부산대학교 도서관

Molecular dynamics simulation is used to study ε(k,ω), the frequency and wave vector dependent dielectric permittivity of a three-site model of methanol in which the methyl group is represented as a single site. The effects of induced dipoles are taken into account using perturbation theory and a three-site molecular polarizability model. The data are analyzed in terms of projected variables which allow us to distinguish between local field factors which renormalize the permanent-dipole contribution to the permittivity from the ‘‘collision induced’’ dipole relaxation. We find that induced dipoles significantly enhance the static permittivity, mainly through the local field factors. The time correlation functions for the longitudinal and transverse components of the collective dipole moments, evaluated at several of the smallest wave vectors in the system, present rapid oscillations at short times, followed by a nonexponential relaxation regime at intermediate times. At long times, for which the longitudinal component has essentially decayed to zero, the transverse component decays exponentially. Collision-induced dipole fluctuations are a major contributor to the longitudinal collective dipole relaxation and a much smaller one to transverse component dynamics. The far-infrared absorption spectrum, evaluated from the smallest k transverse component, exhibits three well resolved absorption bands, with maxima located at frequencies 60, 150, and 645 cm-1, in qualitative agreement with recent experimental measurements. We find that the fluctuations of the induced dipoles contribute in a nontrivial way to the shape of the spectra at frequencies above 50 cm-1. The permanent dipole contribution to ε(k,ω) was obtained also for partially deuterated methanol, CH3OD, in order to aid in the analysis of the hydrogen-bond dynamics. In addition, we briefly describe an approach to the study of solvation dynamics using the MD... [ABSTRACT FROM AUTHOR]