Theoretical Studies of Proton-Coupled Electron Transfer Reactions

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Sharon Hammes-Schiffer of Pennsylvania State University is supported by an award from the Theoretical and Computational Chemistry Program for research which seeks to continue her theoretical and computational investigations of proton-coupled electron transfer reactions (PCET). The work takes a three-pronged approach: i) the physical principles dictating temperature dependence and kinetic isotope effects are being elucidated; ii) nuclear-electronic (NEO) orbital methods are being modifed in order to allow for the calculation of vibronic couplings; and iii) theoretical methods for studying PCET reactions in electrochemistry are being developed.PCET reactions are essential for a variety of chemical and biological processes, including electrochemistry, photosynthesis, respiration, and enzyme reactions. The objective of the first project is to determine how temperature affects rates and kinetic isotope effects for PCET reactions in solution. Rate expressions have been derived and will be applied to two quinol oxidation reactions that have been shown experimentally to exhibit qualitatively different temperature dependences. This application will provide insight into fundamental aspects of PCET reactions and will facilitate the analysis and interpretation of other chemically and biologically significant applications. The second project will look at specific quantum mechanical aspects of the electron transfer process and will provide greater predictive power for the theory. The third project is aimed at developing theoretical methods for electrochemistry. The resulting theory will be applied to an experimentally studied PCET reaction consisting of an osmium complex attached to a gold electrode. These calculations will assist in the interpretation of electrochemical PCET data and will generate experimentally testable predictions. The general theoretical formulation for electrochemical PCET will provide the framework for a wide range of technologically important applications including solar energy conversion systems. The work is, thus, having a broader impact on developing technology for sustainable energy as well as through Hammes-Schiifer's mentoring activities and outreach efforts to local schools.