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 π-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 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° in solution and of 27° in vacuo (theoretical value). Besides this, the presence of a couple of C(i) 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.
Dipartimento di Chimica, Università della Calabria, v. P. Bucci, I-87036 Rende (CS), Italy. giorgio.celebre@unical.it
Giorgio
G
Celebre
Giuseppina
G
De Luca
Maria Enrica
ME
Di Pietro