Electron Spin Resonance of the Benzene Positive-Ion Radical

Abstract

The benzene monopositive-ion radical C6H6+ is prepared by photoionization of benzene in a rigid sulfuric acid matrix. This highly symmetrical radical is of special interest because of the occurrence of near degeneracy in the vibronic levels. Electron spin resonance spectra are investigated over a range of temperatures. The hyperfine spacing is 4.44±0.01 G at −150°C and varies with temperature. The temperature coefficient of the total extent of the spectrum is −5.2±3 mG/°C (−110≤t≤−190°C). A computation of the benzene cation splitting using the semiempirical Colpa—Bolton equation [J. Chem. Phys. 43, 309 (1965)], which treats the effect of excess charge density on hyperfine splitting, leads to a value in fairly good agreement with experiment. The temperature dependence of the proton hyperfine splitting is discussed in terms of the expectation values of the Fermi contact interaction in the thermally populated molecular vibronic states. The g value observed for C6H6+ is 2.00242±0.00002. This result is compared with the g value computed from Stone's semiempirical equation [Mol. Phys. 6, 509 (1963); J. Chem. Phys. 43, 4191 (1965)]. Near degeneracy in hydrocarbon radicals is correlated with the occurrence of deviations from the predictions of Stone's equation.