Orientational ordering in the nematic phase of 4-alkenyl-substituted bicyclohexylnitriles

Abstract

Abstract The orientational ordering of a series of 4-alkenyl-substituted bicyclohexyl liquid crystals was studied by natural abundance 13C NMR spectroscopy. A combination of the methods of variable angle spinning (VAS) and separated local field spectroscopy (SLF) was used. Rapid sample spinning about an axis forming an angle of about 65° with respect to the magnetic field forces the nematic director to align parallel to the spinnin axis, leading to narrow peaks in the 13C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled 13C signals in the ω2 dimension and first-order C-H splitting patterns in the ω1 dimension, from which the C-H dipolar coupling constants for individual pairs of nuclei can be obtained. Then, the order parameters for different molecular segments can be calculated. The method was applied to five different 4-alkenyl-substituted cyanobicyclohexanes. For the compounds studied, the dependence of the ring order parameters on the alkenyl chains sh...