Nematic ordering of 4-<i>n</i>-alkyl-4′-cyanobiphenyls studied by carbon-13 NMR with off-magic-angle spinning

Abstract

A method called RHODIUM (removal of homonuclear dipolar coupling and the use of off-magic-angle spinning) can be applied effectively to study the carbon-13 NMR of liquid crystals in natural abundance. In this method, the sample is spun rapidly (∼1 kHz) at an angle (θ) slightly less than the magic angle, so that the nematic director aligns along the spinning axis and the dipolar couplings are reduced by a factor of (3 cos2 θ−1)/2. Proton–proton dipolar couplings are removed by a special decoupling sequence (BLEW-48) so that carbon–proton dipolar couplings can be readily identified. The coupling constants (DC–H) of individual carbons are obtained by using a two-dimensional technique of separated local field spectroscopy. The RHODIUM method has been used to study the molecular ordering in the nematic phase of 4-n-alkyl-4′-cyanobiphenyls (kCB). Three homologs (5CB, 6CB, and 7CB) were investigated at TNI−T =4 K. Values of DC–H of the aliphatic carbons give rich information on the energetics of chain conformations and those of the aromatic carbons can be used to calculate order parameters of the phenyl rings. It was found that the values of DC–H for the α, β, and γ carbons in 5CB, 6CB, and 7CB are almost the same, and so are the values of Szz and Sxx−Syy of the rings. The results for 5CB are in good agreement with those obtained from deuterium and proton NMR studies of specially deuterated compounds.