FOURIER-TRANSFORM FAR-INFRARED SPECTROSCOPY OF THE HYDROGEN-BOND IN 4-N-ALKOXYBENZOIC ACIDS

Far-infrared transmittance spectra (200-30 cm(-1)) of the hydrogen bon d in cyclic dimers of 4-n-heptyl-, octyl- and nonyloxybenzoic acids ar e observed and investigated. The spectral bands at 42, 63, 68 and 82 c m(-1) are assigned to the four deformation vibrations: twisting, bendi ng (in and out of the dimer ring plane) and stretching by halfwidth sp ectral band temperature analysis. The hydrogen band evolution in the i sotropic and nematic phases is investigated. A cyclic <-> open dimer t ransformation at different vibration frequencies and polarizations is shown. The critical character of the temperature T, defined as the te mperature dividing the nematic phase of the nematics with hydrogen-bon ded molecules into high-temperature (classical) and low-temperature (s mectic-like) subphases, is demonstrated.