Vibration Characteristics, Chemical Bonds and Weak Interactions of 4- And 5-Hydroxysalicylic Acid Investigated by Terahertz Spectroscopy and Density Functional Theory

Abstract

The terahertz (THz) absorption spectra of 4- and 5-hydroxysalicylic acid were measured by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4–2.8 THz. The unit cells of both were calculated by density functional theory (DFT), and the theoretical simulation and experimental data were basically in agreement. To further interpret the origin of the characteristic absorption peaks, vibration characteristics and interactions of the unit cell model were analyzed by using potential energy distribution (PED) and interaction region indicator (IRI) method. The analysis indicated that the absorption peaks of 4- and 5-hydroxysalicylic acid were mainly derived from the vibration mode of dihedral angle torsion, and were closely related to the hydrogen bonds. And the intermolecular weak interaction of both were mainly dominated in quantity by van der Waals interaction. The results show that this is an effective solution to identify isomers and explain the formation mechanism of characteristic absorption peaks by using THz-TDS and combining PED and IRI methods.

Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal–Organic Framework

“…Indeed, low-frequency vibrational spectroscopy covering the terahertz range (0.1–5 THz, 3–166 cm –1 ) is a powerful and very general technique for probing long-range weak forces in condensed phase materials. 39 − 42 The phonon modes at these energies often involve motions of entire molecules or large portions of molecules, which explore significant regions of the intermolecular potential energy hypersurface. 43 …”

“…In particular, because of its extreme sensitivity to minor changes in condensed phase structures, THz vibrational spectroscopy is an ideal tool for determining the configuration of water molecules in the pore of MIL-53­(Al)- np . Indeed, low-frequency vibrational spectroscopy covering the terahertz range (0.1–5 THz, 3–166 cm –1 ) is a powerful and very general technique for probing long-range weak forces in condensed phase materials. − The phonon modes at these energies often involve motions of entire molecules or large portions of molecules, which explore significant regions of the intermolecular potential energy hypersurface …”