Short-Time Dynamics of Solvation: Relationship between Polar and Nonpolar Solvation

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Abstract

The microscopic details of how a solution responds to changes in a solute are now becoming experimentally accessible at the kinds of times that should allow us to follow even the earliest events in solvation. For time scales this short there is a genuine chance that one can identify actual elementary events in the solvation process, meaning that one can begin to think about explicit solvation mechanismsthe specific molecular motions that comprise the crucial steps in the process. Most of the current theories of solvation dynamics, however, try to resolve this early-time dynamics by looking at finer and finer details of the 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 barrier between the solvation processes of polar and nonpolar liquids. We suggest that, at least at the times of interest for questions of solvation mechanism, the distinction between polar and nonpolar solvents is superficial. The ultrafast dynamics of both kinds of solvents are more naturally regarded in terms of their instantaneous normal modeswhich can be further dissected into contributions from such mechanistic elements as solvent libration and solvent translation, and even into contributions 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 usually, but not always, the most efficient route to solvationand why the important distinctions are not between the different families of solvents, but between the differing symmetries of the various solute−solvent interactions that one can choose to monitor experimentally. We illustrate these ideas by performing an instantaneous-normal-mode analysis of the manifestly nonpolar situation of I2 dissolved in liquid CO2, an example deliberately chosen to contrast with our previous study of dipolar solvation in CH3CN. In accordance with the predictions of the simple scaling argument, the primary solvation mechanism shifts from libration to center-of-mass translation as the solute−solvent interaction being monitored is changed from being multipolar in character (dipolar or quadrupolar) to something more symmetric. We find, moreover, that the range of the solute−solvent interaction is of no more than secondary importance in understanding the solvation mechanism: Coulombic (1/r) potentials behave little differently than dispersion (1/r6) potentials 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 solute motion is an important part of solvation: although unfreezing the solute will always allow for faster solvent response, we show that the extent of the effect can be quantitatively predicted by comparing the solute's mass and moment of inertia with that of the solvent, leading us to expect that solute motion should be a rather minor component in most current experiments.

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