The reactivity of closed-shell gas phase cluster anions AuTi3O7− and AuTi3O8− with methane under thermal collision conditions was studied by mass spectrometric experiments and quantum chemical calculations. Methane activation was observed with the formation of AuCH3 in both cases, while the formation of formaldehyde was also identified in the reaction system of AuTi3O8−. The cooperative effect of the separated Au+ and O2− ions on the clusters induces the cleavage of the first C–H bond of methane. Further activation of the second C–H bond by a peroxide ion O22− leads to the formation of formaldehyde. This study shows that closed-shell species on metal oxides can be reactive enough to facilitate thermal H–CH3 bond cleavage and the subsequent conversion.

We apply the mixed quantum-classical Liouville (MQCL) equation to investigate the nonadiabatic curve crossing in condensed phases. More specifically, electron transfer rate constants of the spin-Boson model are calculated by employing a rate constant expression using the collective solvent polarization as the reaction coordinate. In the calculation, classical nuclear degrees of freedom are initially sampled at the transition state configuration, and the initial state for the electronic degree of freedom is obtained from a mixed quantum-classical Boltzmann distribution. Different contributions to the electron transfer rate from the diagonal and off-diagonal elements of the initial density matrix, and contributions from trajectories with positive and negative initial velocities are analyzed. It is shown that the off-diagonal elements of the initial density matrix play an important role in the total electron transfer rate. The MQCL results are also compared with those calculated using Ehrenfest dynamics. It is found that, although the Ehrenfest dynamics is inaccurate when the reactive flux rate expression is used directly, it can give reasonably accurate results when individual contributions from the diagonal and off-diagonal elements of the initial density matrix are calculated.

Recent experimental and theoretical studies have revealed that quantum coherence plays an important role in the excitation energy transfer in photosynthetic light-harvesting (LH) complexes. Inspired by the recent single-molecule two-color double-pump experiment, we theoretically investigate the effect of pulse shaping on observing coherent energy transfer in the single bacterial LH2 complex. It is found that quantum coherent energy transfer can be observed when the time delay and phase difference between the two laser pulses are controlled independently. However, when the two-color pulses are generated using the pulse-shaping method, how the laser pulses are prepared is crucial to the observation of quantum coherent energy transfer in single photosynthetic complexes.

Observations of oscillatory features in the 2D spectra of several photosynthetic complexes have led to diverged opinions on their origins, including electronic coherence, vibrational coherence, and vibronic coherence. In this work, effects of these different types of quantum coherence on ultrafast pump–probe polarization anisotropy are investigated and distinguished. We first simulate the isotropic pump–probe signal and anisotropy decay of the Fenna–Matthews–Olson (FMO) complex using a model with only electronic coherence at low temperature and obtain the same coherence time as in the previous experiment. Then, three model dimer systems with different prespecified quantum coherence are simulated, and the results show that their different spectral characteristics can be used to determine the type of coherence during the spectral process. Finally, we simulate model systems with different electronic-vibrational couplings and reveal the condition in which long time vibronic coherence can be observed in systems like the FMO complex.

We investigate theoretically the effect of hydrogen bond bending motion on the proton coupled electron transfer (PCET) reaction, using a model system where an intramolecular hydrogen-bonded phenol group is the proton donor. It is shown that, in a two-dimensional (2D) model of the PCET reaction, the bending and stretching vibrational motions are separated, and due to the hydrogen bond configuration and anharmonicity of the potential energy surface, the bending vibration can play a role in the PCET reaction. The results are also compared with two different sets of one-dimensional models (1D-linear and 1D-curved). Due to contributions of the bending motion, the rate constants in the 2D model are larger than those in the 1D-linear model, although the differences between the total rate constants and KIEs for 2D and 1D models are not major. Results from the 1D-curved model lie between the 2D- and 1D-linear models, indicating that it can include some effect of bending motion in reducing the potential energies along the reaction path.

