Low-Frequency Vibrations of Indole Derivatives by Terahertz Time-Domain Spectroscopy

Abstract

Several indole derivatives with different 3- substituents have been investigated by terahertz (THz) time-domain spectroscopy. The low-frequency absorption spectra and refractive indices were obtained in the range of 0.2 THz to 2.5 THz (7 cm-1 to 83 cm-1). These derivatives with different substituents present distinct features, which suggests that THz spectroscopy is sensitive to different structures and components of these chemicals. The density functional theory was employed to calculate the low-frequency vibrational properties of indole-3-carboxylic acid and indole-3-propionic acid based on their crystal structures, and the intermolecular interactions were involved. Meanwhile, the temperature dependence of the spectra agreed with the calculated results. The quantitative analysis of a ternary mixture was studied by taking the THz fingerprints into account, and the results demonstrate THz spectroscopy has great potential for the practical applications in biochemistry and pharmaceutics.