Theoretical And Experimental Research On Terahertz Spectra Of Food Additives Vanillin And Its Derivatives

Terahertz(THz)wave is an electromagnetic wave with frequencies in the range of0.1?10 THz and wavelength of 30?3000 ?m.It is located between the microwave and infrared region of the electromagnetic spectrum and exhibits several unique merits,such as high penetration,low energy.Terahertz time-domain spectroscopy(THz-TDS)is a crucial technology for the THz application.As a new emerging spectral detection technology,it has a broad application prospect in maintaining food quality and safety,chemical identification and characterization,etc.This paper investigates the food additives Vanillin and its derivatives using a combination of THz-TDS technology and quantum chemical calculation approaches and obtaining their theoretical and experimental THz absorption spectra.While rapidly and non-destructively identifying these compounds with similar molecular structures,we also thoroughly research the formation mechanism of characteristic absorption peaks in the spectra and analyze the effects of particular molecular structure systems on materials' spectra and properties.The main research contents and results of this paper are as follows:1.The experimental study is performed on the THz spectra of Vanillin and its derivatives.The characteristic absorption spectra of Vanillin and its derivatives in the frequency range of 0.4?2.0 THz are measured by the THz-TDS system.The experimental spectra show that Vanillin and its derivatives have multiple characteristic absorption peaks in the frequency band of 0.4?2.0 THz.These absorption peaks with different intensities and positions can be used as the characteristic fingerprints of Vanillin and its derivatives in this low-frequency band and to distinguish these structurally similar materials.Simultaneously,the experimental results also provide a valuable reference for applying THz-TDS technology to identify and investigate compounds with subtle differences in the chemical structures in the field of food additives.2.The theoretical study is performed on the THz spectral of Vanillin and its derivatives.We analyze the advantages and disadvantages of various quantum chemical theory methods and computational models and establish the scheme of theoretical spectrum calculation based on density functional theory(DFT)and cluster structure.We use visualization and modeling software Gauss View and VMD to construct the cluster models of Vanillin and its derivatives.The theoretical THz spectra of Vanillin and its derivatives are obtained by DFT simulation calculation,and the differences between the theoretical spectra and the experimental spectra are analyzed in detail.The results show that the theoretically calculated spectra of Vanillin and its derivatives are mostly consistent with the experimentally measured spectra.3.The vibrational characteristics of THz spectra of Vanillin and its derivatives are investigated.To assign the absorption peaks in the spectra and analyze the vibrational properties of particular molecules,we utilize the vibrational mode automatic relevance determination(VMARD)method to examine the vibrational characteristics of Vanillin and its derivatives.The vibrational modes corresponding to the characteristic absorption peaks are assigned,and the percentage contribution of each characteristic vibrational mode is provided.The VMARD method results suggest that the vibrational modes of Vanillin and its derivatives are mainly dihedral angle torsion.Moreover,we use the Gauss View software to view the atomic vibrational animation corresponding to each absorption peak and via VMD software to draw the vibrational mode graph corresponding to the absorption peak.4.Research on the intermolecular weak interactions of the THz spectra of Vanillin and its derivatives.The existence of weak interactions in a particular molecular structure system will affect the absorption response of materials in the THz band.To better understand the origin of the characteristic absorption peaks on a molecular level,we use the independent gradient model based on Hirshfeld partition of molecular density(IGMH)method to analyze the intermolecular weak interaction of Vanillin and its derivatives.The IGMH results indicate that the types of intermolecular weak interactions of Vanillin and its derivatives are dominated by van der Waals interaction,and some of these compounds have a little hydrogen bonding and steric effect.These weak interactions will have some extent influence on the physical and chemical properties and the structural stability of Vanillin and its derivatives.Meanwhile,they are also the key to the existence of absorption characteristics of Vanillin and its derivatives in the THz band.The IGMH method provides a new way to analyze further the effects of weak interactions in molecular structure on the spectra and properties of materials.