Ry. Dong et al., MOLECULAR ORDER AND DYNAMICS OF THE NEMATOGEN MBBA - MODELING OF DEUTERIUM NMR OBSERVABLES, Molecular physics, 81(5), 1994, pp. 1017-1038
We report on the measurements of quadrupolar splittings and spectral d
ensities in the nematic phase of a partially deuterated liquid crystal
p-methoxy-benzylidene-p-n-butylaniline (MBBA). The Zeeman and quadrup
olar spin-lattice relaxation times have been measured at 15.3 and 46 M
Hz using a broad-band multiple-pulse sequence. Correlated internal mot
ions of the alkyl chain may be superimposed onto the reorientation of
the 'average' molecule. Such a model has recently been proposed and is
further tested in the present study. The quadrupolar splittings are u
sed in conjunction with the additive potential method to determine the
strength of the nematic mean field. The equilibrium probability of ea
ch conformer and conditional probabilities for transitions among diffe
rent conformers are evaluated to yield spectral densities of motion fo
r deuterons at different sites of the molecule. We use 'realistic' geo
metry to generate all possible conformations in the chain. Two differe
nt motional models for molecular reorientation are examined in detail.
The third-rate model appears to work better than the small step rotat
ional diffusion model in describing reorientation of an 'average' MBBA
molecule. We find that the third-rate model plus three different bond
-motions in the chain can successfully explain the observed temperatur
e, site and frequency dependences of the relaxation data in the nemati
c phase of MBBA. It also predicts discontinuities in the Zeeman spin l
attice relaxation rates at the nematic-isotropic transition.