Nonlinear response of solvent molecules induced by instantaneous change of solute electronic structure: studied by RISM theory

Abstract

We applied the reference interaction-site model (RISM) theory for estimation of dynamic response function of the average-energy relaxation, SS(t), of solute–solvent systems induced by an instantaneous change of the solute electronic structure in polar solvents. We selected solutes with square- or cubic-like structures: initially all the atoms were electrically neutral and, then we applied δ+ and δ− charges instantaneously. For the multipoles, SS(t)s show apparent dependence on δ, which predicts nonlinear response of the solvent, because SS(t) is a normalized quantity irrespective of δ as far as the linear response of solvent dynamics is assumed. On the other hand, previously we have performed transient hole-burning and time-resolved fluorescence spectroscopy. We have found the difference between the dynamic response functions obtained from spectral widths and that from spectral peak shifts implying a nonlinear solvent-response, as these two kinds of the response functions should again correspond to each other within a linear response regime. Our present results based on the RISM theory can be another indication to confirm nonlinearity suggested by our experiments. At the moment, we ascribe the origin of the nonlinear solvent behavior to the larger local-density fluctuation around the solute.