Radical cations of trialkylphosphine oxides, trialkylphosphates, hexamethylphosphoramide, dimethyl sulphoxide and various sulphones, sulphites and sulphates. An electron spin resonance and radiation chemical study

Abstract

Exposure of dilute solutions of the title compounds in trichlorofluoromethane to ⁶⁰Co γ-rays at 77 K or to X-rays at ca. 4 K gave species identified as radical cations by e.s.r. spectroscopy. The parent cations were formed from dimethyl sulphoxide, dimethyl sulphite, tetramethylene sulphone and hexamethylphosphoramide. Trimethylphosphate and dimethylsulphate gave well defined solvent adducts with large hyperfine coupling to a single chlorine nucleus. This result is surprising for the latter cation since it implies localisation of the semi-occupied molecular orbital (SOMO) on one of the two oxygen ligands, which should share the unpaired electron equally in the uncomplexed cation. In the other cases hydrogen-atom migration dominated, giving carbon-centred radicals. This change was also found for the cation complex of trimethylphosphate and dimethylsulphate on photolysis or annealing. For the hexamethyl phosphoramide cation, the SOMO appears to be localised on one of the three Me₂N moieties. This species was stable up to ca. 160 K. The phenyl derivatives, Ph₃PO and (PhO)₃PO gave aromatic radical cations in preference to oxygen-centred cations or their solvent complexes.