Model Proton-Coupled Electron Transfer Reactions in Solution:  Predictions of Rates, Mechanisms, and Kinetic Isotope Effects

This paper presents a comprehensive theoretical study of model systems directed at predicting the effects of solute and solvent properties on the rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer (PCET) reactions. These studies are based on a multistate continuum theory in which the solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the active electrons and transferring protons are treated quantum mechanically. This theoretical formulation is capable of describing a range of mechanisms, including single electron transfer and sequential or concerted EPT mechanisms in which both an electron and a proton are transferred. The probability of the EPT mechanism is predicted to increase as (1) the electron donor−acceptor distance is decreased, (2) the proton donor−acceptor distance is decreased, (3) the proton transfer reaction becomes more exothermic, (4) the electron transfer reaction becomes more endothermic (in the normal Marcus region), (5) the temperature decreases, (6) the solvent polarity decreases, and (7) the size of the electron donor and acceptor increases. The rates are predicted to increase with respect to these properties in a similar manner, with the exception that the rates will increase as the temperature increases and as the electron transfer reaction becomes more exothermic in the normal Marcus region. The kinetic isotope effects are predicted to increase as the probability of the EPT mechanism increases and as the localization and the distance between the reactant and product proton vibrational wave functions increase. Unusually strong kinetic isotope effects may be observed due to strong coupling between the transferring electron and proton. These theoretical studies elucidate the fundamental principles of PCET reactions and provide predictions that can be tested experimentally.