Monitoring the α → β solid–solid phase transition of RDX with Raman spectroscopy: A theoretical and experimental study

Raman spectroscopy has been used to investigate the α → β solid–solid phase transition of the energetic material hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) in real time. The thermal conversion of α → β at atmospheric pressure occurred at 477 K. Temperature-induced spectroscopic changes in the Raman spectrum of α-RDX, such as peak shifting and broadening and disappearance of spectral bands confirm the phase transition. For the interpretation of the experimental results, the structural and spectral characteristics of α- and β-RDX have been studied by performing DFT calculations for energies, geometries, vibrational frequencies and NMR shielding constants (CSGT method). The theoretical data, using density functional theory (DFT) approximation, and experimental results were consistent with each other. The results from these experiments show that once the β-RDX is formed by heating at slow rates, crystal transformation occurs near the melting point and no reversible change in the symmetry is observed. Moreover the differences in the Raman spectra of both solid phases are evident and persistent. In addition, the results of calculated and experimental NMR chemical shifts (13C and 15N) confirm the agreement between de AAA conformer and the β-RDX structure.