Analysis of Intermolecular Weak Interactions and Vibrational Characteristics for Vanillin and Ortho-Vanillin by Terahertz Spectroscopy and Density Functional Theory

Abstract

In this article, the characteristic absorption spectra of Vanillin and ortho-Vanillin were investigated by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4–2.0 THz. Using quantum chemistry tools, density functional theory calculations were performed to obtain the theoretical THz spectra. To better understand the THz absorption spectra on a molecular level and to systematically and comprehensively interpret the origin of the characteristic absorption peaks, the vibrational characteristics and the intermolecular weak interactions of these two compounds were analyzed using a cluster model. We used the vibrational mode automatic relevance determination method to assign the vibrational modes of all absorption peaks and provide the percentage contribution of these modes. The vibrational modes of Vanillin and ortho-Vanillin were mainly dihedral angle torsion. The intermolecular weak interactions were analyzed using the independent gradient model based on Hirshfeld partition of molecular density method. The results indicated that the weak interaction types of Vanillin and ortho-Vanillin were dominated by van der Waals interaction. This work has demonstrated that using THz-TDS combined with quantum chemical calculations is an effective way to identify and investigate isomers. This method has significant potential applications for exploring the relationship between the biomolecular structure and the material performance.