Starting from the mixed quantum-classical Liouville (MQCL) equation, we derive a new trajectory branching method as a modification to the conventional mean field approximation. In the new method, the mean field approximation is used to propagate the mixed quantum-classical dynamics for short times. When the mean field description becomes invalid, new trajectories are added in the simulation by branching the single trajectory into multiple ones. To achieve this, a new set of variables are defined to monitor the deviations of the dynamics on different potential energy surfaces from the reference mean field trajectory, and their equations of motion are derived from the MQCL equation based on the method of first moment expansion. The new method is tested on several one-dimensional two surface problems and is shown to correctly solve the problem of the mean field approximation in several cases.

We investigate the applicability of the Ehrenfest and surface hopping methods to calculate electron transfer rates using the spin–boson model with different parameters. Rate constants are obtained from short time dynamics performed in both the diabatic and adiabatic basis sets. Numerical results and theoretical analysis show that these two methods can be reasonably accurate in the nonadiabatic limit, by staying close to an approximate Fermi’s golden rule. Beyond the nonadiabatic limit, the calculated mixed quantum classical rates are compared with numerical exact results, and similar accuracy was found as in the nonadiabatic limit. The relation between the current finding and recent studies using the surface hopping method based on long time dynamics is also discussed. It is found that the short time dynamics could be more accurate in calculating rate constants using the mixed quantum classical methods.

By combining molecular dynamics (MD) simulation and density functional theory (DFT) calculations, we investigate the surface doubly resonant sum-frequency vibrational spectroscopy (SFVS) for a monolayer of R-1,1′-bi-2-naphthol (R-BN) molecules on water surface. MD simulations indicate that the R-BN molecules stand perpendicularly on the water surface due to hydrogen bonding with the water molecules. DFT and time-dependent density functional theory (TDDFT) methods are employed to obtain potential energy shifts, transition dipoles, and their derivatives, which are then used to calculate both the Franck–Condon and Herzberg–Teller terms to the surface hyperpolarizabilities. The theoretical SFVS agrees well with the experimental result. The origin of the SFVS peaks and symmetry properties of the hyperpolarizabilities tensor are also analyzed, which indicates that theoretical computations can obtain the most important components that are helpful for experimental SFVS analysis. The current work shows that theoretical calculations can provide useful detailed molecular information, such as molecular structure, orientation, and excited structure at interfaces, in studies of surface doubly resonant SFVS.

This contribution provides a summary of proposed theoretical and computational studies on excited state dynamics in molecular aggregates, as an important part of the National Natural Science Foundation (NNSF) Major Project entitled “Theoretical study of the low-lying electronic excited state for molecular aggregates”. This study will focus on developments of novel methods to simulate excited state dynamics of molecular aggregates, with the aim of understanding several important chemical physics processes, and providing a solid foundation for predicting the opto-electronic properties of organic functional materials and devices. The contents of this study include: (1) The quantum chemical methods for electronic excited state and electronic couplings targeted for dynamics in molecular aggregates; (2) Methods to construct effective Hamiltonian models, and to solve their dynamics using system-bath approaches; (3) Non-adiabatic mixed quantum-classic methods targeted for molecular aggregates; (4) Theoretical studies of charge and energy transfer, and related spectroscopic phenomena in molecular aggregates.

Resonance Raman spectra for C60 molecules in vacuum and benzene solutions have been studied based on density functional theory calculations under Ih symmetry. The displacement parameters of the potential energy minimum along normal coordinates between the ground and excited states for totally symmetric modes are determined, which are small. The solvent effect is found to have a slight influence on Raman intensities. Also, resonance Raman excitation profiles have been investigated.

Raman spectra of the CO stretch for liquid methanol and its aqueous solutions were simulated using the combined electronic structure and molecular dynamics simulation method. The instantaneous vibrational frequencies were obtained from an empirical mapping to the electrostatic potentials, while vibrational couplings between different molecules were calculated using the transition dipole coupling model. It is found that noncoincident effects (NCEs) at high concentrations are dominated by the intermolecular couplings of CO stretch and decrease monotonically as the methanol concentration decreases. This behavior is explained as the effect of reduced methanol–methanol hydrogen bonding with the addition of water. A non-monotonic change of the NCEs defined by the peak position of the CO stretch as a function of methanol mole fraction is found, which is ascribed to band asymmetry caused by reorientational dynamics.

