Unstable intermediates. Part CLXIII. Methyl radical anion adducts: radiation effects on dimethyl sulphoxide, dimethyl sulphone, and their solutions in water and methanol

Abstract

Exposure of pure dimethyl sulphoxide to 60Co γ-rays at 77 K gave no detectable e.s.r. features for methyl radicals, but solutions of (CH3)2SO in (CD3)2SO gave methyl radicals with normal e.s.r. parameters. In contrast, as shown by Chung et al., ĊD3 radicals in this medium have an abnormally low 2H hyperfine coupling (2.99 G) which is taken as evidence for ‘adduct’ formation (ĊD3⋯SOCD3–). Both sulphoxides in methanol gave normal methyl radicals (ĊH3 and ĊD3), but in neutral and alkaline aqueous glasses ĊH3 and ĊD3 radicals formed as adducts. Dimethyl sulphone gave abnormal methyl radicals in the pure state, and both normal and abnormal methyl radicals in methanolic solutions. A 33S hyperfine coupling of 22 G was detected for these adducts [·ĊH3⋯ S(O)2CH3–]. A previously unidentified radical in irradiated trimethylphosphine oxide, having a(CH3)ca. 20 and a(31P)ca, 72 G is identified as the adduct H3C⋯:P(O)(CH3)2–. A variety of other radicals were detected in irradiated dimethyl sulphoxide and its solutions, including MeṠO, Me2ṠO+, and the dimer cation Me2S(O)-S(O)Me2+. The parent anion, Me2ṠO2–, has been identified as a primary product in irradiated dimethyl sulphone.