Electronically adiabatic reaction field approach to solvation .2. Solvent effects on electronic spectra

The theoretical formulation developed in the preceding article [H. J. Kim, J. Chem. Phys. 105, 6818 (1996)] is analyzed via a second-order perturbation method and applied to the static electronic spectra of polarizable solutes in solution. In the Born-Oppenheimer (BO) framework of the solvent electronic polarization <(P)over arrow (el)>, the solute electronic wave functions, together with their (free) energy levels and associated Franck-Condon (FC) energies, are examined in the presence of a spherical cavity of arbitrary size and a nonequilibrium solvent orientational polarization configuration <(P)over arrow (or)>. It is found that the solute electronic structure and its free energetics vary strongly with both <(P)over arrow (or)> and the cavity size. The solute dipole enhancement due to solvation decreases with increasing cavity size. Comparison with the self-consistent (SC) reaction field theory predictions shows that classical <(P)over arrow (el)> is more effective in polarizing the solute than quantum <(P)over arrow (el)> couched in the BO description. This is due to the dispersion stabilization mechanism present in the latter. The static electronic spectroscopy is studied to linear order in the solute polarizability and in the cavity size difference between the lower and upper electronic states involved in the FC transition. In the case of the vanishing cavity size difference, our analytic results for the solvent spectral and Stokes shifts are compared with various existing theories and the sources of the discrepancies are briefly discussed. The effects of the cavity size variation on the electronic spectra are illustrated by using a simple two-state model description for the solute. It is found that even in a nonpolar solvent, there can be a significant Stokes shift arising from the cavity size relaxation subsequent to the FC transition. Also the cavity size fluctuations can make a non-negligible contribution to the spectral line broadening. (C) 1996 American Institute of Physics.