Theoretical study of photoinduced proton-coupled electron transfer through asymmetric salt bridges

The theoretical study in this paper is based on the experimental result that the rate of photoinduced electron transfer is ~ 10² times slower through a donor-(amidinium-carboxylate)-acceptor salt bridge than through the corresponding switched interface donor-(carboxylate-amidinium)-acceptor complex (Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997, 119, 9230). This experimental result indicates that the proton-transfer interface plays an important role in these electron-transfer reactions. In this paper, a multistate continuum theory for proton-coupled electron transfer (PCET) is applied to analogues representing the chemical systems studied in these experiments. The solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the active electrons and transferring proton(s) are treated quantum mechanically on equal footing. The application of this theory to these PCET systems provides adiabatic free energy surfaces that depend on two scalar solvent variables corresponding to the proton and electron transfer reactions. The theoretical analysis indicates that the experimentally observed difference in rates for the two chemical systems is due to differences in solute electrostatic properties, solvation energies, solvent reorganization energies, and electronic couplings. Moreover, this theoretical study provides insight into the underlying fundamental principles of PCET reactions.