The electron spin resonance spectrum of trapped phenyl radicals prepared by chemical reaction at low temperature

Phenyl radicals have been prepared and trapped in a variety of solid matrices by the reaction of either sodium or potassium atoms with iodobenzene on the surface of the matrix material which was frozen at 77°K. A rotating cryostat was used to accumulate many layers of the products. The same e. s. r. spectrum was obtained in every case when water, benzene, deuterobenzene and perfluorocyclohexane were used as matrices. This spectrum has been attributed to the phenyl radical. When camphane, hexamethylethane and cyclohexane were used as matrices, different spectra were obtained in each case, and these have been attributed to the radicals formed by the abstraction of a hydrogen atom from a molecule of the matrix. This demonstrates the exceptional reactivity of the phenyl radical. When iodobenzene and xenon were used as matrices the spectra obtained may have been those of the phenyl radical, but modified by the surrounding matrix. The spectrum of the phenyl radical consists of nine hyperfine lines which can be attributed to a major hyperfine coupling of 18·1 Oe to the two ortho protons and a smaller coupling of 6·4 Oe to the two meta protons. It indicates that the unpaired electron remains in an sp 2 orbital on the carbon atom at which bond scission has occurred, to give a σ type radical. Such a structure contrasts with those of many aromatic radicals, such as benzyl, where the unpaired electron is delocalized over the π electron system, and is in accord with the high reactivity of the phenyl radical. This reactivity, however, is even higher than that of alkyl radicals, where the unpaired electrons are localized in π orbitals, and is attributed to the projection of the sp 2 orbital radially outwards from the ring of carbon atoms.