Vibrational relaxation of a dipolar molecule in water

Abstract

The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau–Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH3Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH3Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH3Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH3Cl molecule and that most of these important molecules are at positions perpendicular to the CH3Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray.