Electron Spin Resonance during Photolysis of p-Benzoquinone and Its Derivatives in Ethanol

Electron spin resonance measurements were carried out at room temperature during photolysis of solutions containing p-benzoquinone, 2-methyl-p-benzoquinone, 2,5-dimethyl-p-benzoquinone and 1,4-naphthoquinone in ethanol. From all quinones, the semiquinone radicals and semiquinone anions are found to form simultaneously. When the solution is made acidic, the semiquinone anion transforms into the semiquinone radical, so that the spectrum of the latter is easily determined. For benzoquinone, the semiquinone radical is formed efficiently by light of λ≈300 nm, and has a life of about 0.05 sec. The semiquinone anion is formed by light of λ≈460 nm and is rather long-lived (about 0.5 sec). These results suggest that the semiquinone anion may be formed mostly by photodissociation of charge-transfer complex between benzoquinone and alcohols. Observed proton hyperfine coupling constants were subjected to the molecular orbital treatment. The coupling constants in all the semiquinone anions examined are well interpreted by McLachlan’s approximate SCF-MO method and Colpa-Bolton’s relation with Q=27G and K=12.8 G However, good agreement is not attained between the observed coupling constants and the calculated ones for the semiquinone radicals.