Inspired by the recent observation of correlated excitation energy fluctuations of neighboring chromophores (Lee et al. Science2007,316, 1462), quantum chemistry calculations and molecular dynamics simulations were employed to calculate the electronic–vibrational coupling in the excited states of the photosynthetic reaction center of purple bacteria Rhodobacter (Rb.) sphaeroides. The ground states and lowest excited (Qy) states of isolated bacteriochlorophyll a (BChl a) and bacteriopheophytin (BPhe) molecules were first optimized using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Normal mode analyses were then performed to calculate the Huang–Rhys factors of the intramolecular vibrational modes. To account for intermolecular electronic–vibrational coupling, molecular dynamics simulations were first performed. The ZINDO/S method and partial charge coupling method were then used to calculate the excitation energy fluctuations caused by the protein environment and obtain the spectral density. No obvious correlations in transition energy fluctuations between BChl a and BPhe pigments were observed in the time scale of our MD simulation. Finally, by comparing the calculated absorption spectra with experimental ones, magnitudes of inhomogeneous broadening due to the static disorder were estimated. The large amplitude of the static disorder indicates that a large portion of the spectral density and their correlations may still be hidden in the inhomogeneous broadening due to the finite MD simulation time.

The density functional theory and hopping model were employed to calculate the charge carrier mobility of four planar polycyclic aromatic hydrocarbon fused tetrathiafulvalene derivatives. The effect of halogen substitution and nitrogen substitution were also investigated. Dinaphtho-tetrathiafulvalene (DN-TTF) and diquinoxalino-tetrathiafulvalene (DQ-TTF) were revealed to be primarily hole transport materials due to their high electron injection barrier relative to the work function of the Au electrode. Halogen substituted TFDQ-TTF and TClDQ-TTF were found to have lower HOMO and LUMO energy levels, such that the electron injection barriers are lowered and the hole injection barriers are elevated. The large transfer integral and small reorganization energy for electron transport also suggest that they have relatively large electron mobilities. The calculated results were in good agreement with the experiment ones. Our result shows that withdraw-electron groups introduced in aromatic fused tetrathiafulvalene derivatives is a rational way to obtain good n-type organic semiconductors.

Using a combined electronic structure and molecular dynamics simulation method, we calculated the infrared and Raman spectra for the OH vibrations in liquid CH3OH. The vibrational frequencies, transition dipole moments, and transition polarizabilities are obtained from density functional theory calculations and then mapped into an empirical relation to the electric field on the H atom along the OH bond. Vibrational couplings between OH chromophores on different molecules are treated using transition dipole interactions. The simulated infrared and Raman line shapes are in good agreement with experimental observations. We have also shown that the vibrations of non-hydrogen-bonded OH groups contribute significantly to the difference between the IR and Raman line shapes.

The electronic coupling between adjacent molecules is an important parameter for the charge transport properties of organic semiconductors. In a previous paper, a semiclassical generalized nonadiabatic transition state theory was used to investigate the nonperturbative effect of the electronic coupling on the charge transport properties, but it is not applicable at low temperatures due to the presence of high-frequency modes from the intramolecular conjugated carbon–carbon stretching vibrations [G. J. Nan et al., J. Chem. Phys., 2009, 130, 024704]. In the present paper, we apply a quantum charge transfer rate formula based on the imaginary-time flux–flux correlation function without the weak electronic coupling approximation. The imaginary-time flux–flux correlation function is then expressed in terms of the vibrational-mode path average and is evaluated by the path integral approach. All parameters are computed by quantum chemical approaches, and the mobility is obtained by kinetic Monte-Carlo simulation. We evaluate the intra-layer mobility of sexithiophene crystal structures in high- and low-temperature phases for a wide range of temperatures. In the case of strong coupling, the quantum charge transfer rates were found to be significantly smaller than those calculated using the weak electronic coupling approximation, which leads to reduced mobility especially at low temperatures. As a consequence, the mobility becomes less dependent on temperature when the molecular packing leads to strong electronic coupling in some charge transport directions. The temperature-independent charge mobility in organic thin-film transistors from experimental measurements may be explained from the present model with the grain boundaries considered. In addition, we point out that the widely used Marcus equation is invalid in calculating charge carrier transfer rates in sexithiophene crystals.

