Molecular motion and molecular interaction in the nematic and isotropic phases of a liquid crystal compound

Abstract

The far infra-red absorption (6–200 cm^–1) of a typical liquid crystal compound [N-(p-methoxy-benzylidene)-p-n-butylaniline: MBBA] has been studied over the nematic range (295–320 K) and beyond (up to 358 K); ten per cent concentrations in cyclo-octane (297 to 338 K); fifty per cent concentrations in n-octane (297 to 334 K); down to four per cent concentrations in benzene (296 K); and in d.c. fields up to 9 kV cm^–1. The far infra-red is dominated by a strong feature centred near 130 cm^–1, both sharper and of higher frequency than the Poley absorptions characteristic of most isotropic polar liquids to which the MBBA tends on dilution with increasing temperature. Qualitative and quantitative aspects show the 130 cm^–1 absorption to arise from the librational mode about the long axis of the MBBA molecules. The contour of the absorption (including that of the microwave region) is fitted in terms of both the Brot and Wyllie molecular-dynamic models with closely comparable apparent energy-well depths (16 to 19 kJ mol^–1). This appraisal is supported by a priori calculations of the form and depth of the energy wells in terms of the parameters established for phenyl-ring interactions in the Rae—Mason evaluations of the lattice energy of benzene.