The potential of mean torque for flexible mesogenic molecules. Determination of the interaction parameters from carbon–hydrogen dipolar couplings for 4-n-alkyl-4′-cyanobiphenyls

The potential of mean torque for the flexible molecules comprising liquid crystals which was proposed by Emsley et al. [Proc. R. Soc. London Ser. A 381, 117 (1982)] contains interaction parameters for rigid fragments of the molecule. These are determined for four members of the series 4-n-alkyl-4′-cyanobiphenyls (NCB) with N=5, 6, 7, and 8, at TNI −T=4 K, by comparing calculated values of SCH, the local order parameter for C–H bonds at each position in the alkyl chain, with values determined from dipolar couplings between carbon-13 and protons. The potential of mean torque characterized in this way is then used to calculate vicinal C–H dipolar coupling constants for nuclei in the alkyl chain for 5CB and 6CB; these are found to be in reasonable agreement with experimental values.