Reactions of cerium oxide cluster anions with carbon monoxide are investigated by time-of-flight mass spectrometry and density functional theory computations aided with molecular dynamics simulations. Interesting size-dependent reactivity of the CenO2n+1– cluster series with n = 1–21 is observed: (1) the small n = 1–3 clusters have no or very low reactivity toward CO, (2) the large n = 4–21 clusters can oxidize CO to produce CO2, and (3) the n = 4 (Ce4O9–), 6 (Ce6O13–), 7 (Ce7O15–), and 12 (Ce12O25–) clusters have relatively higher reactivity than their neighboring systems Ce3O7–, Ce5O11–, Ce8O17–, etc. Theoretical study indicates that the CenO2n+1– clusters contain oxygen-centered radicals (O–•) and the nature of the spin density distributions within the clusters controls the experimentally observed size-dependent reactivity. The experiment and theory in this study suggest that the metal oxide clusters as large as Ce21O43– can contain the reactive O–• centers, at which the size may be large enough to mimic related active sites in condensed phase catalysts. Oxidation of CO by O2 at low temperature is of widespread importance and reactive oxygen species including O–• are usually involved. The nature of the O–• radicals is demonstrated to be able to further address the goodness of nanocrystalline CeO2 in the low-termperautre CO oxidation.

Theoretically we study the doubly resonant IR−UV hyper-Raman scattering where the IR light is resonant to the vibrational transition and the UV/visible light is resonant to the electronic transition between the ground and excited states. Based on the Taylor expansion of the electric transition dipole moments with respect to the normal coordinates, we have derived the expressions for the hyper-Raman A, B, and C terms. Using quantum chemistry calculations, we have estimated the magnitudes for all the three terms. Due to double resonance, contributions from all the three terms should be detectable in experiments.

The intermolecular interactions between two meso-tetraphenylporphyrin diacid H4TPPCl2 monomers are investigated by density functional theory with the PBE1PBE functional and 6-31G* basis set. Structures of five stable isomers of (H4TPPCl2)2 are determined. It is found that the interaction (IMHB-1) of Cl with orthoH atoms in two phenyl groups of H4TPPCl2 is unique in that it is the strongest interaction between two H4TPPCl2 monomers. Natural bond orbital analysis is carried out to explain the subtle differences of hydrogen bondings in these isomers. To understand the interactions of water molecule with H4TPPCl2 and (H4TPPCl2)2, structures of H4TPPCl2·H2O and nine isomers of (H4TPPCl2)2·H2O are also determined. The binding energy of H4TPPCl2·H2O is 36.47 kJ mol−1, less than that of the most stable structure of (H4TPPCl2)2 dimers, 41.59 kJ mol−1. The dimers containing the special IMHB-1 interactions may be the elementary building blocks for the aggregation of H4TPPCl2, which is supported by the crystal structure of H4TPPCl2·H2O·2CH3CN. Thus, study of the interactions between two or a small number of gas molecules can provide important information for understanding the main interactions and stable structures in related condensed-phase systems.

Using time-dependent density functional computations we calculate the doubly resonant IR−UV sum-frequency vibrational spectroscopy and sum-frequency vibrational spectroscopy off electronic resonance for d-arabinose solutions. In comparison with the experimental detection limit, the calculated doubly resonant IR−UV sum-frequency vibrational spectroscopy is strong enough to be detectable.

•The laser pulses are included explicitly in two-dimensional electronic spectra simulation.•Effects of the finite laser pulse width are examined.•Two-color two-dimensional electronic spectra are simulated.•To observe quantum beats, the related excitonic states should be excited simultaneously.We combine the hierarchical equations of motion method and the equation-of-motion phase-matching approach to calculate two-dimensional electronic spectra of model systems. When the laser pulse is short enough, the current method reproduces the results based on third-order response function calculations in the impulsive limit. Finite laser pulse width is found to affect both the peak positions and shapes, as well as the time evolution of diagonal and cross peaks. Simulations of the two-color two-dimensional electronic spectra also show that, to observe quantum beats in the diagonal and cross peaks, it is necessary to excite the related excitonic states simultaneously.

