Deuterium NMR relaxation in the smectic-A phase of a chiral smectogen.

Abstract

Correlated internal rotations are studied for a deuterated decyloxy chain in the chiral liquid crystal, 1-methylheptyl 4(')-(4-n-decyloxy-benzoyloxy)biphenyl-4-carboxyate (10B1M7) using deuteron spin-lattice relaxation and quadrupolar splitting measurements. The present study is limited to the uniaxial smectic-A phase of 10B1M7, in which the observed deuterium nuclear magnetic resonance spectra show well-resolved peaks. The deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times are simultaneously determined for the methylene deuterons using the Wimperis broadband pulse sequence at two different Larmor frequencies. The quadrupolar splittings at each temperature are modeled by the additive potential method to obtain the orienting potential of mean torque. A decoupled model using 683 conformations for the decyloxy chain is used to explain the observed relaxation rates. In this model, the overall reorientation is described by a small step rotational diffusion model, while both gauche migration and gauche pair production as well as the so called one- and two-bond motions are allowed to occur in the chain. Transition rates for the chain dynamics and the overall rotational diffusion constants are obtained using a global target analysis of the relaxation data of 10B1M7.