Solvent effects on electron-transfer kinetics: a correlation of rate constants with solvent acidity, basicity, and polarizability parameters

The solvent-dependent dielectric constant term (1/{epsilon}{sub 0}-1/{epsilon}) in the Marcus expression for reorganizational energy can be replaced by a measure of the solvent-solvent-solute interactions from the Taft theory. The Taft theory describes the acidity, basicity, and polarizability of the solvent. It provides a very good model for the variation of electron-transfer rate constants of outer-sphere reactions in a variety of solvents. The improved correlation between predicted and theoretical rate constants for the Taft model over the dielectric continuum model arises because of the former model's recognition of specific solvent-solute interactions in the formation of the transition state. Several examples of electron-transfer reactions in different solvents are analyzed to demonstrate the greatly improved correlation between observed and predicted rate constants using the Taft model.