SOLVATION BY NONPOLAR-SOLVENTS - SHIFTS OF SOLUTE ELECTRONIC-SPECTRA

It is only relatively recently that it has become possible to use spec troscopy to track the solvation of a molecule as one proceeds from sma ll solute-plus-solvent clusters, through bulk liquids, and into cryoge nic matrices. One of the more surprising findings of such studies is t hat, in a number of noteworthy instances - such as with benzene dissol ved in Ar - the solvent shifts of spectral lines in even apparently si zable clusters seem not to go smoothly into the bulk results. In this and the following paper we consider just what level of theoretical tre atment is necessary in order to be able to account for the solvent shi ft of electronic spectra consistently in environments ranging from clu sters to the bulk. As we discuss in some detail, neither continuum die lectric approaches nor sums of pair potentials can adequately describe the solvation. What we propose here, instead, is that the effects of nonpolar solvents can be treated fully microscopically by a model inco rporating both local repulsive effects and longer-ranged dielectric ef fects. The latter contribution, resulting from the solvent's polarizab ility, is formulated in terms of the so-called polarization modes of t he solvent, which change with the detailed arrangement of the solute's environment. We illustrate the ideas by showing that one can understa nd the optical spectroscopy of benzene in liquid Ar more or less quant itatively by using this model, and we point out some connections with analogous time-dependent solvation studies. The application of this sa me approach to clusters is described in the succeeding paper.