Nematic and Smectic Ordering of 4-n-Octyl-4′-cyanobiphenyl Studied by Carbon- 13 NMR

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Abstract
A new NMR method called RHODIUM (Removal of HOmonuclear DIpolar-couplings and the Use of off-Magic-angle-spinning) has been applied to study the molecular ordering of 4-n-octyl-4′-cyanobiphenyl (8CB) as a function of temperature. C-H dipolar coupling constants of various carbon atoms in 8CB were obtained from natural abundance carbon-13 NMR. Order parameters of the benzene rings and those of the C-H bonds in the octyl chain were calculated from the dipolar coupling constants. The order parameter of the -CN axis can also be calculated from the chemical shift of the -CN group. The results are consistent with the second order nature of the smectic A-nematic phase transition in 8CB.