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 (6OBA, HOBA, OOBA, NOBA and DOBA) are used for the present investigations. Far IR spectroscopy is 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 reversible reaction open dimer transformation is indicated at different vibration frequencies and polarizations. The activation energy for the dimer and monomer reorientation is calculated and compared with that per hydrogen bond. The correlation lengths parallel and perpendicular to the nematic director ( xi sub( parallel ) and xi sub( perpendicular )) are calculated and an anisotropy is found in the temperature dependence of the critical index of the correlation length. A comparison with nonoriented nematics with hydrogen bonded molecules is presented.