DIRECTOR FLUCTUATIONS AND ANISOTROPIC REORIENTATION OF A LIQUID-CRYSTAL - A DEUTERIUM NMR-STUDY

The spectral densities of motion were determined by deuteron NMR relax ation measurements in the nematic and smectic-A and -C phases of 4-n-p entyloxybenzylidene-d(1)-4'-heptyline and pentyloxybenzylidene-4'-hept ylaniline-2,3,5,6-d(4) at two different Larmor frequencies. These spec tral densities can only be interpreted by invoking director fluctuatio ns and molecular reorientation as relaxation mechanisms for the deuter on spins at the methine and ring sites. We compared the model of Nordi o and co-workers [Nordio and Busolin, J. Chem. Phys. 55, 5485 (1971)] and the third-rate model of molecular reorientation [Void and Void, J. Chem. Phys. 88, 1443 (1988)] and found that the third-rate model is m ore appropriate in the present study. Rotational diffusion constants w ere determined as a function of temperature in the mesophases of 4-n-p entyloxybenzylidene-4'-heptylaniline.