FAR IR SPECTROSCOPIC INVESTIGATIONS OF NEMATIC LIQUID-CRYSTALS WITH HYDROGEN-BONDED MOLECULES - COMPARISON WITH NONORIENTED SYSTEMS

The 6 - 10th homologues of 4-n-alkoxybenzoic acid, i.e. hexyl-, heptyl -, octyl-, nonyl- and decyloxybenzoic acids (60BA, HOBA, OOBA, NOBA an d DOBA) are used for the present investigations. Far IR spectroscopy i s used to study the state of hydrogen bonding at temperature variation of the nematic Liquid crystal phase. The observed spectral bands are assigned to different OH deformation vibrations. From the analysis of the temperature dependence of the spectral band widths a cyclic <-> op en dimer transformation is indicated at different vibration frequencie s and polarizations. The activation energy for the dimer and monomer r eorientation is calculated and compared with that per hydrogen bond. T he correlation lengths parallel and perpendicular to the nematic direc tor (xi(parallel to) and xi(perpendicular to)) are calculated and an a nisotropy is found in the temperature dependence of the critical index of the correlation length. A comparison with nonoriented nematics wit h hydrogen bonded molecules is presented.