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 pi-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 degrees torsional steps). Our theoretical and experimental outcomes agree in indicating the nonplanarity 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 C-2 symmetry conformations, where both the rings are disrotated, with respect to the vinyl group, of about 17 degrees in solution and of 27 degrees in vacuo (theoretical value). Besides this, the presence of a couple of C-i local minima, with both the rings conrotated of 17 degrees (fluid phase) or of 27 degrees (MP2/6-31G** calculations for the isolated molecule) has been determined.