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

We applied the reference interaction-site model (RISM) theory for estimatio n of dynamic response function of the average-energy relaxation. Ss(t), of solute-solvent systems induced by an instantaneous change of the solute ele ctronic structure in polar solvents. We selected solutes with square- or cu bic-like structures: initially all the atoms were electrically neutral and, then we applied delta (+) and delta (-) charges instantaneously. For the m ultipoles, Ss(t)s show apparent dependence on delta, which predicts nonline ar response of the solvent, because Ss(t) is a normalized quantity irrespec tive of delta 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 betw een the dynamic response functions obtained from spectral widths and that f rom spectral peak shifts implying a nonlinear solvent-response, as these tw o kinds of the response functions should again correspond to each other wit hin a linear response regime. Our present results based on the RISM theory can be another indication to confirm nonlinearity suggested by our experime nts. At the moment, we ascribe the origin of the nonlinear solvent behavior to the larger local-density fluctuation around the solute. (C) 2001 Elsevi er Science B.V. All rights reserved.