| Imidazole,its isomers and its derivatives are widely used in pesticides,antimicrobial agents,growth regulators and other agrochemicals as important basic raw materials for pesticide production,and their biological activity and chemical properties are directly related to the effects of agrochemicals.The identification of imidazole isomers and derivatives with similar structures is of high scientific research value because they have different objects and effects in agricultural production,and improper selection even have opposite effects,which may cause economic losses and endanger human health and safety.Because the different effects in production are related to the microstructure of the isomers,it is necessary to study the microstructure information of imidazole,its isomers and derivatives.Terahertz(THz)waves is an electromagnetic wave with frequencies in the range of 0.1～10 THz(wavelength of 30～3000 μm),which have unique advantages such as strong penetrability,low energy,and good stability.Terahertz time-domain spectroscopy(THz-TDS)can identify isomers with similar structures without sample damage quickly and effectively with high sensitivity and high time resolution.It is a very efficient tool for identifying and analyzing isomers.In addition,microstructural changes such as vibration characteristics and intermolecular weak interactions are distributed in the terahertz band,making THz-TDS an effective means to study microstructural information such as molecular structure and intermolecular weak interactions.Calculation and analysis based on quantum chemistry occupies an important position in the application of terahertz spectroscopy,which can predict the terahertz experimental spectrum of crystals.In this paper,the experimental and theoretical THz absorption spectra of imidazole,its isomer pyrazole and its derivatives benzimidazole,2-methylbenzimidazole and2-aminobenzimidazole in the range of 0.4～2.4 THz were obtained by THz-TDS system and quantum chemical calculation.By identifying the relationship between the microstructure and the THz vibration characteristics,the absorption properties and dispersion properties of terahertz spectra are explained in terms of differences in vibration characteristics and differences in intermolecular weak interactions,and the generation mechanism of terahertz characteristic absorption peaks is revealed.While achieving rapid and non-destructive identification of isomers,the microstructural information of the characteristic absorption peaks have been thoroughly investigated.The main content of this paper is summarized as follows:1.Experimental studies on imidazole,its isomer and its derivatives were carried out.The THz time-domain spectral data of each experimental sample were measured by THz-TDS spectrometer CCT-1800,and the absorbance was used as an optical parameter to characterize the absorption intensity to terahertz waves.The experimental results show that imidazole has three characteristic absorption peaks at 1.94 THz,2.18 THz and 2.33 THz,and its isomer pyrazole has three characteristic absorption peaks at 1.63 THz,1.92 THz and 2.12 THz.The imidazole derivative benzimidazole has five characteristic absorption peaks at 1.24 THz,1.39 THz,1.58 THz,1.91 THz and 2.23 THz.The 2-methylbenzimidazole has three characteristic absorption peaks at 0.91 THz,1.61 THz and 1.95 THz.The2-Aminobenzimidazole had two characteristic absorption peaks at 1.17 THz and 2.29 THz.The isomers of imidazole and pyrazole have their own absorption peaks at 2.33 THz and1.63 THz.Benzimidazole and its cis-substituted derivatives 2-methylbenzimidazole and2-aminobenzimidazole also differed significantly in the number and frequency of THz characteristic absorption peaks,showing their own unique fingerprint characteristics,so the absorption peak frequencies can be used as a basis to distinguish them.2.The molecular structure models of imidazole,its isomer and its derivatives were constructed and calculated.Unimolecule and molecular cluster models were extracted from known crystal structure files,and the extracted molecular structures were subjected to structure optimization and frequency calculation based on density function theory(DFT),and the theoretical calculated predicted spectra were compared with the experimental spectra.The the theoretical calculation results showed that the molecular cluster model using the B3LYP-D3 function and the 6-311+G(d,p)basis group could predict the THz experimental absorption spectra better,and the trends of the theoretical spectra and the experimental spectra were consistent basically,which verified that the optimized theoretical cluster model met the research requirements.In addition,the unimolecules of the five substances have no characteristic absorption peaks in the range of 0.4～2.4 THz,indicating that the characteristic absorption peaks related to intermolecular weak interactions.3.The vibration