Modeling Internal Chain Dynamics in Nematogens of Different Chain Lengths: Deuteron NMR Study

Abstract

Aligned liquid crystals with deuterated chains show well-resolved deuterium NMR spectra. The site specificity of these quadrupolar doublet signals allows measurements of both static and dynamic observables, which can provide information of the rotation dynamics within the flexible end chain. We use a realistic geometry and the rotational isomeric model of Flory to generate all possible configurations and a set of C-D bond orientations in the chain. A master rate equation is used to describe transitions among all allowed configurations of the chain. In this paper, we report on motional parameters derived from quantitative fittings of quadrupolar splitting and spectral density data in three different nematogens of various chain lengths. The deuterated samples are p-methoxy-d₃-benzylidene-d₁-p-n-butyl-d₉-aniline (10.4-d₁₃), 4-n-pentyl-d_n-4‘-cyanobiphenyl-d₄ (5CB-d ₁₅) and 4-n-hexyloxy-d₁₃-4‘-cyanobiphenyl-d₈ (6OCB-d₂₁).