted under various kinetics models with the harmonic or anharmonic approximation included. The feasible kinetics model accounting for experimental results is shown to include anharmonic effect in describing dissociation, while adopting harmonic approximation for isomerization. Thus, the anharmonic effect is found to play important roles affecting the dissociation reaction, while isomerization rates are shown to be insensitive to whether the anharmonic or harmonic oscillator model is being applied. [ABSTRACT FROM AUTHOR]

High resolved absorption and fluorescence spectra of zinc complexes of phthalocyanine (ZnPc) and tetrabenzoporphyrin (ZnTBP) in the region of Q states were reported. Few theoretical investigations were performed to simulate the well-resolved spectra and assigned the vibrational bands of the large molecules, especially for high symmetrical characteristic molecules, on account of the difficulties to optimize the excited states and analyze a large number of final vibrational-normal modes. In the present work, the S0 ↔ S1 absorption and fluorescence spectra (that is, the Q band) of ZnPc and ZnTBP were simulated using time-dependent density functional theory with the inclusions of Duschinsky and Herzberg-Teller contributions to the electronic transition dipole moments. The theoretical results provide a good description of the optical spectra and are proved to be in excellent agreement with experimental spectra in inert-gas matrices or in supersonic expansion. This study focused attentions on the optical spectral similarities and contrasts between ZnPc and ZnTBP, in particular the noticeable Duschinsky and Herzberg-Teller effects on the high-resolved absorption and fluorescence spectra were considered. Substitution of meso-tetraaza on the porphyrin macrocycle framework could affect the ground state geometry and alter the electron density distributions, the orbital energies that accessible in the Q band region of the spectrum. The results were used to help interpret both the nature of the electronic transitions in Q band region, and the spectral discrepancies between phthalocyanine and porphyrin systems. [ABSTRACT FROM AUTHOR]

The electronic spectrum of a cold molecular beam of zirconium dioxide, ZrO2, has been investigated using laser induced fluorescence (LIF) in the region from 17 000 cm-1 to 18 800 cm-1 and by mass-resolved resonance enhanced multi-photon ionization (REMPI) spectroscopy from 17 000 cm-1-21 000 cm-1. The LIF and REMPI spectra are assigned to progressions in the Ã1B2(ν1, ν2, ν3) ← X1A1(0, 0, 0) transitions. Dispersed fluorescence from 13 bands was recorded and analyzed to produce harmonic vibrational parameters for the X1A1 state of ω1 = 898(1) cm-1, ω2 = 287(2) cm-1, and ω3 = 808(3) cm-1. The observed transition frequencies of 45 bands in the LIF and REMPI spectra produce origin and harmonic vibrational parameters for the Ã1B2 state of Te = 16 307(8) cm-1, ω1 = 819(3) cm-1, ω2 = 149(3) cm-1, and ω3 = 518(4) cm-1. The spectra were modeled using a normal coordinate analysis and Franck-Condon factor predictions. The structures, harmonic vibrational frequencies, and the potential energies as a function of bending angle for the Ã1B2 and X1A1 states are predicted using time-dependent density functional theory, complete active space self-consistent field, and related first-principle calculations. A comparison with isovalent TiO2 is made. [ABSTRACT FROM AUTHOR]

By using three different hybrid exchange-correlation functionals containing 20%, 35%, and 50% of exact Hartree-Fock (HF) exchange of the density functional theory and its time-dependent extension plus the Hartree-Fock and the configuration interaction of single excitation methods, equilibrium geometries, and their 30 vibrational-normal-mode frequencies of the ground S0(1A1) and the first excited S1(1B2) states of fluorobenzene (FB) were calculated. The dispersed fluorescence spectrum and internal conversion (IC) rate of the A 1B2 → X 1A1 transition were simulated by Franck-Condon (FC) calculations within the displaced harmonic oscillator approximation plus anharmonic and distorted corrections. The simulated spectral profile is primarily described by the Franck-Condon progression from the ring-breathing modes v9 and v10 which belong to totally symmetry modes. Anharmonic corrections simultaneously improve the intensity order of 910 and 1010 bands and diminish 110 transition that is fairly strong in harmonic simulations. It is concluded that the amount of Hartree-Fock exchange does impact the geometries and vibrational frequencies of FB molecule, but not the relative intensities of the transitions. It is anharmonic corrections that make the relative intensities of the transitions in good agreement with experimental results. Distorted corrections could assign most of the dominant overtones of out-of-plane nontotally symmetry modes, and the results agree well with the experimental assignments. Furthermore, it was found that the internal conversion rate is dominated by three promoting modes that are computed with lowing symmetry to C1. By choosing dephasing width as 10 cm-1 that is consistent with spectral simulation, we obtained the lifetimes of the A 1B2 → X 1A1 de-excitation as 11 and 19 ns, respectively, from TD(B3LYP) and HF/CIS calculations in comparison with the experimental value 14.75 ns. [ABSTRACT FROM AUTHOR]

