Concerning the Radical Character of Superoxide. The H—O Bond Energy of HO−2

The strength of the H-O bond in H−O2 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 HO2 is known. A recent experimental determination of the electron affinity of HO2 [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 HO2. 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 HO2 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.