High pressure study of hydrogen-bonded energetic material 4-nitropyrazole

The study investigates the structural stability of the hydrogen-bonded energetic material 4-nitropyrazole (4-NP, C3H3N3O2) from ambient pressure to 10 GPa at room temperature using diamond anvil cell (DAC) and Raman spectroscopy. Based on the changes observed in the high-pressure Raman spectra, it is proposed that the hydrogen-bonded energetic material 4-NP undergoes a phase transition within the pressure range of 2.7-6.4 GPa. Upon complete pressure release from 10 GPa, the resulting spectrum is consistent with the Raman spectrum observed under ambient pressure, indicating that the phase transition is reversible. Ab initio calculations were used to explain the changes in molecular arrangements and hydrogen-bonded networks. Hirshfeld surfaces and fingerprint plots were used to directly compare the variations in packing patterns and intermolecular interactions. Based on the experimental and calculated results, it is proposed that the phase transition of 4-NP was likely caused by the rearrangements of the hydrogen-bonded networks.