Using high-pressure infrared methods, we have investigated close interactions of charge-enhanced C-H-O type in ionic liquid/dimethyl sulfoxide (DMSO) mixtures. The solvation and association of the 1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF4-) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMM+BF4-) in DMSO-d6 were examined by analysis of C-H spectral features. Based on our concentration-dependent results, the imidazolium C-H groups are more sensitive sites for C-H-O than the alkyl C-H groups and the dominant imidazolium C-H species in dilute ionic liquid/DMSO-d6 should be assigned to the isolated (or dissociated) structures. As the dilute mixtures were compressed by high pressures, the loss in intensity of the bands attributed to the isolated structures was observed. In other words, high pressure can be used to perturb the association-dissociation equilibrium in the polar region. This result is remarkably different from what is revealed for the imidazolium C-H in the BMM+BF4-/D2O mixtures. DFT-calculations are in agreement with our experimental results indicating that C4-H-O and C5-H-O interactions seem to play non-negligible roles for BMM+BF4-/DMSO mixtures. [ABSTRACT FROM AUTHOR]

We consider a system of two coupled Brownian particles fluctuating between two states. The fluctuations are produced by both equilibrium thermal and external nonthermal noise, the transition rates depending on the interparticle distance. An externally induced modulation of the transition rates acts on the internal degree of freedom (the interparticle distance) and generates reciprocating motion along this coordinate. The system moves unidirectionally due to rectification of the internal motion by asymmetric friction fluctuations and thus operates as a dimeric motor that converts input energy into net movement. The properties of the motor are primarily determined by the properties of the reciprocating engine, represented by the interparticle distance dynamics. Two main mechanisms are recognized by which the engine operates: energetic and informational. In the physically important cases where only one of the motion-inducing mechanisms is operative, exact solutions can be found for the model with linearly coupled particles. We focus on the informational mechanism, in which thermal noise is involved as a vital component and the reciprocating velocity exhibits a rich behavior as a function of the model parameters. An efficient rectification method for the reciprocating motion is also discussed. [ABSTRACT FROM AUTHOR]

The performance of an analytical expression for algorithmic decoherence time is investigated for non–Born–Oppenheimer molecular dynamics. There are two terms in the function that represents the dependence of the decoherence time on the system parameters; one represents decoherence due to the quantum time-energy uncertainty principle and the other represents a back reaction from the decoherent force on the classical trajectory. We particularly examine the question of whether the first term should dominate. Five one-dimensional two-state model systems that represent limits of multidimensional nonadiabatic dynamics are designed for testing mixed quantum-classical methods and for comparing semiclassical calculations with exact quantum calculations. Simulations are carried out with the semiclassical Ehrenfest method (SE), Tully’s fewest switch version (TFS) of the trajectory surface hopping method, and the decay-of-mixing method with natural switching, coherent switching (CSDM), and coherent switching with reinitiation (CSDM-D). The CSDM method is demonstrated to be the most accurate method, and it has several desirable features: (i) It behaves like the representation-independent SE method in the strong nonadiabatic coupling regions; (ii) it behaves physically like the TFS method in noninteractive region; and (iii) the trajectories are continuous with continuous momenta. The CSDM method is also demonstrated to balance coherence well with decoherence, and the results are nearly independent of whether one uses the adiabatic or diabatic representation. The present results provide new insight into the formulation of a physically correct decoherence time to be used with the CSDM method for non–Born–Oppenheimer molecular dynamic simulations. [ABSTRACT FROM AUTHOR]

Population transfers in degenerate (or almost degenerate) two-level systems interacting with the few-cycle laser pulse are investigated. A simple and analytical formula of nonadiabatic transition probability is derived with completely degenerate condition, demonstrating the sensitive dependence of the transition probability on the phase of the few-cycle pulses. As one of the applications of this formula, a new way of controlling the nuclear wave packet dynamics at a potential curve crossing by 1 cycle laser pulse is proposed. [ABSTRACT FROM AUTHOR]

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system. [ABSTRACT FROM AUTHOR]

