Dependenceof Vibronic Coupling on Molecular Geometryand Environment: Bridging Hydrogen Atom Transfer and Electron–ProtonTransfer

摘要

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, correspondingprimarily to an HAT mechanism. Consequently, a single molecular systemcan exhibit both HAT and EPT character. The dependence of the vibroniccoupling on the solvent or protein configuration is examined for thesoybean lipoxygenase enzyme. The calculations indicate that this PCETreaction is electronically nonadiabatic with a vibronic coupling thatdoes not depend significantly on the protein environment. Thus, theCondon approximation is shown to be valid for the solvent and proteinnuclear coordinates but invalid for the solute nuclear coordinatesin certain PCET systems. These results have significant implicationsfor the calculation of rate constants, as well as mechanistic interpretations,of PCET reactions.