The rate constant for vibrational energy relaxation of the H-Cl stretch in liquid HCl (T = 188K, ρ = 19.671 nm(-3)) is calculated within the framework of the Landau-Teller formula. The force-force correlation function is calculated via the recently introduced force-derivative-free linearized semiclassical method [Vázquez et al. J. Phys. Chem. A2010, 114, 5682]. The calculated vibrational energy relaxation rate constant is found to be in excellent agreement with experiment, and the electrostatic force is found to contribute significantly to the high frequency component of the force-force correlation function. In contrast, the corresponding classical vibrational energy relaxation rate constant is found to be 2 orders of magnitude slower than the experimental value, and the classical force-force correlation function is found to be dominated by the Lennard-Jones forces. These observations suggest that quantum delocalization, enhanced by the light mass of hydrogen, amplifies the contribution of repulsive Coulombic forces to the force-force correlation function, thereby making electrostriction an unlikely mechanism for vibrational energy relaxation in the case of hydrogen stretches. This interpretation is reinforced by the results of a similar calculation in the case of the D-Cl stretch in liquid DCl under the same conditions. In this case, the quantum enhancement of the vibrational energy relaxation rate constant is observed to be greatly diminished in comparison to HCl, thereby giving rise to a reversal of the isotope effect in comparison to that predicted by the corresponding classical treatment (i.e., whereas the classical vibrational energy relaxation rate of DCl is faster than that of HCl, the opposite trend is predicted by the linearized semiclassical treatment). It is also shown that the vibrational energy relaxation of DCl is completely dominated by the Lennard-Jones forces within either classical and semiclassical treatments, thereby suggesting that electrostriction is the underlying mechanism in this case.

The lifetimes of the first vibrational state of (12)C(14)N(-) and (13)C(15)N(-) dissolved in H2O or D2O were calculated. The calculations were based on the Landau-Teller formula that puts the vibrational lifetimes in terms of the autocorrelation function of the force exerted on the C-N stretch by the remaining degrees of freedom. The force autocorrelation functions were calculated from classical molecular dynamics simulations of the four cyanide/water isotopomer combinations ((12)C(14)N(-)/H2O, (12)C(14)N(-)/D2O, (13)C(15)N(-)/H2O, (13)C(15)N(-)/D2O). The cyanide ion was described by a polarizable force field, and the water was described by either the rigid SPC/E model or the flexible SPC/Fw model, in order to compare two different types of accepting modes, namely, (1) intermolecular (translational and rotational) solvent accepting modes (rigid SPC/E water) and (2) intramolecular (vibrational) solvent accepting modes (flexible SPC/Fw water). Since quantum effects are expected to increase in size with increasing frequency mismatch between relaxing and accepting modes, different quantum correction factors were employed depending on the identity of the accepting modes, more specifically, the harmonic/Schofield quantum correction factor in the case of intermolecular accepting modes and the standard quantum correction factor in the case of intramolecular accepting modes. The lifetimes with either the rigid SPC/E or flexible SPC/Fw water models were found to be in good quantitative agreement with the experimentally measured values for all isotopomer combinations. Our results suggest that taking into account quantum effects on the vibrational energy relaxation of cyanide in aqueous solution can make the intermolecular pathway at least as likely as the intramolecular pathway.

The single-bond cZt-tZt isomerization rate constants of 1,3,5-cis-hexatriene dissolved in a series of explicit alkane (cyclohexane, n-heptane, and cycloheptane) and alcohol (methanol, ethanol, and n-propanol) solvents were calculated via reactive flux theory, from classical molecular dynamics simulations, at different temperatures (275-325 K). We find that the isomerization rate constants in alcohol solvents are slower than those in alkane solvents, in accord with the observed experimental trend (Harris, D. A.; Orozco, M. B.; Sension, R. J. J. Phys. Chem. A 2006, 110, 9325-9333). We also find that the same trend is obtained when the transition state theory limit of the reactive flux expression for the reaction rate constant is employed. The solvent dependence of the reaction rate constant is then traced back to the fundamentally different structure of the solvation shell in alcohol and alkane solvents. Whereas in alcohol solvents, hexatriene fits inside a rigid cavity formed by the hydrogen-bonded network, which is relatively insensitive to conformational dynamics, alkane solvents form a cavity around hexatriene that adjusts to the conformational state of hexatriene, thereby increasing the entropy of transition state configurations relative to reactant configurations and giving rise to faster isomerization.

The mechanism for homogeneous nucleation of the liquid phase in Lennard-Jones solids is studied by combining the Landau free energy approach with some of the methodology developed to characterise transition path ensembles. The second-order bond orientational order parameter, Q 6 which indexes the overall degree of crystalline order, is shown to provide a dynamically significant collective coordinate describing the melting process. Trajectories generated from configurations sampled in the vicinity of the maximum in the Landau free energy curve, F(Q 6 ), are shown to have equal likelihood of teminating in either the solid or liquid-like free energy minima. It is also demonstrated that Q 6 is necessary but not sufficient as a dynamical coordinate to describe melting and it is necessary to explore possiblities for additional coordinates which are critical for initiating melting. Our sudy suggests that the additional coordinates for describing the melting process would be some type of localised defect, much smaller in spatial extent than the size of the critical nucleus predicted by classical nucleation theory.