Free Energy Gap Law under Diffusion Control

The free energy gap law in forward and back electron transfer (bimolecular and geminate) is examined. Both processes are specified by position-dependent rates of remote electron transfer. These rates are used within encounter theory to calculate the bimolecular rate constants of ionization and recombination as well as the quantum yield for photochemical ion separation. The free energy gap law is shown to be a property of kinetic-controlled reactions. In the diffusion-controlled limit this law for either forward or backward reaction constants is violated at the top of Marcus' parabola. This conclusion pertains equally to an analogous quantity extracted from the separation quantum yield for geminate recombination.