BIASED MOTION ABOUT THE LONG MOLECULAR AXIS IN ORDERED SMECTIC PHASESAS STUDIED BY DEUTERIUM NMR RELAXATION

Spectral densities of motion were determined by deuteron NMR relaxatio n measurements in the 'ordered' smectic-B and -G phases of 4-n-pentylo xybenzylidene-d(1)-4'-heptylaniline and pentyloxybenzylidene-4'-heptyl aniline-2.3.5.6-d(4) at two different Larmor frequencies. Different fo rms of motional behaviour are involved in these phases in comparison w ith those in the high temperature 'disordered' phases. Specifically, i nternal ring rotation and direction fluctuation are not effective in t he ordered smectic phases. In addition, the fast rotation of the molec ule about its long molecular axis is now strongly hindered to give a l ibration motion of angular amplitudes of about 100 degrees. The third- rate model is again used to describe the molecular reorientation, taki ng the restricted gamma motion into account. The effects of phase biax iality on spin relaxation in the smectic-G phase are also discussed.