Conformational Distribution of trans-Stilbene in Solution Investigated by Liquid Crystal NMR Spectroscopy and Compared with In Vacuo Theoretical Predictions

The basic question about the structure and the conformational distribution of a -conjugated, flexible organic molecule (interesting in itself, in relation to the balance of forces determining its torsional equilibrium) becomes a really intriguing problem in the case of trans-Stilbene (t-St), a “fundamental” molecule from a chemical point of view, as well as the prototype fragment of a series of derivatives endowed with several important biological and technological properties. As a matter of fact, the problem of t-St planarity when the molecule is isolated or in solution is a particularly debated question. In the present paper we studied the conformational distribution of t-St in solution, by resorting to the powerful technique of Liquid Crystal NMR Spectroscopy (LXNMR), and we compared the obtained experimental results with accurate theoretical calculations carried out in vacuo, by using the MP2/6-31G** method (allowing for bond lengths and angles relaxation every 3° torsional steps). Our theoretical and experimental outcomes agree in indicating the non-planarity of the molecule which, on the contrary, exhibits the coexistence of four stable rotamers, two by two symmetry related. In particular, we have found a couple of global minima corresponding to propeller-like C2 symmetry conformations, where both the rings are “disrotated”, with respect to the vinyl group, of about 17° in solution and of 27° in vacuo (theoretical value). Besides this, the presence of a couple of Ci local minima, with both the rings “conrotated” of 17° (fluid phase) or of 27° (MP2/6-31G** calculations for the isolated molecule) has been determined