Ab initio calculation of homogeneous outer sphere electron transfer rates:: Application to M(OH2)63+/2+ redox couples

Ab initio density functional theory calculations are applied to the prediction of homogeneous outer sphere electron transfer rates within the classical Marcus formalism for a series of transition metal hexaquo ions in a background electrolyte. Reorganization energies, frequency factors, electronic transmission coefficients, and the effective electron transfer distances are calculated. Theoretical inner sphere contributions to the reorganization energies correlate very well with total reorganization energies estimated from experimental self-exchange rates. important energy contributions arising from Jahn-Teller distortions are accurately included in the inner sphere term. Effective electron transfer distances are found to be only slightly longer than the sum of the average calculated M-O distances. Calculated adiabatic self-exchange rates agree well with observed self-exchange rates. The driving force for bimolecular electron transfers, calculated from total energy differences, is found to compare well with estimations using experimental reduction potentials to within 4 kJ/mol. The choice of basis set is found to be very important in these calculations, and for this system, the 6-311+G basis set outperforms DZVP. The methods presented provide a convenient means to produce usefully accurate parameters for Marcus theory to predict outer sphere electron transfer rates.