MOLECULAR ORDER AND DYNAMICS OF THE NEMATOGEN MBBA - MODELING OF DEUTERIUM NMR OBSERVABLES

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.