Mapping Proton-Coupled Electron Transfer With Real Space Coordinates.

Both electron transfer (ET) and proton transfer (PT) are common steps in energy conversion across chemistry and biology. Coupling of these two transfer events reduces the energy demand relative to either individual process, but introduces a fundamentally new process-proton-coupled electron transfer (PCET)-with its own demands for a suitable theoretical description. Conceptualization of PCET usually involves a square scheme representing ET, PT, and PCET steps, each generating states with different energies. While intuitive, these square schemes do not offer structural insights such as the identities and contributions of nuclear motions to PCET. Herein, we present a computational approach that maps these square schemes onto real space coordinates (Å), from which ground and excited-state potential energy surfaces can be generated. This mapping involves the identification of PT and ET coordinates and reconstruction of the potential energy surfaces in orthogonalized coordinates. We find qualitative differences in key features of the surfaces for two distinct processes within PCET, namely concerted proton-electron transfer and hydrogen atom transfer, which may help to distinguish different PCET scenarios.