The reactivity of closed-shell gas phase cluster anions AuTi3O7− and AuTi3O8− with methane under thermal collision conditions was studied by mass spectrometric experiments and quantum chemical calculations. Methane activation was observed with the formation of AuCH3 in both cases, while the formation of formaldehyde was also identified in the reaction system of AuTi3O8−. The cooperative effect of the separated Au+ and O2− ions on the clusters induces the cleavage of the first C–H bond of methane. Further activation of the second C–H bond by a peroxide ion O22− leads to the formation of formaldehyde. This study shows that closed-shell species on metal oxides can be reactive enough to facilitate thermal H–CH3 bond cleavage and the subsequent conversion.

Using a combined electronic structure and molecular dynamics simulation method, we calculated the infrared and Raman spectra for the OH vibrations in liquid CH3OH. The vibrational frequencies, transition dipole moments, and transition polarizabilities are obtained from density functional theory calculations and then mapped into an empirical relation to the electric field on the H atom along the OH bond. Vibrational couplings between OH chromophores on different molecules are treated using transition dipole interactions. The simulated infrared and Raman line shapes are in good agreement with experimental observations. We have also shown that the vibrations of non-hydrogen-bonded OH groups contribute significantly to the difference between the IR and Raman line shapes.

The intermolecular interactions between two meso-tetraphenylporphyrin diacid H4TPPCl2 monomers are investigated by density functional theory with the PBE1PBE functional and 6-31G* basis set. Structures of five stable isomers of (H4TPPCl2)2 are determined. It is found that the interaction (IMHB-1) of Cl with orthoH atoms in two phenyl groups of H4TPPCl2 is unique in that it is the strongest interaction between two H4TPPCl2 monomers. Natural bond orbital analysis is carried out to explain the subtle differences of hydrogen bondings in these isomers. To understand the interactions of water molecule with H4TPPCl2 and (H4TPPCl2)2, structures of H4TPPCl2·H2O and nine isomers of (H4TPPCl2)2·H2O are also determined. The binding energy of H4TPPCl2·H2O is 36.47 kJ mol−1, less than that of the most stable structure of (H4TPPCl2)2 dimers, 41.59 kJ mol−1. The dimers containing the special IMHB-1 interactions may be the elementary building blocks for the aggregation of H4TPPCl2, which is supported by the crystal structure of H4TPPCl2·H2O·2CH3CN. Thus, study of the interactions between two or a small number of gas molecules can provide important information for understanding the main interactions and stable structures in related condensed-phase systems.

The electronic coupling between adjacent molecules is an important parameter for the charge transport properties of organic semiconductors. In a previous paper, a semiclassical generalized nonadiabatic transition state theory was used to investigate the nonperturbative effect of the electronic coupling on the charge transport properties, but it is not applicable at low temperatures due to the presence of high-frequency modes from the intramolecular conjugated carbon–carbon stretching vibrations [G. J. Nan et al., J. Chem. Phys., 2009, 130, 024704]. In the present paper, we apply a quantum charge transfer rate formula based on the imaginary-time flux–flux correlation function without the weak electronic coupling approximation. The imaginary-time flux–flux correlation function is then expressed in terms of the vibrational-mode path average and is evaluated by the path integral approach. All parameters are computed by quantum chemical approaches, and the mobility is obtained by kinetic Monte-Carlo simulation. We evaluate the intra-layer mobility of sexithiophene crystal structures in high- and low-temperature phases for a wide range of temperatures. In the case of strong coupling, the quantum charge transfer rates were found to be significantly smaller than those calculated using the weak electronic coupling approximation, which leads to reduced mobility especially at low temperatures. As a consequence, the mobility becomes less dependent on temperature when the molecular packing leads to strong electronic coupling in some charge transport directions. The temperature-independent charge mobility in organic thin-film transistors from experimental measurements may be explained from the present model with the grain boundaries considered. In addition, we point out that the widely used Marcus equation is invalid in calculating charge carrier transfer rates in sexithiophene crystals.