A high resolution investigation of the visible absorption spectrum of p-benzoquinone-H 4 and -D 4

Abstract

The visible absorption spectrum of p-benzoquinone is shown to consist of two electronic systems, an allowed (by spin-orbit interaction) 3 A u ← 1 A g system and an electronically forbidden 1 B 1 g ← 1 A g system. The latter is made vibronically allowed through the activity of two a u vibrations (a ring-twisting and a CH vibration) and one (or possibly two) b 3 u vibrations. The activity of the a u (CH) vibration contributes most of the intensity. These conclusions contradict previous analyses of the vapour spectrum but agree well with Sidman's conclusions from the low-temperature crystal spectrum [8]. The a u vibrations produce parallel bands which show regularly diverging R-branch heads from about K = 16 to K = 48. Computation of rotational contours for parallel and perpendicular bands, assuming that p-benzoquinone is a symmetric top (in fact κ = −0·79 in the ground state), has confirmed the identification of parallel bands and also aided the identification of perpendicular-type (type B) bands involving b 3 u vibrations. In the 1 B 1 g excited state the ∠HCC decreases by 12° and it is possible that Δ r cc = + 0·10Å and Δ r cc = −0·09 Å. There is no evidence for any deviation from planarity in either the 1 B 1 g or 3 A u excited states.