Electronic States and Spectra of p-Benzoquinone

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Abstract
A complete assignment of the electronic transitions involved in the visible and near ultraviolet spectra of p-benzoquinone is given. The following excited states are observed: π-π* 1B2u at 41 330 cm−1 and π-π* 1B1g at 32 800 cm−1 (in the vapor phase); n-π* 1Au at 20 315 cm−1, n-π* 1B2g at 20 060 cm—1, n-π* 3Au at 18 940 cm−1, and n-π* 3B2g at 18 620 cm−1 (in the pure crystal). The π-π* transitions in the near ultraviolet have already been analyzed earlier and the influence of substitution and the solvent effect on these transitions have been studied. The above work is critically reviewed and confronted with the results of recent CNDO calculations which explain qualitatively quite well the experimental data. In the visible spectra only the n-π* 1B2g and n-π* 3Au states had been located unambiguously in earlier work. A compilation of the data on these spectra is given and discussed. New measurements at 4.2°K of the polarized single crystal spectra of p-benzoquinone and its methyl derivatives are presented. They permit us an unambiguous assignment of both n-π* singlet and triplet states. The following splittings of the two n-π* states are obtained: 255 cm−1 for the singlet states and 320 cm−1 for the triplet states. Our results are in contradiction with the most recent views expressed on this subject [W. Hug, J. Kuhn, K. Seibold, H. Labhart, and G. Wagniére, Helv. Chim. Acta 54, 1451 (1971)].