Polymorphism of Energetic Materials: A Comprehensive Study of Molecular Conformers, Crystal Packing, and the Dominance of Their Energetics in Governing the Most Stable Polymorph

Polymorphism is universal in energetic crystals and brings much complexity in revealing the underlying mechanism for energetic materials against external stimuli. This work comprehensively studies molecular conformers (MCs), molecular stacking, and related MC energy (MCE) and lattice energy (LE) of polymorphs of six common energetic materials (EMs), including 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), 1,3,5-trinitro-1,3,5-triazinane (RDX), 2,2-dinitroethylene-1,1-diarnine (FOX-7), 2,4,6-trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN). We find the dominance of MCE or LE in determining the most stable polymorph at common conditions is variable; i.e., LE dominates CL-20, RDX, PETN, and FOX-7, while MCE governs HMX, and for the two polymorphs of TNT at below -150 degrees C, they have almost the same stability without a difference in either MCE or LE. The variability of the dominance is responsible for the current difficulty in crystal structure predictions. Moreover, the temperature elevation reduces the molecular volume (V-m) of beta-FOX and PETN-II by weakening intermolecular interactions or crystal field effects to relax molecules to be smaller ones. Besides, the high pressure of several GPa mainly shortens the intermolecular distances, rather than compresses V-m. Finally, the heat-induced polymorphic transformation of FOX-7 from alpha- to beta- and gamma-forms allows it to be more and more closely face-to-face pi-pi stacked and serves as a reason for its low impact sensitivity. All these findings are expected to deepen insight into the response mechanism of EMs against external stimuli.