Mapping proton-coupled electron transfer with real space dimensions

Both electron transfer (ET) and proton transfer (PT) are common steps in energy conversion. Coupling of these two transfer events alleviates the energy demand relative to either individual process, but introduces a fundamentally new process—proton-coupled electron transfer (PCET)—with its own demands for theoretical description. Conceptualization of PCET usually involves a square scheme representing, ET, PT and PCET, each generating state of different energy. While intuitive, these square schemes do not offer detailed chemical insight—such as the identity and contribution of nuclear motions to PCET. Herein, we present a computational approach that maps these square schemes onto real space coordinates (Å), from which corresponding potential energy surfaces can be generated. This mapping involves the identification of PT and ET coordinates. Using the relative orientations of these coordinates, we propose a new criterion for ‘concerted’ proton-electron transfer, as well as disambiguate the concepts of concerted and coupled proton and electron transfer.