THz molecular science in condensed phases
The terahertz (THz) frequency region is located between microwave and IR regions, and molecular motions and intermolecular interactions in the THz region reflect unique properties of materials, which are important to understand structural stabilization, chemical reactions, and functionalities of molecular systems. We have been applying spectroscopic methods in the THz region to various problems in the research field of molecular science. Molecular crystals in the solid state show vibrational structures even in the THz region at low temperatures, and mode assignment can be performed by using solid-state density functional theory and crystal structures obtained by X-ray diffraction measurement. The agreement between the experiment and theoretical calculations depends on how we treat the intermolecular interactions in the calculations. The normal modes are generally a mixture of intermolecular and intramolecular vibrational modes.