Kinetic isotope effects in proton coupled electron transfer

Abstract

Simple mathematical expressions describing the thermodynamic and kinetic dependencies of proton coupled electron transfer (PCET) processes have been used to examine the effects of electrolyte composition. Two charge transfer mechanisms have been considered, (i) a stepwise (sw) process and (ii) a concerted (cc) process involving the initial formation of a complex between the redox center and a proton donating/accepting species from the electrolyte. The hydrogen/deuterium kinetic isotope effect (KIE) for both the stepwise and concerted PCET mechanisms has been discussed by considering the variation in pKa for solutes dissolved in light water and heavy water. Simulations demonstrate that different types of kinetic isotope effects can be observed for both stepwise and concerted pathways and, under certain conditions, the KIE of the former can be expected to be of equal magnitude to the latter. The KIE in a monolayer system of aminobenzoquinone has been studied in buffered water and heavy water electrolytes. The results are strongly in accordance with the fact that acid dissociation constants shift in D2O compared to H2O, and this leads to an appreciable KIE even when the stepwise PCET mechanism is operative.