SHORT-TIME DYNAMICS OF SOLVATION - RELATIONSHIP BETWEEN POLAR AND NONPOLAR SOLVATION

The microscopic details of how a solution responds to changes in a sol ute are now becoming experimentally accessible at the kinds of times t hat should allow us to follow even the earliest events in solvation. F or time scales this short there is a genuine chance that one can ident ify actual elementary events in the solvation process, meaning that on e can begin to think about explicit solvation mechanisms-the specific molecular motions that comprise the crucial steps in the process. Most of the current theories of solvation dynamics, however, try to resolv e this early-time dynamics by looking at finer and finer details of th e dielectric response of the bulk solvent, an approach which not only seems to be starting from the opposite extreme of the behavior one is trying to understand, but which erects an artificial conceptual barrie r between the solvation processes of polar and nonpolar liquids. We su ggest that, at least at the times of interest for questions of solvati on mechanism, the distinction between polar and nonpolar solvents is s uperficial. The ultrafast dynamics of both kinds of solvents are more naturally regarded in terms of their instantaneous normal modes-which can be further dissected into contributions from such mechanistic elem ents as solvent libration and solvent translation, and even into contr ibutions from individual solvent shells surrounding the solute, if so desired. We show how, from the perspective of this kind of analysis, a simple scaling argument makes it clear why solvent libration is usual ly, but not always, the most efficient route to solvation-and why the important distinctions are not between the different families of solve nts, but between the differing symmetries of the various solute-solven t interactions that one can choose to monitor experimentally. We illus trate these ideas by performing an instantaneous-normal-mode analysis of the manifestly nonpolar situation of I-2 dissolved in liquid CO2, a n example deliberately chosen to contrast with our previous study of d ipolar solvation in CH3CN. In accordance with the predictions of the s imple scaling argument, the primary solvation mechanism shifts from Li bration to center-of-mass translation as the solute-solvent interactio n being monitored is changed from being multipolar in character (dipol ar or quadrupolar) to something more symmetric. We find, moreover, tha t the range of the solute-solvent interaction is of no more than secon dary importance in understanding the solvation mechanism: Coulombic (l /r) potentials behave little differently than dispersion (l/r(6)) pote ntials in their libration/translation preferences, and both exhibit a prompt solvation process dominated by the first solvation shell, Much the same kind of analysis can be applied to the question of whether so lute motion is an important part of solvation: although unfreezing the solute will always allow for faster solvent response, we show that th e extent of the effect can be quantitatively predicted by comparing th e solute's mass and moment of inertia with that of the solvent, leadin g us to expect that solute motion should be a rather minor component i n most current experiments.