Infrared spectroscopic study of molecular hydrogen bonding in chiral smectic liquid crystals

We report the use of Fourier-transform infrared (IR) spectroscopy to probe intermolecular and intramolecular hydrogen bonding in thermotropic liquid-c rystal phases. Infrared spectra of aligned smectic liquid crystal materials vs temperature, and of isotropic liquid-crystal mixtures vs concentration were measured in homologs both with and without hydrogen bonding. Hydrogen bonding significantly changes the direction and magnitude of the vibrationa l dipole transition moments, causing marked changes in the IR dichroic abso rbance profiles of hydrogen-bonded molecular subfragments. A GAUSSIAN94 com putation of the directions, magnitudes, and frequencies of the vibrational dipole moments of molecular subfragments shows good agreement with the expe rimental data. The results show that IR dichroism can be an effective probe of hydrogen bonding in liquid-crystal phases.