Radiation mechanisms. Part 21. Electron spin resonance studies of the effect of ionising radiation on halogenobenzenes: the formation of σ* anions and ipso addition radicals

Abstract

Various chloro-, bromo-, and iodo-benzene derivatives have been exposed to ⁶⁰Co γ-rays as pure materials and in a range of solvents at 77 K and the radical products have been studied by e.s.r. spectroscopy at this temperature before and after annealing. Two important types of radical have been characterised. Electron addition to certain bromo- and iodo-derivatives gave e.s.r. features characteristic of σ* radicals, the excess electron being largely localised to the C–Hal σ* orbital. It was previously thought that only π* anions of this compound were stable under these conditions. We suggest that the σ* anions are precursors to dissociative electron capture. Competing with σ* anion formation in certain cases was π* anion formation followed by ring protonation. However, the normal substituted cyclohexadienyl radicals with two interacting protons in a CH₂ group were not detected. Instead PhBr and PhI gave radicals containing CH–Hal groups, exhibiting strong hyperfine coupling to one proton and the halogen nucleus. Use of deuteriated compounds and solvents showed complete scrambling of the ring protons, probably induced by migration of the halogen atoms. para-Bromo- and -iodo-phenol gave hydrogen atom adducts containing C[graphic omitted] or C[graphic omitted] groups, with no strongly coupled protons. We suggest that halogen atom migration led to their formation and hence that these ipso-structures are more stable than the alternative CH₂ or CH–Hal structures. E.s.r. parameters for phenyl radicals having para-iodo or para–O^– substituents were similar to those for normal phenyl radicals.