Dependence of Vibronic Coupling on Molecular Geometry and Environment: Bridging Hydrogen Atom Transfer and Electron–Proton Transfer

Abstract

The rate constants for typical concerted proton-coupled electron transfer (PCET) reactions depend on the vibronic coupling between the diabatic reactant and product states. The form of the vibronic coupling is different for electronically adiabatic and nonadiabatic reactions, which are associated with hydrogen atom transfer (HAT) and electron–proton transfer (EPT) mechanisms, respectively. Most PCET rate constant expressions rely on the Condon approximation, which assumes that the vibronic coupling is independent of the nuclear coordinates of the solute and the solvent or protein. Herein we test the Condon approximation for PCET vibronic couplings. The dependence of the vibronic coupling on molecular geometry is investigated for an open and a stacked transition state geometry of the phenoxyl-phenol self-exchange reaction. The calculations indicate that the open geometry is electronically nonadiabatic, corresponding to an EPT mechanism that involves significant electronic charge redistribution, while the stacked geometry is predominantly electronically adiabatic, corresponding primarily to an HAT mechanism. Consequently, a single molecular system can exhibit both HAT and EPT character. The dependence of the vibronic coupling on the solvent or protein configuration is examined for the soybean lipoxygenase enzyme. The calculations indicate that this PCET reaction is electronically nonadiabatic with a vibronic coupling that does not depend significantly on the protein environment. Thus, the Condon approximation is shown to be valid for the solvent and protein nuclear coordinates but invalid for the solute nuclear coordinates in certain PCET systems. These results have significant implications for the calculation of rate constants, as well as mechanistic interpretations, of PCET reactions.

Hydrogen Atom Transfer (HAT) Processes Promoted by the Quinolinimide-N-oxyl Radical. A Kinetic and Theoretical Study

“…This finding is consistent with the calculated structure for the HAT identity reaction for the toluene/benzyl 27 and phenol/phenoxyl 28 couples. As was discussed in ref 8, the overlap of the aromatic moieties results in some degree of noncovalent bonding interaction between them within the TS structure, thereby stabilizing the cisoid TS structure preferentially with respect to the transoid structure.…”

Diabatic model for electrochemical hydrogen evolution based on constrained DFT configuration interaction

“…version of the open source quantum chemistry software CP2K. 44,45 We validated our implementation by reproducing some of the results of Hammes-Schiffer et al [46][47][48] for the PCET self-exchange reaction in the phenoxyl-phenol system. The exact details and results of these simulations are presented in the Supplementary Material.…”

Methane activation by gold-doped titanium oxide cluster anions with closed-shell electronic structures

“…Such a transfer mode can be called an electron–proton transfer (EPT). 45 It was proposed that a key element in the theoretical characterization of the mechanisms of proton and electron transfer is the formulation of their localized diabatic states. 46 However, the electron and proton described by standard quantum mechanical methods tend to be delocalized, and the analysis of the electron or proton acceptors such as a molecular orbital or a chemical bond depends on the adopted computational level of theory.…”