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.