Investigation Of Intermolecular Interactions And Vibrational Mode Based On THz-TDS And DFT

In recent years,terahertz time-domain spectroscopy(THz-TDS)has been developed continuously,as a rapid and effective non-destructive testing technology,it has been applied to many fields such as biology,chemistry,medicine,materials and so on.As we know,the stretching and torsion of the weak intermolecular interactions as hydrogen bonds lie in the terahertz band,so that the terahertz vibration spectrum can show the weak intermolecular interactions clearly.These interactions play an important role in many fields such as crystal engineering,polymer materials,pharmacy,biochemistry,life science and so on.The calculation and analysis based on density functional theory(DFT)plays an important role in the application of terahertz spectroscopy,which can reproduce the terahertz experimental spectrum of crystals and explain the source of terahertz vibration absorption characteristics from a microscopic perspective.However,at present,only some simple theories are applied to the analysis,not to achieve a detailed analysis of the spectrum.In this paper,the terahertz spectra of some crystals are calculated and analyzed by potential energy distribution(PED)and reduced density gradient(RDG)methods on the basis of density functional theory calculation,and the contribution of intermolecular forces in terahertz vibration mode is obtained,so as to realize accurate identification of terahertz spectra.The main research contents of this paper are as follows:Firstly,THz-TDS experiments were carried out on urea with simple molecular structure at room temperature.In order to reproduce the experimental spectrum and explain the terahertz absorption characteristics,the density functional theory was used to calculate the monomolecules and clusters of urea.The results showed that the experimental spectra could not be reproduced by single molecule calculation,and the results of molecular cluster calculation using m06-2x method were in good agreement with the experimental spectra.By PED analysis and combining with Gauss View software,all vibration modes of urea crystals in the low-frequency terahertz range are identified,and it is found that these vibration modes are related to intermolecular hydrogen bonds.In order to better study these intermolecular hydrogen bond interactions,RDG analysis was used to judge the strength of these interactions,and the results were displayed in the form of graphics by VMD software.Secondly,with the increase of molecular structure complexity,the urea derivatives of 1,1-dimethyl urea and 1,3-dimethyl urea isomers were studied experimentally,and the THz vibration absorption spectra were obtained in the range of 0.6 ~ 1.8 THz at room temperature.The results showed that the experimental spectra of urea derivatives was quite different from that of urea,which fully confirmed the sensitivity of terahertz wave to molecular structure.In addition,the experimental spectra of these two isomers are also significantly different,which indicates that THz-TDS can effectively distinguish them.To explore the nature of terahertz vibration,DFT calculations using the freq=intmodes keyword were performed.The results showed that the B3LYP-D3 method can predict the experimental spectrum well,and with the help of Gauss View software the terahertz vibration modes can be identified,which are related to intermolecular hydrogen bonds.RDG analysis was used to study these intermolecular hydrogen bond interactions,and VMD software was used to visualize the position and strength of intermolecular hydrogen bond interactions in these two isomers.In order to verify the sensitivity of terahertz wave to structural isomers,the crystals of fumaric acid and maleic acid isomers were also studied.The experimental and theoretical results showed that the terahertz time-domain spectrum can effectively identify these two isomers,and the terahertz absorption characteristics they exhibit are related to intermolecular hydrogen bonds.Finally,the terahertz experiment and theoretical calculation of urea and fumaric acid cocrystals with complex molecular structure were studied.The cocrystals of fumaric acid and urea were obtained by solution recrystallization and grinding.It was found that the positions of their terahertz absorption peaks are basically the same,which means that the cocrystals prepared by the two methods above should be the same substance,and both methods can effectively prepare cocrystals.Compared with the experimental spectra of fumaric acid and urea crystals,the absorption peak of the cocrystals have the same position as that of fumaric acid,but the intensity is obviously enhanced.Importantly,a new characteristic absorption peak appeared.In order to find the source of this absorption peak,the bimolecule and tetramolecular models were derived from the chain layers with different molecular arrangement structures,and the culculational results showed they also haved completely different terahertz absorption characteristics,which confirmed the claim that terahertz waves are sensitive to changes in molecular configurations.According to the theoretical calculation and analysis results,the two vibration modes of cocrystals refer to the wing vibration caused by the expansion of intermolecular hydrogen bond and the torsion mode of fumaric acid and urea unit.