Phase transition and trigger mechanism of initial reaction under pressure for benzofuroxan energetic materials: Raman spectroscopy and first‐principle calculations

Low‐pressure (below 2 GPa) Raman spectra of benzofuroxan are investigated using a gasketed Mao–Bell‐type sapphire anvil cell and Raman spectrometer to clarify the pressure‐induced structural change and molecular vibration behavior. The first‐principle calculations are performed to compare with the experimental data and to analyze the phase transition and trigger mechanism of initial reaction. Variations of pressure‐induced Raman band width, shift, and intensity are examined. The results show that the benzofuroxan molecule will become nonplanar with increasing pressure. A phase transition occurs because of an abrupt redshift in shift–pressure relationship and reduction of the cell volume, appearing of a new vibration band above 0.13 GPa. Several active vibration modes are found, and the effects of the active mode vibrations on the initial decomposition of benzofuroxan are analyzed using the relaxed scan method for the main change in bond length, bond angle, or dihedral angle to obtain the optimal reaction channel leading to initial decomposition. The results demonstrate that the initial decomposition is the open‐loop reaction in N(O)–O position, which is originated from the increase of dihedral angle O‐C‐N(O)‐O from the out‐of‐plane torsional vibration of furoxan ring (518 cm−1). The scanning energy barrier related to dihedral angle O‐C‐N(O)‐O is about 22.6 kcal/mol, which is consistent with the calculated activation barrier (18.1 kcal/mol) of open‐loop reaction. This proves the reliability of our conclusions. The pressure‐induced phase transition and trigger mechanism of benzofuroxan energetic compound are analyzed by Raman spectroscopy and first‐principle calculations and demonstrate that a phase transition occurs above 0.13 GPa and its initial reaction is the ring‐opening of furoxan ring, whose trigger channel is the increase of dihedral angle O‐C‐N‐O caused by the out‐of‐plane torsion vibration of furoxan ring located at 518 cm−1.