Similarities And Differences Analysis Of Coumarin And Its Derivatives Studied By Terahertz Spectroscopy

Terahertz(THz)radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum with a frequency of 0.1-10 THz,covering a wavelength range of0.03-3 mm,which belongs to the interdisciplinary of macro-electronics and micro-photonics.Its spectrum can provide detailed information about structure and intermolecular of molecule,especially for biological macromolecules,the feature help Terahertz time-domain spectroscopy(THz-TDS)technology to be a powerful tool for molecular science research.In this paper,the absorption spectra of coumarin and its derivatives in THz band were studied by means of THZTDS system and density functional theory(DFT),and systematically explored the characteristics and sources of absorption peaks in the spectrum.The specific research are as follows:1.Measurement of experimental spectra of coumarin and its derivatives.With the help of THz-TDS system,the time-domain waveform of coumarin and its derivatives were measured from 0.4 to 2.8 THz at room-temperature,and the THz spectra in the frequency-domain were obtained by discrete Fourier transform.By comparing the characteristics of the spectra between four compounds,it was found that the experimental spectral feature of coumarin and 6-methylcoumarin are similar but different.On the contrary,the positional isomerism of hydroxyl group exhibits significant differences in the experimental spectra between 4-hydroxycoumarin and 7-hydroxycoumarin.These results confirm that THz wave is quite sensitive to molecular structures,so its spectroscopy is an effective measure to distinguish compounds with similar structures.2.Theoretical spectra calculation of coumarin and its derivatives.Using three different theoretical methods,the paper optimized the stable spatial configurations of the seven-molecule cluster model of coumarin and its derivative,the characteristic waveforms in the THz band were further calculated on the basis.The results show that the B3 LYP functional with dispersion correction(B3LYP-D3)is the ideal choice for simulating the trend of experimental waveform,the position and intensity of the absorption peaks.By matching the experimental spectra with three different theoretical spectra,it is found that electrostatic interaction dominates the formation of absorption peaks in the low frequency region,and the absorption peaks in the higher frequency are mainly affected by van der Waals interaction.Theoretical calculations perfectly reproduce the optical absorption properties appearing in the experimental spectra,providing reliable data for further spectral analysis.3.Study on spectral vibration modes of coumarin and its derivatives.In order to deeply interpret the information carried by the THz spectrum,the paper attributed and analyzed the source and vibrational mode of the characteristic absorption peaks in the theoretical spectra by combining the potential energy distribution(PED)analysis and the vibrational vector diagram of molecular cluster provided by Gauss View software.The analysis results indicate that most vibrational modes of coumarin,6-methylcoumarin and 7-hydroxycoumarin were belong to bond angle bending,4-hydroxycoumarin mainly attributed to the dihedral angle on the molecular plane.At the same time,it was found that the molecules relying on electrostatic interaction attract or move away from each other in the same plane,while the absorption peaks dominated by van der Waals interaction are arranged in an up-down parallel manner between molecules.4.The spectral similarities analysis of coumarin and 6-methylcoumarin.By analyzing the distribution of electrostatic potential and van der Waals potential on the molecular surface,the paper explained the reasons for the spectral similarities from the level of weak intermolecular interactions.Analysis results demonstrate that the substitution of methyl group did not affect the electrostatic potential distribution of single-molecule and multi-molecular clusters,therefore,the same intermolecular hydrogen bonding modes and molecular binding forms determine the similar optical absorption properties of coumarin and 6-methylcoumarin in the THz band.In addition,the distribution of van der Waals potential is closely related to the carbon atoms of the benzene and oxygen heterocycles,which is one of the reasons for the similarity of the THz spectra of coumarin and 6-methylcoumarin.5.The spectral differences analysis of 4-hydroxycoumarin and 7-hydroxycoumarin.According to the distribution characteristics of the electrostatic potential and van der Waals potential on the molecular surface,the source of the spectral difference between 4-hydroxycoumarin and 7-hydroxycoumarin was analyzed from the point of view of molecular binding mode.The analysis results show that although the van der Waals potential distributions of 4-hydroxycoumarin and 7-hydroxycoumarin are similar because of the 9 carbon atoms in the central region of the benzene ring and the oxygen-containing heterocyclic region.The positional isomerization of hydroxyl groups changes the electrostatic potential distribution characteristics of single molecules and multi-molecules,the different number of hydrogen bonds between molecules and the way of molecular bonding lead to obvious differences in the spectra of 4-hydroxycoumarin and 7-hydroxycoumarin.These results highlight the significant role of electrostatic potential complementarity in stabilizing the spatial structure of molecules the auxiliary role of van der Waals interaction in molecular spatial configuration.