Biased motion about the long molecular axis in ‘ordered’ smectic phases as studied by deuterium NMR relaxation

Spectral densities of motion were determined by deuteron NMR relaxation measurements in the ‘ordered’ smectic-B and -G phases of 4-n-pentyloxybenzylidene-d 1-4′-heptylaniline and 4-n-pentyloxybenzylidene-4′-heptylaniline-2,3,5,6-d 4 at two different Larmor frequencies. Different forms of motional behaviour are involved in these phases in comparison with those in the high temperature ‘disordered’ phases. Specifically, internal ring rotation and direction fluctuation are not effective in the ordered smectic phases. In addition, the fast rotation of the molecule about its long molecular axis is now strongly hindered to give a libration motion of angular amplitudes of about 100°. The third-rate model is again used to describe the molecular reorientation, taking the restricted γ motion into account. The effects of phase biaxiality on spin relaxation in the smectic-G phase are also discussed.