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.