Determination of orientational order parameters for nematics by deuterium nuclear magnetic resonance. The influence of internal rotation

Abstract

Deuterium N.M.R. spectra of 4,4′-dimethoxyazoxybenzene-d₁₄ have been recorded over the entire temperature range of the nematic phase. Analysis of the spectra yield partially averaged quadrupolar tensors, [qtilde]_i , and dipolar tensors, [Dtilde]_ij , which are used to test models for the singlet orientational distribution function produced by both internal motion and reorientation of the molecule in the mesophase. It is argued that ratios of [qtilde] values reveal the inadequacy of descriptions of the mesophase as composed of axially symmetric, rigid particles. The data are interpreted in terms of a model for which the internal rotation about the phenyl-methoxy bonds is correlated with reorientation of the whole molecule.