Molecular order and dynamics of the nematogen MBBA

Abstract

We report on the measurements of quadrupolar splittings and spectral densities in the nematic phase of a partially deuterated liquid crystal p-methoxybenzylidene-p-n-butylaniline (MBBA). The Zeeman and quadrupolar spin-lattice relaxation times have been measured at 15·3 and 46 MHz using a broadband multiple-pulse sequence. Correlated internal motions 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 used in conjunction with the additive potential method to determine the strength of the nematic mean field. The equilibrium probability of each conformer and conditional probabilities for transitions among different conformers are evaluated to yield spectral densities of motion for deuterons at different sites of the molecule. We use ‘realistic’ geometry to generate all possible conformations in the chain. Two different motional models for molecular reorientation are examined in detail. The third-rate model appears to work better than the small step rotational 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 temperature, site and frequency dependences of the relaxation data in the nematic phase of MBBA. It also predicts discontinuities in the Zeeman spin-lattice relaxation rates at the nematic-isotropic transition.