Hydrogen-abstraction reactivity patterns from A to Y: The valence bond way

"Give us insight, not numbers" was Coulson's admonition to theoretical chemists. This Review shows that the valence bond (VB)-model provides insights and some good numbers for one of the fundamental reactions in nature, the hydrogen-atom transfer (HAT). The VB model is applied to over 50 reactions from the simplest H + H ₂ process, to P450 hydroxylations and H-transfers among closed-shell molecules; for each system the barriers are estimated from raw data. The model creates a bridge to the Marcus equation and shows that H-atom abstraction by a closed-shell molecule requires a higher barrier owing to the additional promotion energy needed to prepare the abstractor for H-abstraction. Under certain conditions, a closed-shell abstractor can bypass this penalty through a proton-coupled electron transfer (PCET) mechanism. The VB model links the HAT and PCET mechanisms conceptually and shows the consequences that this linking has for H-abstraction reactivity. Give us insight and good numbers: Is it possible to understand one of the fundamental reactions in nature, the hydrogen-atom transfer (HAT), using a single unifying theory? The valence bond diagram model is such a unifying theory. It enables the barriers to be estimated from raw data (see plot), thereby creating a natural bridge to the Marcus equation, addresses the relationships between HAT to proton-coupled electron transfer (PCET), and shows how H-abstractions by closed-shell molecules can occur.