A comparative study of 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under high pressures using Raman spectroscopy and DFT calculations

• Raman spectroscopy used to study pressure effects on RDX and HMX.
• DFT used to analysis vibrational modes of RDX and HMX molecules.
• The α-γ transition of RDX involved molecular deformation and rearrangement.
• HMX showed no phase transition but gradual structure adjustment.

High pressure Raman experiment was performed to compare RDX and HMX crystals. Ab initio calculations using B3LYP Density Functional Theory method with Sadlej's medium-sized polarized basis set (SadlejpVTZ) were carried out for Caae RDX and 1,5-diaxial-3,7-diequatorial chair HMX molecules. Our calculations and measured Raman vibrational spectra reveal both molecules have similarities on bonding and vibrational properties at ambient pressure. However, high pressure responses for both molecules aren't the same. For RDX, at pressure near 4 GPa, a number of changes become apparent in the Raman spectra, such as modes splitting, intensity modification, and discontinuity of pressure-dependence of frequency shifts, which are consistent with previous experiment and believed to associate with α-γ phase transition. For HMX, only slight conformational deformation involving NO2 group was observed, and was considered as an onset of β-ε phase transition. It is proposed that the markedly different behavior under high pressure for these two molecules results from different molecular packing in unit cell.

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