Theoretical studies of proton-coupled electron transfer in interfacial and photochemical energy conversion processes

Abstract

Renewable energy technologies, particularly solar, rely on processes that convert energy from and store energy in chemical bonds. Therefore, understanding and optimizing these reactions is critical to the efficient transduction of solar energy. Intrinsic to these technological reactions as well as those nature is the directed translocation of electrons and protons. These two fundamental chemical reagents often transfer concertedly and as such the kinetics of these reactions require special attention. In this dissertation, computational and theoretical characterizations of proton-coupled electron transfer (PCET) reactions in electro- and photochemical energy conversion processes are presented. A Co complex that performs highly selective, low-overpotential oxygen reduction to hydrogen peroxide was analyzed with density functional theory calculations. These calculations along with experimental kinetics elucidated a likely mechanism for this efficient transformation based on a rate-determining second proton transfer to a Co-hydroperoxo intermediate. In heterogeneous catalysis, a density functional theory characterization of the promising NiFe oxyhydroxide oxygen-evolving electrocatalyst is presented. Spectroelectrochemistry and calculations demonstrated the likely involvement of Fe4+ sites under catalytic current-carrying conditions. The evolution of hydrogen on Au electrodes by triethylammonium in acetonitrile solvent demonstrates a uniquely potential-dependent kinetic isotope effects. Numerical models for the nonadiabatic cathodic rate constant of proton discharge on an electrode were employed to understand this phenomenon as differing contributions of vibronic states for a transferring proton and deuteron. In addition, computational and theoretical models of anthracene-phenol-pyridine unimolecular triads complemented transient spectroscopies in an investigation of photoinduced PCET. Experiment and nonadiabatic rate theory jointly demonstrated the Marcus inverted region for the charge recombination reaction that follows photoexcitation of the triad. A model system study further described the physical criteria for observing inverted region kinetics for PCET. Computational and numerical models are very valuable to understanding experimental phenomena and elucidating molecular and materials design principles for even more efficient and selective energy transduction. This dissertation studies many aspects of energy conversion reactions including electronic structure, atomistic molecular and interfacial structures, reaction thermodynamics and mechanisms, and nonadiabatic rate theories in various limits. Theoretical understandings of each of these components of PCET reactivity, many in conjunction with or inspired by novel experiments, are presented herein.