Study on the Stability of Hydrogen-Bonded Energetic Material 3-Nitropyrazole under High Pressure

The stability of hydrogen-bonded energetic material 3-nitropyrazole (3-NP, C3H3N3O2) under high pressure was studied using a diamond anvil cell, Raman spectrometer, and simulation calculation. Under the hydrostatic experimental pressure and simulated pressure of 10 GPa, the optical Raman spectra, the change of cell parameters, and the Hirshfeld surface energy of 3-NP were obtained respectively. The analysis of the measured Raman spectra shows that the structure is still in original phase at 10 GPa pressure. The effects of pressure on cell parameters, molecular arrangement, and hydrogen bonding were observed by simulation calculation. Systematic analysis of the subtle structural changes, anisotropic characteristics, and various interactions of 3-NP shows that the stability of 3-NP is related to the special trimer structure.


Introduction
Energetic materials refer to a class of materials that can release a large amount of heat and pressure under certain stimuli (such as impact, heat, high pressure, etc.), [1][2] which are widely used in fireworks, explosives, propellants, and other fields.Among them, the exploration and research of energetic materials with high energy, low sensitivity, and green environmental protection has gradually become the focus of the present.[3][4] Pyrazole energetic materials have entered everyone's field of vision because of their high energy, low sensitivity, and green environmental protection.[5]As one of them, nitropyrazole energetic materials have good applications in the fields of high-energy insensitive explosives, propellants, pyrotechnics, and so on.[6][7]In 1970, Habraken C L et al.synthesized 3-NP by rearrangement of 1-NP.[8]As a typical hydrogen bond energetic material, 3-NP is an important intermediate for the synthesis of new explosives such as 3,4-dinitropyrazole (DNP).[9] has strong hydrogen bonds in a solid state.
[10]Strong hydrogen bonds usually lead to a more tightly packed crystal structure, thereby increasing density and making it more stable.
As shown in figure 1

Experimental Section
The experimental sample was 3-NP with a purity of 99%, which was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.The instruments used are a laser confocal Raman spectrometer with an excitation light source wavelength of 638 nm from HORIBA and a symmetrical diamond anvil cell (DAC) with an anvil surface of 400 um.
Before the experiment, the ambient pressure spectrum of 3-NP was tested.The measured spectra were compared with the Raman spectra of 3-NP measured by Nageswara Rao et al. [13]It was found that the peaks of the two groups of spectra were consistent.The T301 gasket in the diamond anvil was preloaded to a thickness of 40-50 μm, and a hole with a diameter of about 130 μm was punched as a sample chamber.The sample is filled in the sample chamber, and 1-2 ruby balls are filled in the sample chamber.Ruby has two strong fluorescence peaks under ambient pressure, and the pressure is calibrated according to the shift of the fluorescence spectrum line R1 of ruby with pressure.[14]The CASTEP module in Materials Studio is used to optimize the structure, to simulate the accurate lattice parameters.The simulated data were imported into Crystal Explorer to draw the surface energy and fingerprint of Hirshfeld, to analyze the intermolecular interaction.With the increase of pressure, the vibration peak shifts to the right.The blue shift is caused by the decrease of intermolecular distance and the enhancement of intermolecular interaction.[15]In the internal modes part, there are two peaks arranged side by side at about 250 cm -1 (Peak 1 is shown on the left and peak 2 on the right).During the pressurization process, cross-overlap occurs due to different movement rates.At 3.3 GPa, peak 1 moves to the right side of peak 2. With the increase of pressure, peak 2 moves to the right normal blue.The signal intensity of peak 1 gradually decreases with the increase of pressure and merges with the peak at 261 cm -1 at 6 GPa.There is no obvious change in the external mode vibration mode, indicating that there is no structural phase transition in the crystal.

