Spectral Density of Motion in a Reentrant Nematic by Deuterium NMR

Abstract

We report on a deuterium spin relaxation study of a 26 wt. % sample of perdeuterated hexyloxycyanobiphenyl (60CB-d ₂₁) in octyloxycyanobiphenyl as a function of temperature at a Larmor frequency of 13.81 MHz. The binary mixture exhibits the following phase sequence: isotropic—nematic—smectic A—reentrant nematic—crystal. The DMR spectrum of this sample is relatively well-resolved and permits the measurement of the Zeeman (T _1Z) and quadrupole (T _1Q) spin-lattice relaxation times for several distinct deuterons in 60CB-d ₂₁. The present study considers only the ring deuterons. In fact, there are two sets of ring deuterons and we restrict our study to the set with the smaller quadrupolar splitting to avoid spectral overlap from the methyl deuterons. These relaxation times were simultaneously measured by the phase-cycled Jeener-Broekaert sequence, and the spectral densities J_p (ρω) (p = 1,2) were found to increase continuously with decreasing temperature. Molecular reorientation seems to be a dominant relaxation mechanism for the ring deuterons and is discussed in terms of a small step rotational diffusion model.