Electron-proton nonadiabaticity: characterization and development of non-Born-Oppenheimer electronic structure methods
Nuclear quantum effects such as zero-point energy and hydrogen tunneling play an important role in a wide variety of chemical reactions. Moreover, non-Born-Oppenheimer effects are important in reactions such as proton-coupled electron transfer (PCET), which are integral to various electrocatalytic applications and bioenzymatic processes. The breakdown of the Born-Oppenheimer approximation between electronic and nuclear motions engenders the need for accurate characterization of the degree of nonadiabaticity. Furthermore, in regimes where the inclusion of these effects is vital, as it is for PCET systems, the development of non-Born-Oppenheimer quantum chemical methods is increasingly important. In this dissertation, we present diagnostics of electron-proton nonadiabaticity that can be obtained from standard electronic structure calculations and describe their application to representative systems, highlighting the mechanistic differences between two subclasses of PCET. In addition, we describe the development of new electronic structure methods within the nuclear-electronic orbital (NEO) framework, which is an orbital-based approach that inherently includes electron-proton nonadiabaticity by treating electrons and select protons quantum mechanically on equal footing. Previous studies using NEO involved applying mean-field-based approaches, which lacked sufficient electron-proton dynamical correlation, leading to overlocalized nuclear densities. Subsequent efforts focused on the development of explicitly correlated NEO approaches which, although accurate, were too computationally intractable to be practical for the study of PCET systems. In this dissertation, we describe two approaches to develop tractable NEO methods. Firstly, we describe the formulation of a multi-component density functional theory approach within the NEO framework, which involves the derivation of several electron-proton correlation functionals to accurately account for electron-proton correlation. Secondly, we describe in detail a novel NEO method: the reduced explicitly correlated Hartree-Fock (RXCHF) approach, which is a wavefunction-based approach that accurately accounts for electron-proton dynamical correlation between a subset of the electronic orbitals and the quantum nuclear orbitals. Systematic approximations for the RXCHF methods that afford substantial gains in computational tractability will be described, and model calculations demonstrating the applicability of this method to systems with select protons treated quantum mechanically will be presented. This method will enable accurate calculations on PCET systems as electron-proton nonadiabatic effects are inherently included, and it can provide fundamental insight into these types of mechanisms. In addition, the NEO-RXCHF method readily affords a framework within which experimentally accessible quantities such as rate constants and isotope effects can be determined for PCET reactions.
proton-coupled electron transfer, quantum chemistry, non-Born-Oppenheimer, nonadiabatic, density functional theory, explicitly correlated wavefunctions, hydrogen atom transfer, diabatization, nuclear quantum effects
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign