Crystal Packing of Planar Organic Azide Molecules

Organic azides possess the potential for green primary explosives, gas generators, and propellants. This work focuses on the crystal packing of 34 planar organic azides and presents an understanding of their properties and performances. As the functional group of azides, –N₃ is wholly weakly charged, and individually, the outer, middle, and inner N atoms thereof are weakly negatively, rather strongly positively, and negatively charged, respectively. This weak polarity leads to weak intermolecular interactions, stemming from the generally lower packing coefficients and lower packing densities of azide crystals, compared with some typical hydrogen bond (HB)-containing nitro compounds. –N₃ is mostly lonely in crystal packing, as its outer N atom is seldom observed as an intermolecular HB acceptor, and the inner N atom acts as an intramolecular HB acceptor in only one case out of a total of 16. Introducing strong HB acceptor/donor groups to azides generally facilitates the enhancement of intermolecular interaction and molecular/crystal stability. Ascribed to the extremely low molecular stability, no evident dependence of impact sensitivity on molecular stacking mode is found for the whole azides. The top three azides with the lowest impact sensitivity all contain dense or moderately strong HBs, implying that HB introduction could be a strategy for enhancing the safety of azides.