The hydrophobic hydration of dimethylsulfoxide (DMSO)/D[sub 2]O was explored using a combination of the high-pressure method and ab initio calculations. The frequencies of the C-H stretching vibration of DMSO increase as the mole fraction of D[sub 2]O increases, while no appreciable changes in spectral shapes are observed upon dilution. Interestingly, the infrared spectra of DMSO/D[sub 2]O observed under high-pressure exhibits dramatic changes, while the new spectral features locating at ∼2942 and ∼3033 cm[sup -1] appear. The spectral changes were attributed to the combined effect of C-H-O hydrogen bonding between C-H in DMSO and oxygen atom in D[sub 2]O, a phase change, pressure increase, etc. Ab initio calculation results, forecasting the frequency shift of the C-H stretching vibration as C-H-O is interacting via hydrogen bonding, are discussed. The reorganization of the hydrogen-bond network or geometry may be responsible for spectral changes as the pressure was elevated. [ABSTRACT FROM AUTHOR]

Complex chemical reactions in the gas phase can be decomposed into a network of elementary (e.g., unimolecular and bimolecular) steps which may involve multiple reactant channels, multiple intermediates, and multiple products. The modeling of such reactions involves describing the molecular species and their transformation by reaction at a detailed level. Here we focus on a detailed modeling of the C([SUP3]P)+ allene (C[SUB3]H[SUB4]) reaction, for which molecular beam experiments and theoretical calculations have previously been performed. In our previous calculations, product branching ratios for a nonrotating isomerizing unimolecular system were predicted. We extend the previous calculations to predict absolute unimolecular rate coefficients and branching ratios using microcanonical variational transition state theory (μ-VTST) with full energy and angular momentum resolution. Our calculation of the initial capture rate is facilitated by systematic ab initio potential energy surface calculations that describe the interaction potential between carbon and allene as a function of the angle of attack. Furthermore, the chemical kinetic scheme is enhanced to explicitly treat the entrance channels in terms of a predicted overall input flux and also to allow for the possibility of redissociation via the entrance channels. Thus, the computation of total bimolecular reaction rates and partial capture rates is now possible. [ABSTRACT FROM AUTHOR]

This study provides the evidence for hydrophobic hydration of CH[sub 3]COOD as its aqueous solution was compressed by high pressure. On going from neat CH[sub 3]COOD to diluted CH[sub 3]COOD/D[sub 2]O mixtures, the C-H vibration absorption bands do not reveal the solvent shift in frequency upon dilution, while the C-H absorption bands are altered into one prominent band at ∼2955 cm[sup -1] as the pressure was applied above 0.3 GPa. The possible explanation is the enhancement of C-H-O interactions between C-H and D[sub 2]O under the condition of high pressure. Ab initio calculations predict that the CH[sub 3] stretching frequencies and intensities of acetic acid are hardly affected by the presence of water, if the methyl group does not participate in the complex formation. Nevertheless, the strongest absorption peak at ∼2928 cm[sup -1] is predicted in several low-energy (acetic acid)-(water)[sub χ]complexes with the participation of C-H-O hydrogen bond. The reorganization of hydrogen-bond network or geometry is suggested to be responsible for the formation of C-H-O hydrogen bond. [ABSTRACT FROM AUTHOR]

C-H—O interactions of a self-assembled triple helix based on the 1-acetamido-3(2-pyrazinyl)-imidazolium cation has been probed by high pressure. The infrared spectroscopic profiles and ab initio calculations allow us to make a vibrational assignment of this compound. The C-H bonds forming C-H—O interactions shorten as the pressure was elevated, while free C-H vibration modes show low sensitivity to high pressure. The pressure-dependent results can be attributed to the strengthening of C-H—O electrostatic-dispersion interactions upon increasing pressure. The appearance of the free-NH infrared absorption indicates that the conventional NH—O hydrogen bond does not dominate the inter-strand packing in the compound. It is proposed that the charge-enhanced C-H—O interactions, forming a helical hydrogen-bonding network, disturb the formation of inter-strand N-H—O hydrogen-bonding in order to form a maximum number of hydrogen bonds. Applying high-pressure seems not to change the C=O bond length in contrast to the trend of blue-shift in frequency of C-H vibrations. London dispersion energy is suggested to be required for understanding the pressure-dependent results, although more additional terms, such as the effect in the presence of charge, are needed for the correct explanation. This work demonstrates that high-pressure studies may have the potential to provide insight into the C-H—O structural properties of biological related systems. [ABSTRACT FROM AUTHOR]

