Blended hydrogen atom abstraction and proton-coupled electron transfer mechanisms of closed-shell molecules

The paper addresses the surging topic of H-abstractions by closed-shell molecules, such as MnO ₄ ^-, α-methylstyrene, ketones, metal-oxo reagents, etc. It is found that in the normal hydrogen atom transfer (HAT) regime, closed-shell abstractors require high barriers for H-abstraction. Under certain conditions a closed-shell abstractor can bypass this penalty via a proton-coupled electron transfer (PCET) mechanism. This occurs mainly in the identity reactions, e.g. MnO ₄ ^- abstracting a hydrogen atom from MnO ₄H ^-·, but not in the corresponding non-identity reactions with alkanes. The usage of the valence bond (VB) diagram model allows us to characterize the HAT/PCET mechanistic relationship and bridge their reactivity patterns. It is thus shown that in the normal HAT regime, high barriers for closed-shell abstractors occur due to the additional promotion energy that is required in order to create a radical center and "prepare" the abstractor for H-abstraction. Mixing of the PCET states into the HAT states mitigates however these high barriers. The variable HAT/PCET mixing in a reaction series is discussed and its consequences for reactivity are outlined. It is shown that non-identity reactions sample PCET characters that depend, among other factors, on the C-H bond strength of the alkane, and hence may cause the Marcus analysis to produce different identity barriers for the same identity reaction.