Method for researching vibration mode and weak interaction of methyl p-hydroxybenzoate by combining terahertz spectrum and solid-state density functional

Abstract

The invention provides a method for researching the vibration mode and weak interaction of methyl p-hydroxybenzoate by combining experiments and theories on the basis of a terahertz spectrum technology and a solid-state density functional theory. The method comprises the following steps: firstly, measuring an absorption spectrum of methyl p-hydroxybenzoate in a range of 0.5-3.0 THz by utilizing a terahertz time-domain spectroscopy system; then carrying out structure optimization and frequency calculation on the crystal by adopting solid density functional software CRYSTAL17 based on a periodic system to obtain an infrared absorption spectrum of the sample; and finally, carrying out comparative analysis on the calculated spectrum and the experimental spectrum, distributing the vibration modes by using a vibration mode automatic correlation determination method, carrying out deep analysis on weak interaction based on an independent gradient model of Hirshfeld partition, and exploring the source of an absorption peak in the terahertz spectrum. Through the analysis, the micro-level information of the methyl p-hydroxybenzoate crystal is supplemented, which has profound significance for predicting the molecular structure and exploring the material property.

Description

Technical Field

[0001] The invention belongs to the technical field of terahertz spectrum detection, and particularly relates to a method for studying vibration modes and weak interactions of methyl parahydroxybenzoate. Background Art

[0002] In order to meet the needs of food diversification and extend the shelf life of food, various additives and preservatives are widely used in different fields. In recent years, the illegal use of food additives has become common, and improper use of food additives may endanger human health. Therefore, the identification and study of such substances are particularly important for quality assurance and consumer safety. Methylparaben (MeP), a derivative of p-hydroxybenzoic acid, is a stable non-volatile compound that naturally occurs in cloudberries, white wine and bourbon vanilla. Methylparaben is often used as a preservative in the food, cosmetics and pharmaceutical industries because of its antimicrobial and antifungal effects, and has been used as an antibacterial preservative in food, medicine and cosmetics for more than 50 years. As a molecular crystal, methylparaben generally appears in solid form at room temperature, and the filling forces that assemble its molecular crystals are non-covalent, such as hydrogen bonds and van der Waals interactions. Although these weak intermolecular forces are very weak, they play an important role in various physical, chemical and biological phenomena. However, traditional detection methods such as fluorescence spectroscopy, single crystal X-ray diffraction and liquid chromatography cannot fully describe the intermolecular interactions in crystals.

[0003] Based on the combination of terahertz experimental spectroscopy and solid-state density functional calculations, the vibration modes and weak interactions in methyl paraben can be explored from a microscopic perspective, and the properties and reaction mechanisms of the substance can be inferred. In the past few decades, the development of terahertz technology and first-principles theory for dealing with non-covalent interactions has made terahertz spectroscopy a powerful tool for weak bond detection in crystals. This shows that terahertz spectroscopy combined with first-principles calculations can provide information on weak molecular interactions in crystals in a non-destructive way. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for studying the vibration modes and weak interactions of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory. To achieve the above objectives, the present invention adopts the following technical solutions:

[0005] A method for studying the vibration mode and weak interaction of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory comprises the following steps:

[0006] Step 1: Sample preparation

[0007] The sample sheet and polyethylene are mixed in proportion, ground, and tableted to obtain a methyl parahydroxybenzoate sample tablet with a smooth surface;

[0008] Step 2: Terahertz time-domain spectroscopy measurement

[0009] Using the terahertz time-domain spectroscopy system, the time-domain signals of all samples collected are Fourier transformed to obtain the frequency-domain signals of the samples, and the absorbance of the samples is obtained by calculation;

[0010] Step 3: Terahertz spectrum calculation

[0011] Perform theoretical terahertz spectrum calculations. Perform frequency calculations after structural optimization to obtain theoretically calculated terahertz absorption spectra;

[0012] Step 4: Comparative Analysis

[0013] Compare and analyze the experimental terahertz absorption spectrum in step 2 with the theoretical terahertz absorption spectrum in step 3;

[0014] Step 5: Vibration Mode Analysis

[0015] Assign the vibration modes corresponding to each absorption peak in the theoretical terahertz absorption spectrum and perform vibration analysis;

[0016] Step 6: Weak interaction analysis

[0017] Based on the optimized structure, the position, composition and strength of weak interactions in the structure are investigated.

