Peroxyl radical
A superoxide, also known by the obsolete name hyperoxide, is a compound that contains the superoxide anion with the chemical formula O−
2. The systematic name of the anion is dioxide(1−). The reactive oxygen anion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. Whereas molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, the addition of a second electron fills one of its two degenerate molecular orbitals, leaving a charged ionic species with single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism.
Superoxides are compounds in which the oxidation number of oxygen is − 1⁄2. The O–O bond distance in O−
2 is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O2−
2.
The salts CsO2, RbO2, KO2, and NaO2 are prepared by the direct reaction of O2 with the respective alkali metal. The overall trend corresponds to a reduction in the bond order from 2 (O2), to 1.5 (O−
2), to 1 (O2−
2).
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