Orientational Ordering of 4-n-Alkyl-oxy-4′-Cyanobiphenyls Studied by 2D Carbon-13 NMR

Abstract

The orientational ordering of four 4-n-alkyloxy-4′-cyanobiphenyls (50CB, 60CB, 70CB and 80CB) in their nematic phase at T_NI - T= 10K was studied by carbon-13 NMR. The technique used is a combination of separated local field spectroscopy (SLF) and variable angle spinning (VAS). Rapid sample spinning of these compounds causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 NMR spectra. The 2D method of SLF produces a first order splitting pattern for each carbon signal, from which C-H dipolar coupling constants can be determined. The order parameters for each phenyl ring and each aliphatic C-H segment can then be calculated. Results for the kOCB series are presented.