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

Far-IR spectroscopy (200-30 cm-1) was used to study the temperature evolution of the hydrogen bonds in the nematic liquid crystal phase of 4, n-heptyl-, octyl- and nonyl-oxybenzoic acids (HOBA, OOBA and NOBA). The observed absorption bands (v(OH) = 82, 68, 63 and 42 cm-1) are assigned to the four predominant vibrations respectively - stretching, bending (out of and in the dimer ring plane) and twisting. From a spectral band half-width temperature analysis hydrogen-bond evolution in the isotropic and nematic phases is investigated. A cyclic reversible open dimer transformation at different vibrational frequencies and polarizations is indicated.