Ab initio calculation of homogeneous outer sphere electron transfer rates:Application to M(OH2)(6)(3+/2+) redox couples

Ab initio density functional theory calculations are applied to the predict ion of homogeneous outer sphere electron transfer rates within the classica l Marcus formalism for a series of transition metal hexaquo ions in a backg round electrolyte. Reorganization energies, frequency factors, electronic t ransmission coefficients, and the effective electron transfer distances are calculated. Theoretical inner sphere contributions to the reorganization e nergies correlate very well with total reorganization energies estimated fr om experimental self-exchange rates. important energy contributions arising from Jahn-Teller distortions are accurately included in the inner sphere t erm. Effective electron transfer distances are found to be only slightly lo nger than the sum of the average calculated M-O distances. Calculated adiab atic self-exchange rates agree well with observed self-exchange rates. The driving force for bimolecular electron transfers, calculated from total ene rgy differences, is found to compare well with estimations using experiment al reduction potentials to within 4 kJ/mol. The choice of basis set is foun d to be very important in these calculations, and for this system, the 6-31 1+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.