Proton-Coupled Electron Transfer (PCET) chemical reactions involve movement of electron(s) and proton(s). Many processes involve PCET, including energy conversions in fuel cells and the mitochondrial electron transport chain, chemical processes in the environment, and recent applications in organic synthesis and nanochemistry. Every science student learns that a hydrogen atom is a proton and an electron (H• = H+ + e–) but there are broad implications of that simple statement. Hydrogen atom transfer (HAT) is perhaps the simplest kind of PCET, where the H+ and e– transfer ‘together,’ whatever that means for two quantum particles. PCET can also involve H+ and e– that are separated in space and/or in time.
This Bailar lecture will develop the fundamentals of PCET chemistry in a variety of contexts. In particular, experimental studies of molecular systems will develop the relationships among mechanisms, rates, free energies of reaction, and the component ∆G°’s for H+ and e– transfer. These studies have probed the influence of proton wavefunction overlaps, intrinsic barriers, and other features of Hammes-Schiffer’s PCET theory. Results include first examples of PCET in the Marcus Inverted Region, and of homolytic cleavage of a C–H bond by concerted but separated e– and H+. The insights from these systems are directly applicable to hydrogen at solid/solution interfaces, such as at aqueous gold colloids and nickel oxide thin film electrodes. Hydrogen is ubiquitous on and in materials, but most surface and materials analysis tools are not sensitive to H. Using chemical and electrochemical experiments, we have quantified the stoichiometries, surface–H binding energies, and the reactivity of H on a variety of materials.