Electronic States of p-Benzoquinone. V. Vibrational Analysis of the Vapor Absorption Spectrum around 4500-A Region

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Abstract
The absorption spectrum of p‐benzoquinone vapor in the region between 4080 and 5100 A was photographed and measured. Although the theoretical calculation as well as the experimental crystal spectrum indicate that two singlet‐singlet electronic transitions exist in this region, prominent vibrational structure of the vapor spectrum can be interpreted as caused by a single forbidden electronic transition. If we assume that the hydrogen vibrations are not effective as perturbing vibrations which make the forbidden electronic transitions allowed through the vibrational‐electronic interaction, this fact suggests the following two sets of assignments for the symmetries of the forbidden transition and the perturbing allowed transition: (1) the spectrum is a superposition of 1Au, 1B2u←1Ag transitions perturbed by a 1B2u←1Ag transition, (2) the spectrum is caused by a single 1B1g←1Ag transition being allowed through vibrational‐electronic interaction with the allowed 1B1u←1Ag transition. Vibrational structure alone cannot exclude the second possibility, but a discussion supporting the first assignment is given. This assignment (1) is in agreement with the theoretical calculation of the energy levels. Most prominent bands are assigned to particular vibronic transitions. Fundamental frequencies in the excited electronic state 436, 796, 1109, and 1220 cm—1 are obtained corresponding probably to the ground‐state values of 444, 770, 1144, and 1667 cm—1.

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