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

The electronic coupling element for electron transfer between a donor and a cceptor 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 potentia ls. Following equilibration, molecular dynamics simulations are performed w ith the donor, acceptor, and water at approximately 300 K, under periodic b oundary conditions. In the second phase of the simulation, the electronic c oupling element between the donor and acceptor is calculated for a number o f time slices, in the presence of the intervening water molecules (those ha ving 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 g iven donor-acceptor separation. The value for beta using INDO wave function s 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 suggest s that the ab initio beta value would be in the range of 1.5-1.8 Angstrom(- 1).