From Disappearing Polymorphs to Local Anisotropy of Mechanical Properties: Decoding Impact Sensitivity of 3,4-Dinitro-1H-Pyrazole-1,5-Diamine

The response to mechanical stimuli is an important property of reactive substances, which is most clearly observed in the case of energetic materials. Factors related to the crystal structure are among the important descriptors of the impact sensitivity for energetic species. Here we scrutinized the known, but underexplored compound 3,4-dinitro-1H-pyrazole-1,5-diamine (1). We obtained two new polymorphs and report their X-ray structures. Numerous following attempts to prepare these polymorphs were unsuccessful; only a reported earlier orthorhombic phase was obtained (hence, it is one more example of disappearing polymorphs). The impact sensitivities for various samples of the title compound crystallized from different solvents were measured. We report two levels of impact sensitivity for 1: ∼4 J and ∼30 J, viz., changing the final crystallization step, we obtained the highly sensitive or low sensitive samples. To rationalize this behavior, we thoroughly analyzed the samples at the crystal level: range the crystal faces by elastic modulus (via atomic force microscopy), then introduce a new coefficient that represents the ratio of the surface areas of brittle to ductile faces within a single crystal (from X-ray diffraction). We found that the low sensitivity is associated with the presence of pronounced ductile faces that exhibit weak intermolecular interactions. Under mechanical stress, these faces may undergo plastic deformation, preventing local heating and leading to safety improvement. These findings on the crystal-level origins of mechanical safety for the discussed compound and the employed set of experimental and computational tools should have been applicable to other reactive organic species.