STUDY OF THE MOLECULAR-DYNAMICS FROM DEUTERIUM ZEEMAN AND QUADRUPOLAR NMR RELAXATION OF 4,4'-DI-N-HEPTYLAZOXYBENZENE IN THE NEMATIC AND SMECTIC-A PHASES

The deuterium spin-lattice (Tz) and quadrupolar (T1Q) relaxation times of the liquid crystal 4,4'-Heptyl-azoxybenzene (HAB), deuterated on the aromatic rings and the first methylene group of the chains, have been measured by means of the Jeener-Brockaert pulse sequence at two different frequencies (10 and 46.04 MHz) and throughout the nematic and the smectic A phase. By applying suitable models, we have deter-mined rotational and reorientational diffusion coefficients and evaluated the contribution of the different motions (internal, molecular and collective) to the relaxation. In particular we have found a non negligible contribution from order director fluctuations. A comparison between the T1Z and T1Q measurements performed by means of Jeener-Broekaert and Wimperis pulse sequences is also shown.