Electronic States of p-Benzoquinone. VIII. Vibrational Analysis of the Vapor Absorption Spectrum around 2900 Å

The absorption spectrum of p-benzoquinone vapor was observed at 2900 Å region and the vibrational analysis was carried out. Two absorption groups were found. The bands in group 1 are sharp and single, but the nature of the transition is uncertain. Group 2 is composed of a doublet whose frequency separation is 45 cm−1. It corresponds to the 1B1g←1Ag transition and is allowed by vibronic interaction with the allowed 1Bi2g←Ag transition. The perturbing vibration is a C=O bending vibration belonging to the b3u species whose frequency is 370 cm−1 in the ground state. The vibration frequencies of the main progression are 444 cm−1and 435, 692 and 1560 cm−1 in the ground and excited electronic states respectively.