Radical reactions in aqueous solution: use of the aci-anion of nitromethane as a spin trap

Abstract

The type of radical detected by esr when first-formed short-lived radicals are generated by photolytic, radiolytic, or flow techniques in the presence of nitromethane is shown to depend upon the nature and structure of the transient radical and on the pH. Thus electron-donating radicals may react with CH3NO2 itself under certain conditions to give rise to CH3NO2−• and the alkoxy nitroxides CH3N(OR)O•. At higher pH, a wide variety of radicals (X) react very readily with the aci-anion CH2:NO2− to give adducts XCH2NO2−• (and, from radicals capable of one-electron transfer, O2NCH2CH2NO2−•). Structural characteristics of adducts XCH2NO2−• are described and it is shown how radicals which are normally undetectable in fluid solution can be trapped and recognized in this way. Mechanistic pathways diagnosed with this trap (involving, e.g. alkoxyl radicals, aroyloxyl radicals, aromatic carboxylate cation-radicals) are exemplified.