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.
This project increased fundamental understanding of chemical reactions involving the transfer of electrons and protons. These types of reactions play an important role in biological processes, such as respiration and photosynthesis, and in chemical processes relevant to solar cells and other energy devices. Theoretical equations were derived to allow the calculation of the rates of these reactions, and computational methods were developed to simulate their mechanisms. These methods were applied to experimentally relevant systems to assist in the interpretation of experimental data. In particular, the calculations elucidated the unusual temperature dependence of biomimetic systems for quinol oxidation and the asymmetry of the current versus potential plots for osmium complexes attached to gold electrodes. The theoretical and computational methods developed within this project are applicable to a wide range of other chemical and biological systems. This project provided training for graduate students and postdoctoral researchers, who learned about deriving theoretical equations, computer programming, and algorithm development. The older graduate students and postdoctoral researchers also gained experience in mentoring the younger graduate students. All of the people working on this project have learned how to connect theoretical calculations to experimentally relevant systems and how to obtain experimentally testable predictions from theoretical calculations. They have also learned to present their results through posters and oral presentations. One postdoctoral researcher working on this project helped coach the Science Bowl team at a local high school. He worked with the team on a weekly basis and traveled with them to regional and national competitions. His active involvement brought enthusiasm and intellectual stimulation to the high school students on the team.
