TY - JOUR
T1 - Blended hydrogen atom abstraction and proton-coupled electron transfer mechanisms of closed-shell molecules
AU - Li, Chunsen
AU - Danovich, David
AU - Shaik, Sason
PY - 2012/6
Y1 - 2012/6
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=84864210472&partnerID=8YFLogxK
U2 - 10.1039/c2sc20115a
DO - 10.1039/c2sc20115a
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AN - SCOPUS:84864210472
SN - 2041-6520
VL - 3
SP - 1903
EP - 1918
JO - Chemical Science
JF - Chemical Science
IS - 6
ER -