Brownian-particle survival is considered in the case when killing traps are gathered in spherical clusters distributed in space in a noncorrelated manner. Recently, we [J. Chem. Phys. 111, 711 (1999)] have proposed a mean-field theory of trapping by identical clusters of traps, in which the cluster radius R and the number of traps in a cluster n are the same for all of the clusters. Here we focus on a more realistic model, where the parameters R and n vary from one cluster to another, according to a given probabilistic law. For the model of polydisperse clusters, general expressions are obtained for the particle survival probability and the time-dependent rate coefficient. To exhibit the effect of polydispersity, we compare the trapping kinetics for polydisperse clusters with that for the reference system of identical clusters. It is shown, in particular, that fluctuations in n always lead to a process slowdown. An explicit solution is derived for slightly polydisperse nonoverlapping clusters. [ABSTRACT FROM AUTHOR]

Photodissociation of jet-cooled ethylbenzene at 248 nm was studied using VUV photoionization/multimass ion imaging techniques. The photofragment translational energy distribution obtained at 248 nm showed that after the excitation 75% of the ethylbenzene molecules dissociate from electronic excited state, and the rest 25% of the molecules dissociate through a hot molecule mechanism. This is the first experimental evidence which proves that the dissociation of alkyl-substituted benzenes can occur not only from hot molecule mechanism in this UV region. [ABSTRACT FROM AUTHOR]

The infrared spectroscopic profiles of HCOOD/D[sub 2]O mixtures were measured as a function of pressure and concentration. The C–H bond of HCOOD shortens as the pressure is elevated, while the increase in C–H bond length upon diluting HCOOD with D[sub 2]O was observed. Based on the experimental results, the shift in frequency of C–H stretching band is concluded to relate to the mechanism of the hydration of the C–H group and the water structure in the vicinity of the C–H group. The pressure-dependent results can be attributed to the strengthening of C–H---O electrostatic/dispersion interaction upon increasing pressure. The observations are in accord with ab initio calculation forecasting a blueshift of the C–H stretching mode via C–H---O interaction in HCOOD-water/(HCOOD)[sub 2]-(D[sub 2]O) complexes relative to the noninteracting monomer/dimer. Hydrogen-bonding nonadditivity and the size of water clusters are suggested to be responsible to cause the redshift in C–H stretching mode upon dilution HCOOD with D[sub 2]O. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

C–H–O interaction of acetone and deuterium oxide has been probed by high pressure. High-pressure study provides the first experimental evidence for the enhancement of hydrophobic hydration of acetone as its aqueous solution was compressed to high-pressure ices. Based on the results, we conclude that the C–H–O interaction may be a distinct possibility to understand the origin of the spectral feature located at ∼2950 cm-1, being sensitive to concentration and pressure dependence. Ab initio calculation results, forecasting the frequency red shift of the C–H stretching vibration as C–H–O is interacted via hydrogen bonding, are discussed. This study demonstrates that high pressure can be used as a valuable means of triggering and investigating C–H–O hydrogen-bonding interaction. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In femtosecond laser-pulse experiments the pump pulse, with duration comparable or shorter than a typical period of intra- or intermolecular vibrations, creates a nonstationary wave packet. In this paper we use the density-matrix method to analyze creation of space–time coherences by the pump pulse and their effect on the probe pulse. Expressions for the density-matrix jumps, induced by the probe pulse, have been obtained in a general case. The material equations, determining propagation of the probe pulse, have been derived. [ABSTRACT FROM AUTHOR]

A theory of the explosive desorption, observed by Chuang and Domen [Phys. Rev. Lett. 59, 1484 (1987); J. Vac. Sci. Technol. A 5, 473 (1987); J. Chem. Phys. (90, 3318 1989; Chem. Phys. Lett. (with A. Mödl). (154, 187 (1989)] is developed. When adsorbates are subjected to UV laser action, an explosive, molecular nonselective desorption takes place. This phenomenon is explained as follows: adsorbate molecules are excited by an UV laser to the higher electronic states. The electronic energy is delivered to the low energy vibronic states. The energy differences of these states are resonant to the corresponding phonon modes. This leads to the excitation of these phonons. When the energy input to the phonon modes exceeds their decay, phonon numbers start to increase exponentially. The phonon avalanche takes place. This in turn causes molecular nonselective desorption. The main result of the theory is the finding of the fluence and coverage dependent threshold conditions. [ABSTRACT FROM AUTHOR]

