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Concerning the Radical Character of Superoxide. The H—O Bond Energy of HO − 2
Author(s) -
Liebman Joel F.,
Valentine Joan S.
Publication year - 1983
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198300063
Subject(s) - chemistry , reactivity (psychology) , bromine , radical , bond energy , superoxide , electron affinity (data page) , hydrogen atom , atom (system on chip) , crystallography , molecule , organic chemistry , alkyl , alternative medicine , pathology , computer science , embedded system , enzyme , medicine
The strength of the H‐O bond in H − O 2 should be a good indicator of the chemical reactivity of superoxide, O − 2 , as a hydrogen atom abstractor. This quantity can be calculated if the electron affinity of HO 2 is known. A recent experimental determination of the electron affinity of HO 2 [V. M. Bierbaum, R. J. Schmitt and C. H. DePuy, J. Am. Chem. Soc ., 103 , 6262–6263 (1981)] gave that value as 27.4 ± 0.2 kcal mol −1 . The H‐O bond energy of HO − − 2 can then be calculated from that value to be approximately 66 kcal mol −1 . Various theoretical methods may also be used to approximate the value of the electron affinity of HO 2 . These methods generally give values in accord with the experimental result. Based on the low H—O bond energy of 66 kcal mol −1 for HO − 2 , superoxide is expected to be relatively unreactive, comparable to iodine atoms but less reactive than bromine atoms or HO 2 and much less reactive than chlorine atoms or hydroxyl radicals. Based on our analysis, we predict that superoxide can only react as a hydrogen atom abstractor with substrates that contain relatively weak bonds to hydrogen such as hydroxylamine or hydroquinones.