characteristics of imidazole,its isomer and its derivatives were studied.The vibration modes of different characteristic absorption peaks were decomposed by potential energy distribution(PED),and vibration analysis results were identified and analyzed.The PED analysis results show that the vibration modes of imidazole and isomer pyrazole were different.The vibration modes of imidazole clusters are mainly bond angle bending and dihedral torsion,while the vibration modes of pyrazole clusters are mainly dihedral torsion.The vibration modes of the imidazole derivative benzimidazole and its2-substituted derivatives 2-methylbenzimidazole and 2-aminobenzimidazole are also different.The vibration modes of the benzimidazole clusters are mainly bond length stretching,bond angle bending and dihedral angle torsion,the vibration modes of2-methylbenzimidazole clusters are mainly bond angle bending and dihedral torsion,and the vibration modes of 2-aminobenzimidazole clusters are also mainly bond angle bending and dihedral twisting.The main source of absorption characteristics is the low frequency vibration of molecules.The absorption characteristics are derived from the low-frequency vibrations of molecules,the different low-frequency vibration characteristics produced different frequencies of absorption peaks,while atoms with the same vibrational characteristics are not on the same molecule,so the differences in vibrational characteristics are related to weak intermolecular interactions.4.The absorption properties and dispersion properties of imidazole,its isomer and its derivatives were studied.The intermolecular weak interactions were analyzed qualitatively and quantitatively by interaction region indicator(IRI)graphical analysis,energy decomposition analysis based on force field(EDA-FF)and atoms-in-molecules(AIM)topological analysis,and further studied the intrinsic relationship between the intermolecular weak interactions and vibration characteristics in detail.The visualization analysis results of intermolecular weak interactions show that the vibration characteristics of imidazole are determined by the electrostatic interaction of intermolecular N-H···N hydrogen bond and the dispersion interaction contributed by heavy atoms on the five-membered heterocyclic ring,in which the bond energy of hydrogen bond is-8.34kcal/mol;the vibration characteristics of pyrazole are dominated by the electrostatic interaction of intermolecular N-H···N hydrogen bond,and the bond energies of hydrogen bonds are-5.77kcal/mol,-6.82kcal/mol and-9.74kcal/mol.The overall hydrogen bond strength of pyrazole is weaker than that of imidazole,but the number of hydrogen bonds is more than that of imidazole.The vibration characteristics of benzimidazole are dominated by the dispersion interaction from the heavy atoms on the benzene and imidazole rings.The vibration characteristics of 2-methylbenzimidazole are determined by the electrostatic interaction of intermolecular N-H···N hydrogen bond and the dispersion interaction,where the hydrogen bond energy is-8.29kcal/mol,and the dispersion interaction is mainly contributed by the donor and acceptor weight atoms of the hydrogen bond.The vibration characteristics of 2-aminobenzimidazole are mainly dominated by the electrostatic interaction of intermolecular N-H···N hydrogen bond,and the bond energies of hydrogen bonds are-6.46kcal/mol,-5.44kcal/mol and-8.07kcal/mol,respectively.The overall hydrogen bond strength of 2-aminobenzimidazole are weaker than that of the other two substances,but the number of hydrogen bonds is more than that of the other two substances.In summary,the different types and essences of weak interactions,the number and strength of hydrogen bonds,and the atoms contributing to dispersion make isomers and isosubstituted derivatives have different molecular vibration characteristics.Therefore,there is the significant difference in the characteristic absorption peak frequencies in the0.4～2.4 THz band.This study analyse the origin of the generation of the THz characteristic absorption peaks from the microscopic perspective of vibrational features and weak intermolecular interactions,and further explain the absorption characteristics and dispersion properties of terahertz spectra.The present study analyses the origin of the THz characteristic absorption peaks from the microscopic perspective of vibrational characteristics and intermolecular weak interactions,and further explains the absorption and dispersion characteristics of terahertz spectra,thus revealing that the formation mechanism of characteristic absorption peaks is closely related to the differences in vibrational characteristics and intermolecular weak interactions of molecules,which provides experimental and theoretical references for the identification of pesticide raw materials containing nitrogen heterocyclic isomers and the acquisition of microstructural information. |