The vibronic spectra of ethylene have been studied using ab initio molecular orbital methods. Geometries of the singlet π–π*, π–3s, and π–3p excited electronic states have been optimized at the CIS and CASSCF levels of theory with the 6-311(2+)G* basis set. Vertical and adiabatic excitation energies, calculated by the multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) methods are in quantitative agreement with experiment. Vibrational frequencies and normal coordinates for the ground and excited states are used for the calculations of vibrational overlap integrals and Franck–Condon factors, taking into account distortion, displacement, and normal mode mixing (up to four modes). Major features of the observed absorption spectrum of ethylene have been interpreted on the basis of the computed Franck–Condon factors. The role of each electronic state in the spectra has been clarified; the π–3s transition corresponds to the distinct intensive peaks in the 57 000–61 000 cm-1 energy region, the less intensive distinct bands in the interval of 62 000–65 000 cm-1 are due to the π–3pσ states and the π–π* peaks constitute the continuum underlying the spectrum. The theoretical vibronic spectrum is in qualitative agreement with the experimental one, except of some details. Possible reasons for the discrepancies between theory and experiment are also discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vibrational dephasing of the 1385 cm-1 vibron of host molecules (naphthalene) perturbed by electronically excited guest molecules (pentacene) was studied by the time-resolved coherent Stokes Raman spectroscopy (CSRS) in the temperature region of 6–26 K. The decay time was faster when the excitation frequency was tuned from the off-resonance to the resonance of the electronic transition of pentacene. For the resonance case, longer decay times were observed at higher temperatures (∼26 K) than at low temperatures (∼6 K). Two possible mechanisms were considered for the inverse temperature behavior. The shorter decay time in the impurity perturbed domains may be attributed to the increase of a coupling strength on the decay channel from 1385 cm-1 mode to 1365 cm-1 mode by stimulating phonon emission. [ABSTRACT FROM AUTHOR]

The O([sup 1]D and [sup 3]P)+SiH[sub 4] reactions have been studied using ab initio/Rice-Ramsperger-Kassel-Marcus calculations to investigate possible formation mechanisms of various products in combustion and chemical vapor deposition processes. The relative branching ratios for various products formed through the O([sup 1]D)+SiH[sub 4] reaction involving the long-lived H[sub 3]SiOH intermediate are calculated as 55.5% for the H[sub 2]SiO/HSiOH+2H channel, 28.4% for the SiO+2H[sub 2] channel, 9.9% for the OH+SiH[sub 3] channel, 3.2% for the H[sub 2]O+SiH[sub 2] channel, and 3.0% for the HSiO/SiOH+H[sub 2]+H channel. These results significantly differ from those obtained in experiment, implying that the O([sup 1]D)+SiH[sub 4] reaction can take place through a mechanism other than the insertion mechanism. While the O([sup 3]P)+SiH[sub 4] reaction takes place by the abstraction mechanism, the O([sup 1]D)+SiH[sub 4] reaction can occur through both insertion and addition/abstraction mechanisms. The addition/abstraction mechanism occurring on the first excited potential energy surface is demonstrated to provide a significant contribution to the reaction products and to account for the forward scattering of the OH products observed in experiment. Finally, heats of formation for various species involving Si atom are computed employing the Gaussian 3 theory. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Nonadiabatic coupling matrix elements between the 1 [sup 2]A[sup ′], 2 [sup 2]A[sup ′], and 1 [sup 2]A[sup ″] electronic states of the C[sub 2]H radical are computed using ab initio full valence active space CASSCF method. The line-integral technique is then applied to study possible geometric phase effects. The results indicate the existence of a unique conical intersection due to CCH bending between the 1 [sup 2]A[sup ′] and 2 [sup 2]A[sup ′] states at the linear configuration in the vicinity of r[sub CC]=1.35 Å and r[sub CH]=1.60 Å. The line-integral calculations with ab initio nonadiabatic coupling terms confirm that when a path encircles the conical intersection, the line integral always produces the value π for the topological (Berry) phase and when a path encircles the two (symmetrical) conical interactions or none of them, the line integral produces the value of zero for the topological phase. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH 3 OH/TiO 2 (110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO 2 substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity. In addition, the SFG spectra have obvious changes while the methanol coverage increases, especially for the OH vibration peaks. Our theoretical spectra agree reasonably well with experimental measurements at 1 ML coverage, and an interesting peak which is absent in the theoretical spectra is tentatively assigned to some CH 3 stretch vibration of methanol adsorbed on the oxygen vacancy of TiO 2 .

