Noncontinuum Solvent Effects Upon the Intrinsic Free-Energy Barrier for Electron-Transfer Reactions

摘要

A phenomenological electrochemical approach is outlined by which 'noncontinuum' contributions to the outer-shell intrinsic barrier to electron transfer, 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 ammine and other redox couples involving specific ligand-solvent interactions indicate that the noncontinuum contributions to the outer-shell intrinsic barrier to electron transfer for both homogeneous and electrochemical exchange reactions can be surprisingly small (typically less than or = 1-2 kcal/mol) 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.