Theoretical formulation for electron transfer coupled to multiple protons: Application to amidinium-carboxylate interfaces

This paper presents a theoretical formulation for electron transfer coupled to the motion of multiple protons. This theory is applied to proton-coupled electron transfer (PCET) through amidinium-carboxylate salt bridges, where the electron transfer reaction is coupled to the motion of two protons at the proton transfer interface. The rate for the donor-(amidinium-carboxylate)-acceptor system is found to be substantially slower than the rate for the switched interface donor-(carboxylate-amidinium)-acceptor system. This trend is consistent with experimental data for photoinduced PCET in analogous systems. The calculations indicate that this difference in rates is due mainly to the opposite dipole moments at the proton transfer interfaces for the two systems, leading to an endothermic reaction for the donor-(amidinium-carboxylate)-acceptor system and an exothermic reaction for the donor-(carboxylate-amidinium)-acceptor system. The deuterium kinetic isotope effects are found to be moderate (i.e., k(H)/k(D)<3) for both types of systems. These moderate kinetic isotope effects are due to the dominance of vibrationally excited product states, leading to significant overlap between the reactant and product proton vibrational wave functions. (C) 2001 American Institute of Physics.