High Pressure Experimental Results and Analysis
As shown in figure 2(b), this is the hydrogen bond vibration of 3-NP.At 4.2 GPa, a new peak appears at 3197 cm -1 , which is the result of the decrease of hydrogen bond symmetry caused by pressure.At 6 GPa, the new peak and the adjacent characteristic peaks merge into a broad peak, which may be one of the reasons why the peak broadens with the increase of pressure.Through observation, it was found that the geometric structure of N-H ... N hydrogen bond changed significantly.Because hydrogen bonds play a role in stabilizing the structure, the change of hydrogen bond geometry may affect the sensitivity and performance.[16]Other original peaks blue shift to the right with the increase of pressure, gradually become shorter and wider and change to the amorphous direction.This may be caused by the weakening of the peak intensity or the thinning of the sample during the pressurization process.Raman spectra of oxamide ranging from 400 to 1600 cm -1 and the pressure dependence of these modes are depicted in figure 3. The change of internal modes under pressure can judge the change of relevant information around a specific group.It is accompanied by the emergence of new peaks, the disappearance of original peaks, and the splitting of some peaks.There are obvious peak splittings at 614 cm -1 at 4.2 GPa, 1049 cm -1 at 5.1 GPa, and 563 cm -1 at 6 GPa, respectively.It can be seen from the splitting of the internal model that the pressure process is also accompanied by changes in molecular conformation.At 0 GPa, the NO2 stretching vibration at 1475 cm -1 disappears at 4.2 GPa as the pressure increases.When the pressure reaches 8.2 GPa, the internal mode vibration peak begins to broaden.This means that random arrangement and distortion may occur within and between molecules.Through calculation, the changes in cell parameters, molecular arrangement, and hydrogen bonds were determined.The mechanism of structural stability and the synergistic effect between hydrogen bonds and van der Waals interaction are analyzed.As shown in table 1, anisotropic compression of 3-NP is shown.By comparison, it is found that the lattice parameters at 0 GPa are in good agreement with the previously reported results.[12] The compression behavior under different pressures is obviously different.When the pressure reaches 10 GPa, the compression of the c-axis is the strongest, from 14.378 Å to 11.372 Å, which is reduced to 79% of the original.The unit cell volume was compressed from 1505.33 Å 3 to 941.887 Å 3 , which was compressed to 63% of the original.By observing the Hirshfeld surface energy and fingerprint under different pressures, the changes in intermolecular interactions in the crystal were intuitively compared.Figure 4 compares the intermolecular interactions of 3-NP under ambient pressure and 10 GPa to analyze the effect of pressure on intermolecular.Figure 5 is the Hirshfeld surface energy and fingerprint of 3-NP at atmospheric pressure and 10 GPa.The blue part of the surface energy represents the long-range intermolecular force.As the pressure increases, the gap in the crystal gradually decreases, and the blue region gradually disappears.At the same time, the red area gradually increased, indicating that the short-range intermolecular force was enhanced.These changes in the appearance of the surface can quickly determine the structural changes that occur during compression.The volume change of the fingerprint shows that the 3-NP stacking mode is more compact when the pressure is up to 10 GPa.As shown in figure 4(b), with the increase of pressure, the intermolecular force increases, and the 10 GPa fingerprint moves to the origin.As shown in figure 4(a), the ' fork ' region represents the characteristic morphology of the N−H ... N hydrogen bond.The fork above corresponds to the donor of the hydrogen bond, and the fork below corresponds to the acceptor of the hydrogen bond.Both N and H atoms are positively charged, and there is a repulsion between them.When an N atom acts as a hydrogen bond acceptor, the atom is highly sensitive to the shortening of the distance, which may be the main reason for the torsion of the hydrogen bond under pressure.It can be seen from figure 4 that the H ... O interaction decreases from 47% at atmospheric pressure to 40% at 10 GPa.The 'wing' region represents the C−H interaction, which changes from 5.4% to 4.1% at 10 GPa.

Computational Simulation Results and Analysis
Hydrogen bonds and van der Waals forces are the two main interactions in 3-NP crystal.Its structural stability mainly depends on its special trimer structure, as well as the competition and synergy of these two interactions.Combined with the analysis of the chart, under the action of external pressure, the molecular spacing in the unit cell gradually shortened, forming a close packing, and the hydrogen bond and van der Waals force gradually increased.For 3-NP, the molecules are stably connected by N-H ... N hydrogen bonds.The hydrogen bond network of the ternary ring can release the increased intermolecular interaction by twisting, to maintain the balance of hydrogen bond and van der Waals force and the stability of the structure.

Conclusion
In this study, the stability of 3-NP under high pressure and the change of intermolecular interaction were deeply explored.By analyzing the in-situ high-pressure Raman spectra of 3-NP, it was found that the structure of 3-NP was stable at 10 GPa.By analyzing the changes in hydrogen bond vibration, we believe that the stability of the 3-NP structure is attributed to special three-membered ring structure and hydrogen bond interaction.The high-pressure study of its hydrogen bonds enables us to deeply understand the nature of the structure-property relationship and fills the gap in the study of the behavior of pyrazole energetic materials under high pressure.

, 3 -
NP molecules are connected into trimers by N-H ... N hydrogen bonds.The hydrogen bond increases the molecular stability of 3-NP.The planar 3-NP molecules are arranged in layers, and the layers are combined by van der Waals force.This structure makes the molecular packing more compact and the crystal structure more stable.[11]Related studies have shown that 3-NP crystals belong to the monoclinic structure of the P-1 space group, and each unit cell has 12 molecules.The unit cell parameters are a = 10.102Å, b = 12.501 Å, c = 13.119Å, α = 100.26°, β = 104.41°, γ = 111.31°, V = 1427.2383Å 3 , Z = 12.[12]The systematic study of 3-NP can help us understand the structural stability of 3-NP under pressure, the change of intermolecular interaction force, and the important role of hydrogen bonds in determining the structure and properties of materials.

Figure 1 .
Figure 1.The crystal structure of 3-NP under environmental conditions and 10 Gpa.

Figure 2 .
Figure 2. Raman spectra of 3-NP external mode vibration and hydrogen bond vibration.

Figure 3 .
Figure 3. Raman spectra of 3-NP as a function of increasing pressure in the wavenumber range 400−1600 cm −1 .

Figure 4 .
Figure 4.The proportion of intermolecular interaction under different.

Figure 5 .
Figure 5. Hirshfeld surface energy and fingerprint of 3-NP at atmospheric pressure and 10 GPa.

Table 1 .
The lattice parameters of 3-NP at 10 GPa.