Dance of HMX molecule in conformational space by quasi-static heating: a combined Raman spectroscopy and theoretical study*

The complexity of HMX polymorphs arises from conformers and molecular packing. In this study, HMX single crystal is heated at rates of 0.1 or 0.2 K/min during phase transitions. The sample is equilibrated at least 15 minutes at every step of increasing temperature to achieve the quasi-static condition. Raman spectroscopy is performed to monitor transitions between conformers by fitting position and full width at half maximum of the observed spectra. Conformers, transition states and IRC are verified at the level of DLPNO-CCSD(T)/cc-pVQZ//M062X-D3(0)/def2-TZVP for single HMX molecule. We attribute the response of spectra to ring puckering and nitro vibrations during phase transitions by analyzing Raman spectra and decomposition and identification of vibrational modes. Our results provide evidence of β-α and subsequent α-δ phase transition, which agree with recent thermodynamic model that α-HMX must be thermodynamically stable in phase diagram.