A theoretical study of the electronic coupling element for electron transfer in water

The electronic coupling element for electron transfer between a donor and acceptor in water is examined using simulations combining molecular dynamics and semiempirical quantum mechanics. In the first phase of the simulations a model donor and acceptor are solvated in water, using realistic potentials. Following equilibration, molecular dynamics simulations are performed with the donor, acceptor, and water at approximately 300 K, under periodic boundary conditions. In the second phase of the simulation, the electronic coupling element between the donor and acceptor is calculated for a number of time slices, in the presence of the intervening water molecules (those having a nonnegligible effect on the coupling element at the given distance). Finally, a subset of these configurations is used to investigate the donor-acceptor energy dependence of the coupling by varying the model donor and acceptor. It is found, contrary to a number of previous theoretical results, that water significantly increases the electronic coupling element at a given donor-acceptor separation. The value for beta using INDO wave functions is estimated to be 2.0 Angstrom(-1) and is found to depend weakly on the identity of the donor and acceptor. Comparison with ab initio results for a subset of the configurations or using idealized solvent geometries suggests that the ab initio beta value would be in the range of 1.5-1.8 Angstrom(-1).