X. Shen et Ry. Dong, BIASED MOTION ABOUT THE LONG MOLECULAR AXIS IN ORDERED SMECTIC PHASESAS STUDIED BY DEUTERIUM NMR RELAXATION, Molecular physics, 83(6), 1994, pp. 1117-1131
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.