Unstable intermediates

A recent suggestion that methyl bromide in methyl cyanide when exposed to γ-rays at 77 K gives methyl radicals which are loosely associated with their parent bromide ions has been examined, the previous e.s.r. results have been confirmed, and the bromine hyperfine tensor components obtained. These show that transfer of spin to bromine is small, and indeed it may well be that spin density is acquired mainly by spin polarisation. Similar results were obtained from methyl iodide and ethyl bromide. t-Butyl bromide gave only Me3C· radicals. The same compounds in methyl alcohol gave only the corresponding alkyl radicals with no halide interaction. This difference is explained in terms of the far greater anion solvating power of the alcohol. Methylene bromide in both solvents gave Br2– and HĊBr2 but no species corresponding to the bromide ion adducts. The pure compounds on irradiation at 77 K gave dihalide anions and other products, but no halide ion adducts. In contrast, N-chloro- and N-bromo-amides gave σ* radical anions without loss of halide ion, both in methyl cyanide and in methyl alcohol. These differences are explained in terms of the different structures of the organic radicals formed by loss of halide ions.