NON-CONDON EFFECTS IN ADIABATIC BRIDGE-GROUP-ASSISTED ELECTRON-TRANSFER REACTIONS

The concept of adiabatic bridge-group-assisted electron transfer is important for inner sphere electron transfer reactions, electrochemical electron transfer of transition metal complexes involving adsorbed ligands or surface groups, and for bridge group transfer reactions. Such processes can be handled by a three-level scheme incorporating a high energy intermediate bridge group electronic state in addition to the donor and acceptor states. The level splitting is determined by coupling between the three levels and leads to a lowering of the activation barrier by a second-order, superexchange electron exchange factor in the adiabatic limit of strong interaction. This is equivalent to energy splitting by the first-order electron exchange factor in two-level adiabatic processes. We show here that the saddle-point on the overall Gibbs energy surface representing such a process and the corresponding activation Gibbs energy may not coincide with the same quantities obtained by the Condon approximation otherwise used widely. Deviation from the Condon approximation thus leads to considerable additional lowering of the activation Gibbs energy and easier electron transfer. This is an interesting difference from the case in diabatic reactions where non-Condon effects increase the activation energy.