Possible pre-phase transition of α-HMX crystal observed by the variation of hydrogen-bonding net under high pressures
DFT calculations were performed to investigate the pressure-induced hydrogen bond network in α-HMX crystal by geometrical evolution, electronic structure, Hirshfeld surface, AIM method, IGM model, mutual penetration distance, mechanical property, and vibration spectra. The results show that the hydrogen bond network predominates in the intermolecular interactions in the crystal and becomes more sensitive in low pressure region (below 10 GPa). Compared to the relatively strong C-H···O interactions related to vertical nitro groups, the weak hydrogen bonds located on horizontal N-NO2 are more complicated and may govern the physicochemical property of the metastable -HMX crystal under compression. The abrupt variation of the hydrogen bond network in the crystal at 4 GPa that attributes to shifting of relative position of the molecules indicates an emergence of possible pre-phase transition before the transformation from β- to δ-HMX. Our investigations undercover detailed variation of noncovalent interactions in the -HMX crystal at high pressures, which provides a deeper understanding of the safe performance of nitramine explosives.