Noncontinuum solvent effects upon the intrinsic free-energy barrier for electron-transfer reactions

Abstract

A phenomenological electrochemical approach is outlined by which noncontinuum contributions to the outer-shell intrinsic barrier to electron transfer, ..delta..G/sub os//sup */, resulting from specific reactant-solvent interactions can be estimated from the measured dependence of the formal potential upon the molecular and structural properties of the solvent. A simplified derivation, based on electrochemical half-reactions, of the conventional dielectric continuum expression is given in order to clarify the physical origins of the outer-shell intrinsic barrier and to identify likely additional noncontinuum components. Numerical calculations for amine and other redox couples involving specific ligand-solvent interactions indicate that the noncontinuum contributions to ..delta..G/sub os//sup */ for both homogeneous and electrochemical exchange reactions can be surprisingly small (typically approximately less than or equal to 1-2 kcal mol/sup -1/) even when the thermodynamics of ion solvation are in severe disagreement with the dielectric continuum (Born) predictions. An additional noncontinuum component associated with vibrational distortions of outer-shell solvent may be significant for multicharged aquo complexes and other reactants engaging in strong ligand-solvent hydrogen bonding. 50 references, 3 figures, 1 table.