Radical cations of benzyl chlorides and bromides formed by radiolysis

Exposure of dilute solutions of benzyl chloride, benzyl bromide and various derivatives thereof in trichlorofluoromethane to 60Co γ-rays at 77 K gave the radical cations, characterised by their e.s.r. spectra. The major species obtained from C6H5CH2Cl had hyperfine coupling to three inequivalent protons, assigned to the para ring proton and the two methylene protons, with no detectable coupling to the chlorine nucleus. Only a single isotropic feature was obtained from C6D5CD2Cl, confirming the absence of significant coupling to chlorine. It is suggested that the preferred conformation has the chlorine atom close to, but not in, the plane of the benzene ring, with a significant barrier for the in-plane site. In contrast, only cations with strong hyperfine coupling to bromide nuclei were obtained from benzyl bromide. The p-methyl derivative gave only one structure but benzyl bromide gave two major species thought to be two alternative conformations of the parent cation. All these cations were sensitive to visible light, but we were unable to detect any well defined radical photoproducts. Reasons for the formation of different conformations are discussed and the results are compared with optical studies for matrix-isolated cations and with e.s.r. results for β-chloro- and β-bromo-alkyl radicals. (author)