The structure and conformation of a mesogenic compound between almost zero and almost complete orientational order

The conformational distributions in molecules that form liquid crystalline phases are predicted to depend strongly on orientational order. Results are presented here to test this hypothesis. The mesogen 4-hexyloxy-4'-cyanobiphenyl (6OCB) has been studied by NMR spectroscopy in the isotropic phase and in the nematic phase. In the isotropic phase the field-induced orientational ordering produces small dipolar couplings between C-13 and H-1 nuclei, which were determined from the 13 C spectra. Couplings between H-1 nuclei were also obtained using 2D selective refocusing experiments. In the nematic phase, both H-1-H-1 dipolar couplings and quadrupolar splittings for deuterium nuclei were measured for partially-deuterated samples. Both proton and deuterium spectra were also obtained for 6OCB in an equimolar mixture with 4-(ethoxybenzylidene)-4'-butylaniline (EBBA). This mixture exhibits SmA and SmB phases. The data obtained from these experiments has been analysed to yield the probability distribution of the conformations in this molecule generated by rotations about bonds. It is found that there is a substantial influence of the orientational order of the molecules on these distributions.