Heat-Induced Polymorphic Transformation Facilitating the Low Impact-Sensitivity of 2,2-Dinitroethylene-1,1-diamine (FOX-7)

Abstract

The intermediate structures for an energetic material (EM) loaded until the final decay are often inaccessible and overlooked, while they are a determining factor of property and performance, with a similar importance of the original structure under common conditions. The present work exemplifies the importance by revealing the low impact sensitivity of 2,2-dinitroethylene-1,1-diamine (FOX-7) with a consideration of heat-induced polymorphic transformation. Checking the packing structures of the polymorph at ambient conditions (α-form) and the two heat-induced ones (β- and γ-forms) of FOX-7, we confirm that the heating until the final decay makes the shear sliding increasingly ready. That is, from the α- to β- and γ-FOX-7, the crystal packing varies from a wavelike shape to a face-to-face one, with the increase of molecular planarity, as their maximal torsion angles of O–N–C–C decrease from 35.6 to 25.6 and 20.2°; and their shear-sliding barriers reduce and ready sliding ranges increase in the same order, verified by density functional theory calculations. This heat-induced polymorphic transformation of FOX-7 from wavelike to face-to-face π–π stacking is responsible for its low impact sensitivity, by remedying its disadvantage of relatively low thermal stability. Hardly, we will understand the low impact-sensitivity of FOX-7 if the original α-form is considered alone. This work presents an exact example to show the importance of intermediates produced by external stimuli loaded on an EM for understanding its performance. It also shows the complexity of the sensitivity mechanism of EMs and some possible deficiencies caused by considering the initial unloaded case alone.