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.