Resolution of different conformers of methoxy-trans-stilbenes via rotational coherence spectroscopy

The excitation spectra of 4-methoxy-t-stilbene (MoS) and 4-methoxy,4′-methyl-t-stilbene (MoMeS) both exhibit two electronic origin transitions, separated by 260–280 cm−1. This suggests the presence of two structural forms, as predicted by semi-empirical quantum mechanical calculations. The two forms should be ‘syn’ and ‘anti’, where the methoxy group is coplanar with the mean plane of the aromatic molecule. Rotational coherence spectroscopy has identified the syn and anti rotamers of the two species, establishing in both cases that the syn form has the longer-wavelength absorption.