Experimental and theoretical studies on THz spectra of phenylthiourea compounds

THz spectroscopy is a promising technique to study intermolecular interactions and molecular dynamics in organic crystals. In this work, aromatic thiourea compounds with methyl substituents were used as model systems to study the intermolecular interactions between aromatic rings and the dynamics of the methyl internal rotation which falls into the terahertz region. The THz spectra of two phenylthiourea derivatives, 4-methylthiocarbanilide and 1-(2-hydroxyethyl)-1-methyl-3-phenylthiourea, which are pharmaceutical intermediates, were measured using a TeraView TPS Spectra 3000 spectrometer at different concentrations in the mixture pellets with polyethylene powder. Three well-defined peaks were consistently observed at 1.22, 1.47 and 2.01 THz in the spectra of the former compound, and at 1.22, 1.57 and 2.00 THz in the spectra of the latter compound. The calculations using density functional theory of the isolated molecule model of the compounds reproduced these peaks well. It indicates that the absorption peaks should be caused by the low-frequency intramolecular vibrational modes of organic fragments, such as benzene rings, methyl group and 2-hydroxyethyl group.