Deuteron NMR study of a long-chain smectic liquid crystal: Molecular order and dynamics

Deuteron Zeeman (T-1Z) and quadrupolar (T-1Q) spin-lattice relaxation times and quadrupolar splittings were measured in the nematic and smectic A phases of a chain-deuterated 4-n-octyloxy-4' -cyanobiphenyl (80CB-d(17)) at 15.1 and 46 MHz. To model the NMR observables, the so called pentane effect is used to limit the number of possible conformations in the chain, and is found to be a good approximation. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings. A decoupled model is used to describe correlated internal motions in the chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Nordio (Tarroni and Zannoni), while the former motion is described using a master rate equation. In the nematic phase, order director fluctuations are found to be necessary. The relaxation data in both mesophases were treated using a global target method, and the derived motional parameters are acceptable in comparison with those found for 6OCB. [S1063-651X(99)11711-2].