Effect of Pressure on the Vibrational Structure of Insensitive Energetic Material 5-Nitro-2,4-dihydro-1,2,4-triazole-3-one

The Raman spectra of α form 5-nitro-2,4-dihydro-1,2,4-triazole-3-one (α-NTO, space group P1̄) were measured
in a high-pressure vessel diamond anvil cell (DAC). The pressure was increased to 27.6 GPa. In general,
Raman bands show a blue shift because of the nature of the molecule packing as a high-pressure effect, but
some particular bands exhibited a red shift, disappearance, split, or slight shifting in our experiments. Those
red-shifting bands concerning hydrogen bonds, i.e., carbonyl and amino groups, are likely to work as a stabilizer
against stimuli to the molecule or crystal. This stabilizing nature might characterize the insensitivity of NTO.
Molecular dynamic (MD) calculations were performed to reveal the high-pressure effect of the α-NTO crystal.
The coordinates of individual atoms in the crystal structure were obtained using X-ray diffraction analysis.
The pressure dependence of the power spectra of the correlation functions of the CO bond length in NTO
was calculated. A unique high-pressure effect of the α-NTO crystal was found on the power spectra. The
peak frequency in the power spectrum of the CO stretching vibration exhibited a red shift with an increase
in pressure to 10.0 GPa, while the peak intensity considerably decreased under the same pressure process,
because this bond length increased with an increase in pressure to 10.0 GPa. At a pressure of >20.0 GPa, a
blue shift appeared. These results of the MD calculations are in good agreement with our experimental data.