σ* Radical anions from halogen derivatives of perfluorobenzene

Abstract

Electron addition to C₆F₅I, C₆F₅Br, the corresponding di-iodo and dibromo derivatives, and C₆F₅Cl gave only σ* radicals, as deduced from the e.s.r. spectra. Addition was achieved by exposing dilute solutions at 77 K to ⁶⁰Co γ-rays, and was confirmed by observing a reduction in yield when electron-scavenging solutes were added. The σ* orbital containing the unpaired electron is shown to be largely confined to a single C—Cl, C—Br or C—I bond. However, for the ortho-disubstituted derivatives, the halogen hyperfine parameters were clearly reduced, indicating slight delocalisation into the neighbouring C—halogen bond. No features of the type characteristic of π* anions or the delocalized C₆F₆^– system were observed. The results are compared with those for the corresponding aryl halide σ* anions, which exhibit less delocalisation of the excess electron onto halogen.