Determination of the order parameters of liquid crystals from carbon-13 chemical shifts

Abstract

The orientational ordering of four 4-n-alkyl-4′-cyanobiphenyls (kCB’s: 5CB, 6CB, 7CB, 8CB) and four 4-n-alkoxy-4′-cyanobiphenyls (kOCB’s: 5OCB, 6OCB, 7OCB, 8OCB) in their nematic phase was studied at different temperatures by C-13 NMR. The technique used was a combination of variable-angle spinning and a two-dimensional method called separated local-field spectroscopy. The C-13 chemical shifts for each carbon nucleus were measured with the sample spinning parallel to the magnetic field. For the carbon atoms in the aromatic rings and the cyano group as well as the carbon atoms in the in α and β positions of the aliphatic chain, the order parameters for all four compounds in each homologous series show a linear dependence on the corresponding chemical shifts. For the ω-CH3 group and the (ω−1)-CH2 group, a simple linear dependence of the C–H order parameter on the chemical shift was found for all eight compounds in the two homologous series. The slopes and intercepts of all the results are tabulated. The results offer a simple way to determine the order parameters of liquid crystals directly from C-13 chemical shifts.