The motion of a spherical Brownian particle in an asymmetric periodic channel is considered. Under an external periodic stimulus, the particle switches between two states with different particle radius, every half-period. Using Brownian dynamics simulations, we show that the particle size oscillation, combined with the asymmetry of the channel, induces a drift along the channel axis, directed towards the steeper wall of the channel. The oscillation of the particle size is accompanied by a time variation of the space accessible to the particle and by an oscillation of its diffusion coefficient. The former underlies the drift inducing mechanism of purely entropic nature. The latter, combined with the former, leads to a significant amplification of the effect. The drift velocity vanishes when interconversion between the states occurs either very slow or very fast, having a maximum in between. The position and magnitude of the maximum are discussed by providing an analytical approach based on intuitively appealing assumptions.

The excited-state orientation hydrogen-bonding dynamics, and vibronic spectra of isoquinoline (IQ) and its cationic form IQc in water have been investigated at the time-dependent density functional theory quantum chemistry level plus Franck-Condon simulation and interpretation. The excited-state orientation hydrogen bond strengthening has been found in IQ:H 2 O complex due to the charge redistribution upon excitation; this is interpreted by simulated 1:1 mixed absorption spectra of free IQ and IQ:H 2 O complex having best agreement with experimental results. Conversely, the orientation hydrogen bond in IQc:H 2 O complex would be strongly weakening in the S 1 state and this is interpreted by simulated absorption spectra of free IQc having best agreement with experimental results. By performing Franck-Condon simulation, it reveals that several important vibrational normal modes with frequencies about 1250 cm -1 involving the wagging motion of the hydrogen atoms are very sensitive to the formation of the orientation hydrogen bond for the IQ/IQc:H 2 O complex and this is confirmed by damped Franck-Condon simulation with free IQ/IQc in water. However, the emission spectra of the IQ and IQc in water have been found differently. Upon the excitation, the simulated fluorescence of IQ in water is dominated by the IQ:H 2 O complex; thus hydrogen bond between IQ and H 2 O is much easier to form in the S 1 state. While the weakened hydrogen bond in IQc:H 2 O complex is probably cleaved upon the laser pulse because the simulated emission spectrum of the free IQc is in better agreement with the experimental results.

Damped harmonic oscillators are utilized to calculate Franck-Condon factors within displaced harmonic oscillator approximation. This is practically done by scaling unperturbed Hessian matrix that represents local modes of force constants for molecule in gaseous phase, and then by diagonalizing perturbed Hessian matrix it results in direct modification of Huang-Rhys factors which represent normal modes of solute molecule perturbed by solvent environment. Scaling parameters are empirically introduced for simulating absorption and fluorescence spectra of an isolated solute molecule in solution. The present method is especially useful for simulating vibronic spectra of polycyclic aromatic hydrocarbon molecules in which hydrogen atom vibrations in solution can be scaled equally, namely the same scaling factor being applied to all hydrogen atoms in polycyclic aromatic hydrocarbons. The present method is demonstrated in simulating solvent enhanced X (1)Ag ↔ A(1)B1u absorption and fluorescence spectra of perylene (medium-sized polycyclic aromatic hydrocarbon) in benzene solution. It is found that one of six active normal modes v10 is actually responsible to the solvent enhancement of spectra observed in experiment. Simulations from all functionals (TD) B3LYP, (TD) B3LYP35, (TD) B3LYP50, and (TD) B3LYP100 draw the same conclusion. Hence, the present method is able to adequately reproduce experimental absorption and fluorescence spectra in both gas and solution phases.

We consider a system of two coupled particles fluctuating between two states, with different interparticle interaction potentials and particle friction coefficients. An external action drives the interstate transitions that induces reciprocating motion along the internal coordinate x (the interparticle distance). The system moves unidirectionally due to rectification of the internal motion by asymmetric friction fluctuations and thus operates as a dimeric motor that converts input energy into net movement. We focus on how the law of interaction between the particles affects the dimer transport and, in particular, the role of thermal noise in the motion inducing mechanism. It is argued that if the interaction potential behaves at large distances as x(α), depending on the value of the exponent α, the thermal noise plays a constructive (α > 2), neutral (α = 2), or destructive (α < 2) role. In the case of α = 1, corresponding piecewise linear potential profiles, an exact solution is obtained and discussed in detail.

