The structure, conformation and orientational order of fluorinated liquid crystals determined from carbon-13 NMR spectroscopy

The C-13-{H-1} NMR spectra of two monofluorinated nematic liquid crystals, I35 and I52, have been obtained and analysed to yield sets of dipolar couplings, D-iF, from each carbon in the molecule to the fluorine nucleus. The couplings involving carbons in the fluorinated biphenyl group are used, together with a mean field theoretical model (the Additive Potential, AP), to determine the shape of the potential governing rotation about the inter-ring bond. For both molecules this is found to have a minimum when the two phenyl rings are at either 40 degrees or 140 degrees to one another. For I35 the couplings involving the aliphatic carbons are used to determine that the minimum energy forms for rotation about the aromatic ring to alkyl chain bonds are when the chains are orthogonal to the rings; the data for I52 are consistent with the same conformations. The AP model predicts that the conformer probabilities for the molecules in the nematic phase are substantially different from those in the isotropic phase at the same temperature.