Orientational Ordering Of Phenylcyclohexane-Based Liquid Crystals Determined From Carbon-13 2D NMR Spectroscopy

The orientational ordering of eight liquid crystals in their nematic phase at TNI - T = 4 K and 10K was studied by carbon-13 NMR. The technique applied is a combination of separated local field spectroscopy (SLF) and variable angle spinning (VAS). Using this method, a first order splitting pattern for each carbon signal was obtained and C-H dipolar coupling constants were determined. Order parameters for each aliphatic C-H segment and each phenyl and cyclohexyl ring were calculated for the compounds. Results are presented for four 1-(4'-cyanophenyl)-trans-4-alkenylcyclohexanes and two of their analogous 1-(4'-cyanophenyl)-trans-4-alkylcyclohexanes, as well as a 1-(4'-isothiocyanophenyl)-trans-4-alkylcyclohexane and a 4-alkyl-4'-cyanobiphenyl for comparison. The effects of substitution of a phenyl group for a cyclohexyl group, the presence and position of a double bond, the length of the aliphatic chain, the nature of the phenyl-ring substituent, and temperature are discussed.

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