A Theoretical Assignment of the ESR Spectra of Cyclohexadienyl and Phenyl Radicals

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Abstract
Two kinds of spectra—a triplet with a finer structure of four triplets, and a singlet—are known to be observed in ESR of irradiated benzene, and two different assignments of these spectra have been proposed—one assigning the triplet to the cyclohexadienyl radical and the singlet to the phenyl radical, and the other assigning the triplet to the phenyl radical. In the present paper, in order to give ground for a theoretical choice between the assignments, a valence bond calculation on the spin distribution of cyclohexadienyl and phenyl radicals has been carried out. On the cyclohexadienyl radical, though the numerical value of the spin density for methylenic protons is dependent on the C–Cmethylene bond length, the calculated spectrum is in good agreement with the observed triplet with four triplets. On the other hand, the phenyl radical has been shown theoretically to be an essentially localized radical and to give a singlet spectrum, which also agrees with that of the observed singlet.