By using three different hybrid exchange-correlation functionals containing 20%, 35%, and 50% of exact Hartree-Fock (HF) exchange of the density functional theory and its time-dependent extension plus the Hartree-Fock and the configuration interaction of single excitation methods, equilibrium geometries, and their 30 vibrational-normal-mode frequencies of the ground S(0)((1)A(1)) and the first excited S(1)((1)B(2)) states of fluorobenzene (FB) were calculated. The dispersed fluorescence spectrum and internal conversion (IC) rate of the A(1)B(2)→X(1)A(1) transition were simulated by Franck-Condon (FC) calculations within the displaced harmonic oscillator approximation plus anharmonic and distorted corrections. The simulated spectral profile is primarily described by the Franck-Condon progression from the ring-breathing modes ν(9) and ν(10) which belong to totally symmetry modes. Anharmonic corrections simultaneously improve the intensity order of 9(1)(0) and 10(1)(0) bands and diminish 1(1)(0) transition that is fairly strong in harmonic simulations. It is concluded that the amount of Hartree-Fock exchange does impact the geometries and vibrational frequencies of FB molecule, but not the relative intensities of the transitions. It is anharmonic corrections that make the relative intensities of the transitions in good agreement with experimental results. Distorted corrections could assign most of the dominant overtones of out-of-plane nontotally symmetry modes, and the results agree well with the experimental assignments. Furthermore, it was found that the internal conversion rate is dominated by three promoting modes that are computed with lowing symmetry to C(1). By choosing dephasing width as 10 cm(-1) that is consistent with spectral simulation, we obtained the lifetimes of the A(1)B(2)→X(1)A(1) de-excitation as 11 and 19 ns, respectively, from TD(B3LYP) and HF∕CIS calculations in comparison with the experimental value 14.75 ns., (© 2011 American Institute of Physics.)

The microscopic features of binary mixtures formed by an ionic liquid (EMI(+)TFSA(-) or EMI(+)FSA(-)) and a molecular liquid (acetonitrile or methanol) have been investigated by high-pressure infrared spectroscopy. On the basis of its responses to changes in pressure and concentration, the imidazolium C-H appears to exist at least in two different forms, i.e., isolated and associated structures. The weak band at approximately 3102 cm(-1) should be assigned to the isolated structure. CD(3)CN can be added to change the structural organization of ionic liquids. The compression of an EMI(+)TFSA(-)/CD(3)CN mixture leads to the increase in the isolated C-H band intensity. Nevertheless, the loss in intensity of the isolated structures was observed for EMI(+)FSA(-)/CD(3)CN mixtures as the pressure was elevated. In other words, the associated configuration is favored with increasing pressure by debiting the isolated form for EMI(+)FSA(-)/CD(3)CN mixtures. The stronger C-H...F interactions in EMI(+)FSA(-) may be one of the reasons for the remarkable differences in the pressure-dependent results of EMI(+)TFSA(-) and EMI(+)FSA(-).

Vibrational level structure in the S(0) (1)A(g) and S(1) (1)B(3u) states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm(-1). This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S(1) (1)B(3u) state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0(0) (0) band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S(0) and S(1) states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of M(J) levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S(0) state is also slow. We conclude that the similarity of pyrene's molecular structure and potential energy curve in its S(0) and S(1) states is the main cause of the slow radiationless transitions.

We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S(2) (1)A(1)<--S(0) (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S(2) state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S(0) and S(2) states (C(2v) symmetry). Rotational constants of the S(2) state are almost identical to those of the S(0) state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S(2) (1)A(1) state of azulene was identified to be IC to the S(1) (1)B(2) state. However, this S(2)-->S(1) IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S(1) (1)B(2) state, which is also responsible for the extraordinarily fast S(1)-->S(0) IC in the isolated azulene molecule.

High-pressure infrared measurements were carried out to observe the microscopic structures of two imidazolium-based ionic liquids, i.e., 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [EMI(+)(CF(3)SO(2))(2)N(-), EMI(+)TFSA(-)] and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [EMI(+)(FSO(2))(2)N(-), EMI(+)FSA(-)]. The results obtained at ambient pressure indicate that the imidazolium C-H may exist in two different forms, i.e., isolated and network structures. As the sample of pure EMI(+)FSA(-) was compressed, the network configuration is favored with increasing pressure by debiting the isolated form. For EMI(+)TFSA(-)/H(2)O mixtures, the imidazolium C-H peaks split into four bands at high pressures. The new spectral features at approximately 3117 and 3190 cm(-1), being concentration sensitive, can be attributed to the interactions between the imidazolium C-H and water molecules. The alkyl C-H absorption exhibits a new band at approximately 3025 cm(-1) under high pressures. This observation suggests the formation of a certain water structure around the alkyl C-H groups. The O-H stretching absorption reveals two types of O-H species, i.e., free O-H and bonded O-H. For EMI(+)TFSA(-)/H(2)O mixtures, the compression leads to a loss of the free O-H band intensities, and pressure somehow stabilizes the bonded O-H configurations. The results also suggest the non-negligible roles of weak hydrogen bonds in the structure of ionic liquids.

