Reconciling Imbalanced and Nonadiabatic Reactivity in Transition Metal–Oxo-Mediated Concerted Proton Electron Transfer (CPET)

Recently, there have been several experimental demonstrations
of
how the rates of concerted proton electron transfer (CPET) are affected
by stepwise thermodynamic parameters of only proton (ΔG°PT) or electron (ΔG°ET) transfer. Semiclassical structure–activity
relationships have been invoked to rationalize these linear free energy
relationships, but it is not clear how they would manifest in a nonadiabatic
reaction. Using density functional theory calculations, we demonstrate
how a decrease in ΔG°PT can
lead to transition state imbalance in a nonadiabatic framework. We
then use these calculations to anchor a theoretical model that reproduces
experimental trends with ΔG°PT and ΔG°ET. Our results reconcile
predictions from semiclassical transition state theory with models
that treat proton transfer quantum mechanically in CPET reactivity,
make new predictions about the importance of basicity for uphill CPET
reactions, and suggest similar treatments may be possible for other
nonadiabatic reactions.