Nematic phase nuclear magnetic resonance investigations of rotational isomerism. Part II. Evidence for the SO-cis-conformation in thiophen-2-carbaldehyde

Abstract

The 100 MHz ¹H n.m.r. spectrum of thiophen-2-carbaldehyde partially oriented in the nematic phase of a liquid crystalline solvent has been obtained and interpreted. It has been shown that only the SO-cis-conformer is present whereas the SO-trans-form, an equilibrium between the two rotamers, and free rotation of the formyl group can be unambiguously excluded. The SO-cis-conformation has been found to be planar or very nearly so and the orientational parameters of the molecule with respect to the magnetic field have been determined. Self consistent molecular orbital calculations (CNDO/2) indicate that the SO-cis-conformation is more stable by 2 kcal mol^–1 than the SO-trans, in agreement with the experimental findings. The differences in the conformational behaviour of furan- and thiophen-2-carbaldehyde are discussed in terms of different electronic distribution.