Rotational band contours in electronic spectra of large asymmetric top molecules: the 2750 Å system of phenol

Abstract

The rotational band contour of the 0–0 band of phenol at 2750 Å has been recorded experimentally with a resolving power of 300000. The contour contains many characteristic features of which a series dependent on K a has been used to obtain trial sets of rotational constants A', B' and C' in the electronically excited state. The excited state was assumed to be planar. These data together with rotational selection rules were used in an asymmetric rotor band contour computer program and the rotational constants varied until the com­puted contour matched the observed. The contours were matched only by using type B selection rules. The electronic assignment is therefore 1 B 2 – 1 A 1 (using the C 2 v point group) and the excited state rotational constants are : A ' = 0·1773 ± 0·0002 cm -1 ; B ' = 0·08751 ± 0·00006 cm -1 ; C ' = 0·05859 ± 0·00001 cm -1 . These constants reflect an appreciable interaction of the hydroxyl group with the ring in the excited state whereas microwave data have shown very little interaction in the ground state. In particular, there is a slight overall contraction of the molecule along the long in-plane inertial axis from the ground to the excited state in contrast to an expected expansion if there were no hydroxyl group interaction. The origin of the 2750 Å 0–0 band is at 36 348·7 ± 0·2 cm -1 .