Further theoretical analysis of absorption peaks of perylene-3,4,9,10-tetracarboxylic dianhydride in the terahertz region

Density functional theory calculations of its crystal elucidated that the absorption peaks of perylene 3,4,9,10-tetracarboxylic dianhydride in the THz region are caused by the frequency shift and the peak splitting of the in-phase and out-of-phase vibrations of two molecules in a crystal unit cell. Crystal-packing interactions are found to enable IR-inactive vibration in isolated molecules to become IR-active vibration along with frequency shift and absorption peak splittings.