Study of weak intermolecular interactions and vibrational modes in hydroxybenzoic acid isomers using terahertz spectroscopy and density functional theory techniques

In this study, the terahertz (THz) absorption spectra of 2-, 3-, and 4-hydroxybenzoic acid molecules were recorded in the frequency range of 0.6-2.0 THz using the terahertz time-domain spectroscopy (THz-TDS) technique. The characteristic absorption peaks of these three hydroxybenzoic acid isomers were identified. The peak frequencies of the 2-, 3-, and 4-hydroxybenzoic acid molecules were significantly different from each other. Apart from experimental studies, theoretical studies using the density functional theory (DFT) technique were also conducted on the unit cells of the hydroxybenzoic acid molecules for simulating the experimental THz spectral features. The results were in good agreement with the experimentally obtained results. The vibrational characteristics and the weak intermolecular interactions of the three hydroxybenzoic acid isomers were analyzed by studying the potential energy distribution (PED) and using the energy decomposition analysis (EDA) technique based on the molecular force-field (FF; EDA-FF) methods to better understand the THz absorption spectra on a molecular level. The studies helped to systematically and comprehensively interpret the origin of the characteristic absorption peaks. The data obtained by studying the PED were used to explain the absorption peak mechanism in the theoretical spectra of the three isomers. The theoretical spectra were predicted using the DFT technique. The results obtained using the EDA-FF method revealed that the vibrational modes of the 2-hydroxybenzoic acid system are primarily the angle between the vector and the plane, while the vibrational modes of the 3-hydroxybenzoic acid and 4-hydroxybenzoic acid systems were primarily the dihedral angle (torsion). Results obtained using the EDA-FF analysis and the visual molecular dynamics (VMD) visualization methods revealed that the weak interactions exhibited by the 2-, 3-, and 4-hydroxybenzoic acid systems were primarily dispersive in nature. The results presented in this study revealed that the intermolecular hydrogen bonds (HBs) exhibited an obvious resonance response and played key roles in generating the low frequency vibrations in the THz region.