Unstable intermediates. Part 194. Alkyl radical–halide ion adducts: an electron spin resonance study of the radiolysis of a range of alkyl bromides and iodides

Abstract

Alkyl radicals formed by dissociative electron capture from alkyl bromides or iodides in solids at low temperatures often remain trapped in the vicinity of the halide ion, their e.s.r. spectra exhibiting hyperfine coupling to the halide ion nuclei. By studying the e.s.r. spectra of a range of such adducts in different media as a function of temperature, we have established that the perpendicular hyperfine coupling to the halogen nuclei is negative, the isotropic coupling being close to zero. The extent of charge transfer, estimated from the dipolar hyperfine coupling to the halogen nuclei or from the ¹H coupling of the radical, is 10–20% and is greater for iodides than from bromides. These results do not support a σ* structure for electron adducts of alkyl halides, and this is further supported by the absence of a positive shift for g_⊥, such as is invariably observed for σ* radicals. Some iodides were unusual in that species identified as the non-rotating adducts in adamantane co-existed at certain temperatures with the isotropic species. It is suggested that the latter adducts are associated with double vacancies and the former with single vacancies.