Nonreactive dynamics in solution: The emerging molecular view of solvation dynamics and vibrational relaxation

Two of the more fundamental ways in which molecules change their behavior when they are dissolved are that they can begin to exchange energy with the surrounding liquid and they can induce their surroundings to rearrange so as to provide a significant stabilizing influence. The first of these is typified by the process of vibrational population relaxation of a vibrationally hot species. The second concept - critical to solution chemistry - is what is known as solvation. Both of these processes are sufficiently fundamental that one would really like to know, at the most mechanical and molecular level possible, just what events are required in order to make them happen. But how difficult is it going to be to extract such molecular detail from the complicated many-body dynamics? The most microscopic level of understanding one could ever hope to possess might seem far removed from the finely detailed dynamical information which is available routinely for individual isolated molecules and for molecule - molecule collisions in molecular beams. It might even seem that the broad, almost featureless character of typical solute spectra would condemn us to never being able to measure anything more than a few paultry tidbits of highly averaged data caricaturing the intriguing processes that can take place in liquids. However, spectroscopists have for some time been able to infer at least some aspects of the dynamics of liquids from the spectroscopy of dissolved molecules, and with the advent of novel ultrafast time-resolved spectroscopies and new theoretical perspectives, the likelihood of resolving solution dynamics into genuinely molecular components has increased dramatically. We discuss a few of these recent developments here for the special, but nonetheless illuminating, cases of solvation dynamics and vibrational relaxation and note a few of the more promising directions that future work might take.