[0018] Preferably, the terahertz time-domain spectrometer used in step 2 is a CCT-1800 terahertz time-domain spectrometer independently developed by China Huaxun Ark Technology Co., Ltd.

[0019] Preferably, the geometry optimization and frequency calculation in step 3 are both implemented using the periodic system calculation program CRYSTAL17.

[0020] Preferably, in step 3, PBE density functional theory and pob-TZVP basis set are used, and a semi-empirical Grimme-D3 London dispersion force correction term is added, and the energy convergence criterion of geometry optimization is set to ΔE<10 -8 .

[0021] Preferably, the energy convergence standard of the frequency calculation in step 3 is set to ΔE<10 -9 and the infrared intensity was determined by the Berry phase method.

[0022] Preferably, the vibration mode analysis described in step 5 is implemented by a Vibrational Mode Automatic Relevance Determination (VMARD) method.

[0023] Preferably, the weak interaction analysis described in step 6 adopts an independent gradient model based on Hirshfeld Partition of Molecular Density (IGMH) and "Atoms in Molecules" (AIM) topological analysis theory.

[0024] Through the above scheme, compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention provides a method for studying the vibration mode and weak interaction of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory. First, the time-domain terahertz spectrum of methyl parahydroxybenzoate is obtained; then the crystal structure is optimized and the frequency is calculated using the periodic system calculation program CRYSTAL17, and its theoretical calculation spectrum is further obtained; the calculation results are compared and analyzed with the experimental results; and the origin of the characteristic peaks in the spectrum is further analyzed from the two perspectives of vibration mode and weak interaction. This has far-reaching significance for promoting the study of weak interactions in molecular crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein

[0027] Figure 1 This is a comparison diagram of the terahertz experimental and theoretical spectra of the methyl p-hydroxybenzoate crystal of the present invention.

[0028] Figure 2 The vibration modes of methyl p-hydroxybenzoate of the present invention are simple vibration coordinates at 1.74 THz, 2.62 THz and 2.99 THz respectively.

[0029] Figure 3 It is the IGMH analysis result of methyl p-hydroxybenzoate of the present invention.

[0030] Figure 4 It is the AIM topological analysis result of methyl paraben of the present invention. DETAILED DESCRIPTION

[0031] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not based on limiting the present invention.

[0032] The vibration modes and weak interactions of methyl paraben were studied using the following methods and steps, and the specific implementation steps are as follows:

[0033] 1. Terahertz spectroscopy experiment

[0034] (1) Sample preparation of methyl paraben crystals

[0035] The purity of methyl paraben and polyethylene powder used in the experiment was greater than 99%, and they could be used without further purification. First, the sample and polyethylene powder were weighed separately using an electronic analytical balance, and mixed and ground in a mass ratio of 3:1; secondly, the pressure value of the FW-4A tablet press was set to 10MPa, and the mixture was pressed into a sample with a diameter of 13mm and a weight of about 200mg under this pressure, keeping the surface of the sample smooth and free of cracks; finally, the sample was placed in a 50℃ vacuum oven for 2 hours to remove moisture.

[0036] (2) Terahertz spectroscopy of methyl paraben crystals

[0037] In the experiment, the terahertz time-domain spectrometer CCT-1800 was used to measure the terahertz time-domain spectrum of methyl paraben in the range of 0.5 to 3.0 THz. The spectrometer uses a 780nm ultra-short pulse fiber laser with a signal-to-noise ratio of more than 70dB. The laser beam is divided into pump light and detection light through a cubic beam splitter, which are used for the generation and detection of terahertz waves respectively. To ensure the accuracy of the experimental data, the terahertz optical path is kept closed and filled with dry nitrogen throughout the experiment to ensure that the relative humidity in the sample chamber is less than 2% to reduce the influence of moisture in the air on the absorption of terahertz waves.

[0038] (3) Data processing

[0039] The obtained terahertz time domain signal is converted into a frequency domain signal through fast Fourier transform to obtain the terahertz absorption spectrum in the frequency domain.