The structural organization in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)])/water or methanol was studied by infrared spectroscopy. No drastic change in the concentration dependence of the alkyl C-H band frequency was observed at high concentration of the ionic liquid. This behavior indicates a clustering of the ionic liquid in alkyl regions. Nevertheless, the presence of methanol significantly perturbs the ionic liquid-ionic liquid associations in the imidazolium region. On the basis of the responses to change in pressure and concentration, two different types of O-H species, i.e., free O-H and bonded O-H, were observed in the O-H stretching region. For [bmim][BF(4)]/water mixtures, the compression leads to loss of the free O-H band intensity. It is likely that free O-H is switched to bonded O-H as high pressures are applied. For [bmim][BF(4)]/methanol mixtures, the free O-H is still stable under high pressures.

Unified semiclasical solution for general nonadiabatic tunneling between two adiabatic potential energy surfaces is established by employing unified semiclassical solution for pure nonadiabatic transition [C. Zhu, J. Chem. Phys. 105, 4159 (1996)] with the certain symmetry transformation. This symmetry comes from a detailed analysis of the reduced scattering matrix for Landau-Zener type of crossing as a special case of nonadiabatic transition and nonadiabatic tunneling. Traditional classification of crossing and noncrossing types of nonadiabatic transition can be quantitatively defined by the rotation angle of adiabatic-to-diabatic transformation, and this rotational angle enters the analytical solution for general nonadiabatic tunneling. The certain two-state exponential potential models are employed for numerical tests, and the calculations from the present general nonadiabatic tunneling formula are demonstrated in very good agreement with the results from exact quantum mechanical calculations. The present general nonadiabatic tunneling formula can be incorporated with various mixed quantum-classical methods for modeling electronically nonadiabatic processes in photochemistry.

Photodissociation of pyrimidine at 193 and 248 nm was investigated separately using vacuum ultraviolet photoionization at 118.4 and 88.6 nm and multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C4N2H4 --> C4N2H3 + H and five ring opening dissociation channels, C4N2H4 --> C3NH3 + HCN, C4N2H4 --> 2C2NH2, C4N2H4 --> CH3N + C3NH, C4N2H4 --> C4NH2 + NH2, and C4N2H4 --> CH2N + C3NH2. Only the first four channels were observed at 248 nm. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion at both wavelengths. The dissociation rates were found to be >5 x 10(7) and 1 x 10(6) s(-1) at 193 and 248 nm, respectively. Comparison with the potential energies from ab initio calculations have been made.

We have calculated the state-to-state integral cross sections and rate constants for the rotationally inelastic collision of CH(B (2)Sigma(-), nu=0, N-->N') with Ar using the quantum coupled-state and close-coupling methods on an ab initio potential-energy surface constructed by Alexander et al. [J. Chem. Phys. 101, 4547 (1994)]. Overall the calculated rate constants are in good agreements with the three available experimental results. The rate constants are comparable to the usual gas kinetic and decrease with increasing N and DeltaN. For the multiquantum transition cases, the theory underestimates the experiment. We discuss some possible causes to the discrepancies among the theory and the experiments.

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5 --> C5NH4 + H (10%) and five ring opening dissociation channels, C5NH5 --> C4H4 + HCN, C5NH5 --> C3H3 + C2NH2, C5NH5 --> C2H4 +C3NH, C5NH5 --> C4NH2 + CH3 (14%), and C5NH5 --> C2H2 + C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2x10(3) s(-1). Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made.

We have probed under high pressure the C-H hydrogen bonds formed by N,N(')-disubstituted imidazolium ions having PF(6) (-) and Br(-) counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N-H...F unit) in the infrared spectrum of the PF(6) (-) salt indicates that conventional N-H...O and N-H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C(2)-H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N-H hydrogen bonds in the PF(6) (-) salt. This finding is consistent with the pressure-dependent results, which reveal that the C(2)-H...F interaction is enhanced upon increasing the pressure. In contrast to the PF(6) (-) salt, the imidazolium C-H bonds of the Br(-) salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br(-) and PF(6) (-) ions., ((c) 2004 American Institute of Physics.)