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Issue 6, 2012
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Blended hydrogen atom abstraction and proton-coupled electron transfer mechanisms of closed-shell molecules

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Abstract

The paper addresses the surging topic of H-abstractions by closed-shell molecules, such as MnO4, 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. MnO4 abstracting a hydrogen atom from MnO4H·, 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.

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

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Publication details

The article was received on 27 Jan 2012, accepted on 07 Mar 2012 and first published on 07 Mar 2012


Article type: Edge Article
DOI: 10.1039/C2SC20115A
Citation: Chem. Sci., 2012,3, 1903-1918
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    Blended hydrogen atom abstraction and proton-coupled electron transfer mechanisms of closed-shell molecules

    C. Li, D. Danovich and S. Shaik, Chem. Sci., 2012, 3, 1903
    DOI: 10.1039/C2SC20115A

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