Mechanistic Studies of Solvation Dynamics in Liquids

Abstract

This chapter deals with several aspects of theory and computer simulation of solvation dynamics (SD), the solvent response to a change in solutesolvent interactions brought about by a solute electronic transition. The chapter starts with an overview of recent progress in SD research and with the basic assumptions that are used as a starting point in most SD theories and simulations. Instantaneous normal mode and time-domain methods of analysis of the solvation response applicable within the linear response approximation are then presented. Their use in uncovering several aspects of the molecular mechanism of SD, including the relative contributions of different molecular degrees of freedom, the collective nature of the response, the dependence of the response on the range and symmetry of the perturbation in solute-solvent interactions and the resemblance between SD and other experimentally-accessible solvent dynamics, is illustrated. It is shown how the results of this analysis can be used to develop approximations to SD in terms of single-solvent-molecule and pure solvent dynamics. Several causes of breakdown of the linear response approximation, relevant to SD in real systems, are discussed. Analysis of molecular dynamics simulation data on SD in benzene-acetonitrile mixtures is used to illustrate how to uncover the molecular mechanisms leading to nonlinear response and to show that significant nonlinearities can arise even for modest changes in the solute dipole.