Chapter 13Proton-Coupled Electron Transfers

The coupling between electron and proton transfers has a long experimental and theoretical history inchemistryandbiochemistry.Proton-coupledelectrontransfer(PCET)reactionsalsoplayacriticalroleinawiderangeofbiologicalprocesses,includingenzymereactions,photosynthesis,and respiration.PCETisemployedhereasageneraltermforreactionsinwhichbothanelectronanda protonaretransferred,eitherintwodistinctstepsorinasinglestep[1].Wetermthelattermechanism concerted proton and electron transfer (CPET) [2]. Other terms have been used in the literature todescribethesamemechanism:electrontransfer-protontransfer(ETPT)[3],orelectron-proton transfer(EPT)[4],ormultiplesite-electronprotontransfer(MS-EPT)[5].Reactionsinwhichthe electronandprotontransfersoccurbetweenthesamedonorandacceptor,thatis,hydrogen-atom transfer,arenotconsideredhere.WearemainlyinterestedinelectrochemicalPCETreactionsin which electrons are Žowing into or from an electrode, while protons are transferred between an acid and a base.