18 - Vibrational structure in the electronic spectrum of 1,4-benzodioxan: assignment of low frequency modes

Summary

Concepts illustrated: low frequency vibrations in complex molecules; ab initio calculation of vibrational frequencies; laser-induced fluorescence (excitation and dispersed) spectroscopy; vibrational assignments and Franck–Condon principle.

This Case Study demonstrates some of the subtle arguments that can be employed in assigning vibrational features in electronic spectra. It also provides an illustration of how important structural information on a fairly complex molecule can be extracted. The original work was carried out by Gordon and Hollas using both direct absorption spectroscopy of 1,4-benzodioxan vapour and laser-induced fluorescence (LIF) spectroscopy in a supersonic jet [1]. The direct absorption spectra were of a room temperature sample and were therefore more congested than the jet-cooled LIF spectra. Nevertheless, the direct absorption data provided important information, as will be seen shortly. For the LIF experiments, both excitation and dispersed fluorescence methods were employed (see Section 11.2 for experimental details). Only a few selected aspects of the work by Gordon and Hollas are discussed here; the interested reader should consult the original papers for a more comprehensive account [1, 2].

Possible structures of 1,4-benzodioxan are shown in Figure 18.1. Assuming planarity of the benzene ring, there are three feasible structures that differ in the conformation of the dioxan ring. One possibility is that both C O bonds are displaced above (or equivalently below) the plane of the benzene ring yielding a folded structure with only a plane of symmetry (C_s point group symmetry).