An a b i n i t i o study of aqueous Fe2+–Fe3+ electron transfer

Abstract

The tunneling rate for the transfer of an electron from an Fe2+ to an Fe3+ ion is determined by a Hamiltonian matrix element, which has been calculated for interionic distances in the range 5.10–7.22 Å. The ions were either alone, or surrounded by water molecules. The calculations were made by computing the electronic wave function of the system by the complete active space self-consistent field (CASSCF) method, and then evaluating the relevant matrix element for this wave function interacting with a symmetry image of itself, representing the final state. Basis set dependence, and in particular the effect of bond functions, was investigated, as well as the effect of removing some water molecules or replacing them with point charges. The results seem to indicate that earlier calculated values, which are frequently used, of the tunneling rate may be too large by as much as a factor of 10. If so, then the agreement between earlier calculated electron transfer rates and experimental data may be explained by compensating deficiencies in the assumed statistical distribution of interionic distances.