Terahertz spectral vibrational properties and weak interactions analysis of caffeic acid and ferulic acid

Abstract

Caffeic acid and its derivative, ferulic acid, are a class of natural phenolic compounds with strong antioxidant properties and have good application prospects in the field of medicine. In this paper, the spectral characteristics and the types of intermolecular interactions of caffeic acid and ferulic acid were analyzed using terahertz (THz) detection method combined with quantum chemical correlation analysis methods. The absorption spectra of caffeic acid and ferulic acid in the 0.4–2.0 THz band were measured by terahertz time-domain spectroscopy (THz-TDS) detection system. The theoretical spectra of caffeic acid and ferulic acid in this band were simulated by structure optimization and frequency calculation with the help of density functional theory (DFT), and the theoretical and experimental spectra were in good agreement. To resolve the spectral information in detail, we used the Potential Energy Distribution (PED) method to assign the vibrational modes of the absorption peaks, then identified the types of intermolecular interactions by interaction region indicator (IRI) and energy decomposition analysis based on forcefield (EDA-FF) methods to explain the reasons for the formation of vibrations. The results show that the main vibrational modes of caffeic acid in the absorption peak are dihedral torsion, for ferulic acid are bond angle bending and dihedral torsion. Intermolecular interactions play an important role in these vibrations, with van der Waals effects predominant, while the intermolecular hydrogen bonding model overall vibrates. This work demonstrates that using THz spectroscopy combined with DFT, IRI, and EDA-FF methods, can analyze the weak interaction types of structurally similar substances efficiently.