The Role of Electronic Structure Calculation in Mechanistic Analysis of Electron Transfer Reactions in the Liquid Phase

Model calculations have been employed in elucidating the mechanism of electron transfer reactions in aqueous solution. The contribution of inner shell OH bonds to activation barriers has been estimated from calculation for metal ion hydrates. Calculated electron transfer matrix elements (Haf) for redox processes of the type, ML6 2+ + ML6 3+ = ML6 3+ + ML6 2+, M = Fe, Co, or Ru, L = H2O or NH3, have been analyzed in terms of various orbital concepts. The matrix elements are based on ab initio wavefunctions for model supermolecule clusters of the type, (MLn •••LnM)5+ with n = 1 or 3. The analysis shows that the many-electron Hif quantities can in fact be expressed to good approximation as effective 1-electron expressions of the type, Hif ∝ (λ↑)2 NchLlLr, where λ ’is the metal-ligand covalency parameter, hLlLr is a local 1-electron matrix element for ligand orbitals in contact in the transition state, and Nc is the number of such contacts. A least-squares fit of the data implies a value of ~ 5000 cm-1 for hLlLr showing that significant coupling can occur in the absence of formal bonding between reactants.