High field carbon 13 relaxation of 5CB. Chemical shift anisotropy as a probe of nematic dynamics

Abstract

The carbon 13 spin-lattice relaxation times of the ring carbons at 22·6 MHz in the nematic phase and at 20·0 MHz in the isotropic phase of 5CB have been published previously. An analysis of the results in the nematic phase based on diffusion constants obtained from deuteron relaxation was presented. In this paper measurements of the carbon 13 spin-lattice relaxation times of the ring carbons of 5CB at 125·6 MHz are given along with an analysis which makes use of the wide variation in the field to interpret the relatively simple relaxation of the protonated aromatic carbons. An effort to interpret the more complicated relaxation behaviour of the unprotonated carbons using the simplification in relaxation behaviour at high fields where chemical shift anisotropy dominates the relaxation is less successful. However, the conclusion that order director fluctuations account for almost 50 per cent of the relaxation rate in the nematic phase at high fields suggests that study of high field rotating frame relaxation for these carbons might be a rich source of information on critical fluctuations.