Electron Spin Resonance Study on Ultraviolet Light Irradiated Benzene Adsorbed on Silica Gel and Alumina

Benzene adsorbed on silica gel was irradiated at 77°K with ultraviolet light. Three kinds of samples, with different degree of adsorption, were examined. ESR spectra were strongly dependent on the amount of benzene adsorbed and three different spectra were observed; (1) six-line spectrum with a splitting value of 6 gauss, assigned to phenyl radical, (2) seven-line spectrum with a splitting value of 4.5 gauss, assigned to benzene cation radical, and (3) apparent nine-line spectrum with 2.4 gauss splitting due to benzene dimer cation radical. Upon raising temperature, benzene cation radical was found to transform into benzene dimer cation radical. In UV-irradiated benzene-alumina system, cyclohexadienyl radicals were observed in addition to the ion radicals. It is suggested that the above reaction may be occurred by two photon absorption process involving the excitation of triplet benzene.