Weak Interaction Study Of Terahertz Spectra For Salicylic Acid And Its Derivatives

The hydrogen bonds,halogen bonds,van der Waals forces and ?···? interactions and other weak interactions between molecules in the crystal are all in the terahertz frequency band,and stability.Terahertz time-domain spectroscopy technology,as an emerging fast and effective non-destructive spectroscopy detection technology,has been an effective tool for detecting weak interactions between molecules.Calculations based on density functional theory can reproduce the terahertz experimental spectrum of the crystal,and explain the mechanism of the terahertz vibration absorption characteristics from a microscopic point of view.However,most of the current researches on the terahertz spectrum of substances focus on experimental detection,or only apply some simple theories for analysis,which cannot analyze the resources of characteristic absorption peaks in experimental spectra in detail.In this paper,by combining the THz-TDS with the theoretical calculation of quantum chemistry,the terahertz spectra of salicylic acid and a series of crystals with similar structures are analyzed.The main content of this paper is summarized as follows:THz-TDS technology was used to detect the terahertz characteristic absorption spectra of two isomers(salicylic acid,2,4-dihydroxybenzaldehyde)in the range of 0.8-2.6 THz,and the B3LYP-D3 method in density functional theory was used in the Gaussian software to perform geometric optimization and frequency calculation on the single-molecules and molecular clusters of them.It was found that only one absorption peak appeared in the theoretical simulation calculation of a single-molecule of salicylic acid,while 2,4-dihydroxybenzaldehyde did not have any absorption peak in the 0.8-2.6THz range.However,their molecular cluster theoretical calculations all showed four characteristic absorption peaks,which are in good agreement with the experimental spectra and can reproduce the experimental spectra well.The results indicate that single-molecule calculations can't consider the effects of weak interactions between molecules and reproduce the experimental spectra,while the theoretical calculation spectra of molecular clusters are in good agreement with the experimental spectra,and it can be reproduced well.In addition,the characteristics are mostly derived from the intermolecular vibration mode.By the potential energy distribution analysis(PED)method,the main composition of the normal vibration mode and the cluster molecular vibration vector diagram are obtained,and the vibration mode of the theoretical spectral absorption peak is assigned in detail.In order to visually analyze the weak intermolecular interaction of salicylic acid and its isomers,Hirshfeld surface analysis was carried out with the help of Multiwfn and VMD software,which further proved that the absorption peaks of them are mainly derived from hydrogen bonds.The terahertz spectrum characteristics of the crystals of two derivatives(4-chlorosalicylic acid,5-chlorosalicylic acid)obtained after chlorination of salicylic acid were studied.They are only different in the position of the chlorine atom on the benzene ring,but the THz absorption spectra obtained at room temperature are obviously different.These results show that THz-TDS technology is very sensitive to subtle differences in crystal structure and can be used to identify 4-chlorosalicylic acid and 5-chlorosalicylic acid.In order to find out the cause of terahertz characteristic absorption peaks and reproduce the experimental spectrum,the theoretical simulation calculation based on DFT was performed on its single-molecules and molecular clusters respectively.The results reveal that the singlemolecule calculation can't reproduce the experimental spectrum,and the molecular cluster calculation can better predict the absorption peaks of the experimental spectrum.By using PED method and combining it with the molecular vibration animation displayed in Gauss View software,the vibration modes of the THz absorption spectra of 4-chlorosalicylic acid and 5-chlorosalicylic acid crystals were identified,and it was found that these vibration modes are related to intermolecular hydrogen bonds.The dorm map and 2D fingerprint of Hirshfeld surface analysis were used to visualize the position and type of weak interactions between molecules such as hydrogen bonds in the crystals of these two substances.The results show that the combination of THz-TDS technology and theoretical calculations can effectively identify salicylic acid and its derivatives,and can identify the characteristic absorption peaks in the low-frequency THz band in detail,Which not only promotes the use of THz-TDS technology in medicine,and its application in the identification of chemical isomers and chemical isomers,and also provides an effective reference for detecting weak interactions such as intermolecular hydrogen bonds in crystals with similar structures.