The 100-and 220-MHz nmr spectra of 2,2'-bithienyl partially oriented in the nematic phase of a liquid crystalline solvent have been obtained and interpreted. The values of the direct dipolar couplings (£>,,) were found in disagreement with the presence of solely the s-trans or the s-cis conformation; a number of models taking into account the existence of a rapid equilibrium between the two forms have been discussed. It has been shown that the s-trans is the more stable conformer (70 ±5%) and the energy required for the s-trans, s-cis interconversion is about 5 ± 2 kcal/mol. The results agree with the conclusions of theoretical calculations and are consistent with X-ray and esr experimental evidences.Information on the structures and conformations of molecules with internal motions has been gained by means of the nmr spectroscopy of partially oriented molecules in liquid crystalline solvents.3•4 The possibility of using this technique for conformational analysis appears very interesting but must be tested with many examples in order to know the advantages and the limits of its applications.In the case of some bithienyl derivatives413 it has been possible to show that neither the free rotation nor the existence of a single conformer (twisted or planar) accounts for the parameters obtained from the nematic phase nmr spectrum. By allowing for some approximations it was possible to infer that both the s-trans and s-cis isomers are present with different populations, s-trans s-cis the former being the more stable. However, when symmetrically substituted derivatives of 2,2'-bithienyl are examined,413 the number of observable parameters is rather small so that the results cannot be very accurate; furthermore, no data are available for studying the barrier to the internal rotation.

No abstract

No abstract

In the present work, the quite intriguing question of the temperature dependence of the inter-ring angle of biphenyl in fluid condensed phases has been investigated by the proton liquid crystal nuclear magnetic resonance technique. The spectra of the molecule dissolved in three different thermotropic uniaxial solvents (one of which shows also a smectic A phase) at different values of temperature have been analysed by standard procedures, and the resulting temperature-dependent dipolar coupling sets have been rationalised by the additive potential for the treatment of the ordering interactions method, combined with the direct probability description of the torsional curve, in order to obtain the distribution of the twist angle j for each temperature. The results emphasise a very slight but unequivocal and systematic increase of j M (the most probable value of the dihedral angle) with temperature, so qualitatively confirming what was previously found by a cruder hybrid approach (based on experimental quadrupolar splittings of perdeuteriated biphenyl combined with computer-simulated order parameters) carried out by two of the present authors (Celebre, G.; De Luca, G.; Mazzone, G. J. Mol. Struct. (THEOCHEM) 2005, 728, 209-214).

No abstract