ESR Studies of the Photoreduction of Quinones. II. The Reinter-pretation of Semiquinone Intermediates from Halogenated p-Benzoquinones in Ethanol

Abstract A reinterpretation was made of the electron spin resonance spectrum of semiquinone intermediates during the photolysis of a flowing solution of tetrachloro-p-benzoquinone (chloranil) in ethanol. The photoinduced spectrum with a width of 0.06 mT with an unresolved hyperfine structure was found to be identical with the spectrum of the semiquinone anion generated by reducing chloranil in an alkaline ethanol solution. This and other observations indicated that the photoinduced spectrum was due to the chloranil semiquinone anion formed primarily by the one-electron transfer to chloranil from the solvent ethanol, rather than to the semiquinone radical. The photoinduced one-electron transfer reaction was found, though qualitatively, to proceed more efficiently for chloranil and fluoranil, with a larger electron affinity, than for p-benzoquinone and dichloro-p-benzoquinones.