The stable conformations of 4,4′-dichloro-trans-stilbene in solution by liquid crystal NMR spectroscopy

The mechanisms conferring to the trans-stilbene derivatives their well known important biological properties are not yet completely clear; then, it is reasonable to suppose that their conformational equilibrium could play some kind of role (as found for many other molecules of pharmacological interest). In this scenario, the 4,4′-dichloro-trans-stilbene (DCS) can be considered as the prototype of a series of stilbenes substituted in 4,4′ positions in order to enhance their properties. Following this starting idea, we decided to study the conformational distribution of DCS in solution by Liquid Crystal NMR Spectroscopy (LXNMR), one of the best techniques to obtain this kind of information in the liquid state of matter. Our results show that the molecule is basically not planar: the most populated conformations we found are (a) the couple of symmetry-related, propeller-like structures, belonging to the C2 point group and characterized by the phenyl rings disrotated of about 18.7° with respect to the vinyl plane and (b) the couple of symmetry-related Ci structures (where the rings are parallel, like the pedals of a cycle) having this time the two rings conrotated of an angle of ∼18.7° with respect to the vinyl fragment.