Molecular Ordering in Liquid Crystals Carrying Flexible Hydrocarbon Tails—A Rotational Isomeric State Analysis of D-NMR Data of Emsley et al on 4-n-Alkyl-4'-Cyanobiphenyls (nCB)

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Abstract
Deuterium NMR studies have been extensively carried out for 4-cyanobiphenyls carrying deuterated n-alkyl tails such as 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) by Emsley et al. In this paper, we have performed a shape analysis of D-NMR spectra within the framework of the rotational isomeric state approximation, statistical weight parameters assigned to the individual bond rotations being treated as empirical variables. For simplicity, the molecular axis is taken to lie in the direction parallel to the line connecting both terminals of the molecule (Model II). Values of the statistical weight parameters thus deduced are used in the estimation of the distribution of conformers permitted in the nematic phase. The results obtained for 5CB and 8CB were found to be in reasonable agreement with those reported by Emsley et al. Our model is simpler, and it has an advantage in that no separate consideration for the external potential field is required. Application of our scheme to the dimer and polymer liquid crystals comprising mesogenic cores joined on both terminals of a soft-spacer should be straightforward.