[0040] 2. Terahertz spectroscopy calculation

[0041] (1) Geometry Optimization

[0042] The crystal data required in the calculation process were all from the Cambridge Crystallographic Data Centre (CCDC). In the parameter setting, PBE density functional and pob-TZVP basis set were used, and the semi-empirical Grimme-D3 London dispersion force correction term was added. The energy convergence criterion for geometry optimization was set to ΔE<10 -8 , and obtain the lowest energy structure.

[0043] (2) Frequency calculation

[0044] Based on the optimized structure, CRYSTAL17 was used to perform frequency calculations, and the energy convergence criterion for frequency calculations was set to ΔE<10 -9 and the infrared intensity was determined by the Berry phase method.

[0045] 3. Results Analysis

[0046] (1) Comparison of theoretical and experimental spectra

[0047] pass Figure 1 It can be clearly seen that the experimental spectrum of methyl paraben is between 1.57 and

[0048] There are three characteristic absorption peaks at 2.62 and 2.93 THz. These peaks can be regarded as the characteristic fingerprints of methyl paraben in the range of 0.5-3.0 THz.

[0049] The calculated absorption peaks of methyl paraben at 1.67THz and 2.67THz correspond to the experimental absorption peaks at 1.57THz and 2.62THz. Except for the absorption peak at 2.93THz, the other absorption peaks are in good agreement with the experimental spectrum. The slight frequency difference between the calculated absorption peak and the experimental absorption peak is attributed to the influence of temperature. Due to the thermal effect, the experimental absorption peak at 2.93THz corresponds to the calculated absorption peak above 3THz. In addition, it can be clearly seen that the absorption peak at 2.67THz is caused by the coupling of two vibration modes at 2.58THz and 2.71THz.

[0050] (2) Vibration mode analysis

[0051] The VMARD assignment results in Table 1 show that the absorption peak of methyl paraben at 1.67 THz is mainly caused by bond stretching, and the contribution of the unscreened motion to the system is less than 6.4%. This means that the motion corresponding to this peak is too dispersed to be intuitively represented by the internal coordinate motion. Figure 2 This indicates that the overall molecular motion corresponding to this vibration mode tends to rotate around the a-axis.

[0052] Figure 1It shows that the absorption peak at 2.67THz is mainly formed by the coupling of two vibration modes at 2.58THz and 2.71THz. The vibration mode at 2.58THz is mainly caused by bond angle bending, which is mainly manifested as the rotational bending motion of methyl groups; the vibration mode at 2.71THz is mainly caused by dihedral angle torsion, which is mainly manifested as the rotational bending motion of methyl groups. Figure 2 It can be seen that the molecules exhibit a collective rotation around the a-axis.

[0053] (3) Weak interaction analysis

[0054] like Figure 3 As shown in the figure, HB#1 and HB#2 are in the middle blue and green state, which means that these regions have obvious hydrogen bonding characteristics. HB#3 and HB#4 meet the definition of hydrogen bonding, but their colors are completely green, so we believe that these interactions are mainly van der Waals interactions. Figure 4 It can be seen that HB#1 of methyl paraben is mainly formed by the interaction between -CO- on the ester group of the central molecule and H on the hydroxyl groups of the surrounding molecules. HB#2 is mainly formed by the interaction between H on the hydroxyl group of the central molecule and -CO- on the ester group of the surrounding molecules. HB#1 and HB#2 are classified as the same type of hydrogen bonds. HB#3 is mainly formed by the interaction between O on the hydroxyl group of the central molecule and H on the benzene ring of the surrounding molecules, and HB#4 is formed by the interaction between H on the benzene ring of the central molecule and O on the hydroxyl groups of the surrounding molecules. This shows that the weak interactions are mainly concentrated around the -OH and -COOCH3 functional groups.

[0055] Table 2 shows that the strengths of HB#1, HB#2, HB#3, and HB#4 are all weak to moderate, and their main properties are electrostatic interactions. The binding energies of HB#3 and HB#4 are weaker than those of HB#1 and HB#2, and their binding energy values ​​are very close to -2.5 kcal / mol (which is the boundary between very weak and weak medium). This means that although all weak interactions are dominated by electrostatic interactions, the dispersion interactions of HB#3 and HB#4 should not be ignored compared with HB#1 and HB#2. And the data show that the binding energies of similar hydrogen bonds have a high degree of similarity.

