ORIENTATIONAL ORDERING OF FERROELECTRIC THIOBENZOATE LIQUID-CRYSTALS

The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance 13C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle of c. 45° with respect to the magnetic field, the smectic director aligns parallel to the spinning 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 Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C‒H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order parameters was observed. © 1990 Taylor & Francis Group, LLC.