Deuterium NMR study of molecular order and reorientation in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline

The deuterium quadrupole doublet splittings and spin–lattice relaxation times (T1) are measured as a function of temperature in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline (MBBA-d13). It is shown that a single order parameter tensor is sufficient to fully describe the orientational order of the molecule even though the molecule is nonrigid. The molecular conformational averages, which are different for various C–D bonds, result in resolved deuterium NMR spectra from which the T1 for each deuteron site can be measured. While the simplest approach in treating order director fluctuations, in which molecular reorientations and collective fluctuation modes are assumed to be uncoupled, fails to explain our T1 data for all deuterons, Freed’s treatment of spin relaxation in liquid crystals does give a qualitative explanation. Using this theory, molecular reorientation times for different deuteron sites are estimated.