Bihalide ion combination reactions in solution: electronic structure and solvation aspects

Abstract

The Kim—Hynes theory for electronic structure for reaction systems in solution is applied to the reaction class of the title, focusing on the I - +I → I 2 - reaction in acetonitrile solvent from a valence bond perspective. The transition between a delocalized electronic structure at small internuclear separations r to localized structures at large r is described in terms of a two-dimensional nonequilibrium free energy surface; the second coordinate is a solvent coordinate gauging the state of the solvent orientational polarization. The reaction path on this surface is described, and is contrasted with an equilibrium solvation perspective. An important focus is the polarization force, defined as the force on the diatomic ion coordinate r due to the solvent, which arises from the charge shifting, or electronic structure change, in the solute system along the reaction coordinate. It is argued that this force should play a significant role in the vibrational relaxation of the I 2 - ion.