PROTON-COUPLED ELECTRON TRANSFER: A Reaction Chemist's View

Proton-coupled electron transfer (PCET) reactions involve the concerted transfer of an electron and a proton. Such reactions play an important role in many areas of chemistry and biology. Concerted PCET is thermochemically more favorable than the first step in competing consecutive processes involving stepwise electron transfer (ET) and proton transfer (PT), often by ≥1 eV. PCET reactions of the form X–H + Y → X + H–Y can be termed hydrogen atom transfer (HAT). Another PCET class involves outersphere electron transfer concerted with deprotonation by another reagent, Y⁺ + XH–B → Y + X–HB⁺. Many PCET/HAT rate constants are predicted well by the Marcus cross relation. The cross-relation calculation uses rate constants for self-exchange reactions to provide information on intrinsic barriers. Intrinsic barriers for PCET can be comparable to or larger than those for ET. These properties are discussed in light of recent theoretical treatments of PCET.