[0056] In summary, the high consistency between the experimental spectrum and the theoretically calculated spectrum indicates that solid-state density functional calculations can accurately predict the absorption characteristics of the terahertz band. The appearance of the characteristic peaks is mainly due to the coupling of intermolecular and intramolecular motions, especially the collective rotation of molecules, and its weak interactions are mainly concentrated near the -OH and -COOCH3 functional groups, and are mainly electrostatic interactions.

[0057] Finally, it should be noted that the above embodiments apply specific examples to illustrate the principles and implementation methods of the present invention, and the description of the above embodiments is only used to help understand the method of the present invention and its core ideas. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

[0058] Table 1 Vibration mode assignment results of methyl paraben

[0059]

[0060] a ν,δ,γ,τ represent bond stretching, bond angle bending, out-of-plane angle bending and dihedral angle torsion, respectively. The percentage in brackets represents the contribution ratio of the vibration mode.

[0061] Table 2 Topological analysis results of methyl paraben, predicted hydrogen bond ratio binding energy and classification

[0062]

[0063] b ρ represents the electron density at the critical point.

Claims

1. A method for studying the vibration modes and weak interactions of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory, characterized in that: The following steps are involved: Step 1: Sample preparation The sample sheet and polyethylene are mixed in proportion, ground, and tableted to obtain a methyl parahydroxybenzoate sample tablet with a smooth surface; Step 2: Terahertz time-domain spectroscopy measurement Using the terahertz time-domain spectroscopy system, the time-domain signals of all samples collected are Fourier transformed to obtain the frequency-domain signals of the samples, and the absorbance of the samples is obtained by calculation; Step 3: Terahertz spectrum calculation Perform theoretical terahertz spectrum calculations. Perform frequency calculations after structural optimization to obtain theoretically calculated terahertz absorption spectra; Step 4: Comparative Analysis Compare and analyze the experimental terahertz absorption spectrum in step 2 with the theoretical terahertz absorption spectrum in step 3; Step 5: Vibration Mode Analysis Assign the vibration modes corresponding to each absorption peak in the theoretical terahertz absorption spectrum and perform vibration analysis; Step 6: Weak interaction analysis Based on the optimized structure, the position, composition and strength of weak interactions in the structure are investigated.

2. The method for studying the vibration modes and weak interactions of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory according to claim 1, characterized in that: The terahertz time-domain spectrometer used in step 2 is the CCT-1800 terahertz time-domain spectrometer independently developed by China Huaxun Ark Technology Co., Ltd.

3. The method for studying the vibration modes and weak interactions of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory according to claim 1, characterized in that: The geometry optimization and frequency calculation in step 3 are both implemented using the periodic system calculation program CRYSTAL17.

4. The method for studying the vibration modes and weak interactions of methyl parahydroxybenzoate by combining terahertz spectroscopy with solid-state density functional theory according to claim 1, characterized in that: In step 3, PBE density functional theory and pob-TZVP basis set are used, and the semi-empirical Grimme-D3 London dispersion force correction term is added. The energy convergence criterion of geometry optimization is set to ΔE<10 -8 .

5. The method of combining terahertz spectroscopy with solid-state density functional theory to study vibration modes and weak interactions of methyl parahydroxybenzoate according to claim 1, characterized in that: The energy convergence standard of the frequency calculation in step 3 is set to ΔE<10 -9 and the infrared intensity was determined by the Berry phase method.

6. The method of combining terahertz spectroscopy with solid-state density functional theory to study vibration modes and weak interactions of methyl parahydroxybenzoate according to claim 1, characterized in that: The vibration mode analysis described in step 5 is achieved by a vibration mode automatic relevance determination (VMARD) method.

7. The method of combining terahertz spectroscopy with solid-state density functional theory to study vibration modes and weak interactions of methyl parahydroxybenzoate according to claim 1, characterized in that: The weak interaction analysis described in step 6 adopts the Independent Gradient Model based on Hirshfeld Partition of Molecular Density (IGMH) and the "Atoms in Molecules" (AIM) topological analysis theory.

Images