Theory of Coupled Electron and Proton Transfer Reactions

The theory of coupled electron and proton transfer reactions is discussed. A concerted PCET mechanism is defined as a PCET reaction that does not involve a stable intermediate arising from single electron or proton transfer. Concerted PCET reactions are described in terms of reactant and product states corresponding to the electron being localized on the donor or acceptor, respectively. The proton vibrational states can be calculated for the reactant and product electronic states, leading to two sets of electron-proton vibronic states. The proton donor is lower in energy when the electron is localized on its donor, whereas the proton acceptor is lower in energy when the electron is localized on its acceptor. The proton vibrational ground state wave function is localized near its donor in the reactant state and near its acceptor in the product state. PCET processes can exhibit large kinetic isotope effects and interesting dependence of the rate constants and kinetic isotope effects on temperature, pH, and driving force.