Chemical bonds and weak interactions of methoxysalicylic acid isomers investigated by terahertz spectroscopy and density functional theory

The characteristic absorption spectra of isomers (3-, 4-, and 6-methoxysalicylic acid) were investigated by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4–3.0 THz. In parallel with the experimental study, the unit cells of the three isomers were calculated by density functional theory, and we found that the theoretical spectral absorption peaks were basically consistent with the experimental results. To better comprehensively interpret the origin of the characteristic absorption peaks, the potential energy distribution (PED) method was used to assign the vibrational modes of absorption peaks. The interaction of the system was analyzed qualitatively and quantitatively by using the interaction region indicator (IRI) and energy decomposition analysis based on forcefield (EDA-FF). The analysis indicated the weak interaction types of 3-, 4-, and 6-methoxysalicylic acid were dominated by van der Waals interaction. There are slight differences in intra- and inter-molecular weak hydrogen bonds among the three substances. The weak interaction is mainly contributed by dispersion. The results show that THz-TDS can accurately identify isomers. It provides a novel idea for studying molecular vibration characteristics, chemical bonds, and weak interactions combined with PED, IRI, and EDA-FF analysis methods.