Terahertz vibrational modes of crystalline salicylic acid by numerical model using periodic density functional theory

The terahertz vibrational modes of crystalline salicylic acid at 1-6THz were investigated using a vibrational calculation method in which the diagonalization of force constant matrix was estimated by periodic density functional calculations. The result proved sufficient to enable the terahertz vibrational modes to be assigned to lattice modes at 32.2-64.9 cm^-1 coupled with translational and rotational modes, intermolecular bending modes at 67.0-132.1 cm^-1 including the torsion of carboxyl groups, and out-of-plane intramolecular bending modes at 162.0-175.4cm^-1. The theoretical model presented allows for the interpretation of the terahertz spectra of organic crystals with a hydrogen-bonded dimer lacking the molecular vibrations of a large amplitude, by including effects arising from crystal periodicity.