Form III of 2,2′,4,4′,6,6′-hexanitro-azobenzene (HNAB-III)

The crystal structure of form III of the title compound, HNAB [systematic name: bis(2,4,6-trinitrophenyl)diazene], C₁₂H₄N ₈O₁₂, has finally been solved as a pseudo-merohedral twin (monoclinic space group P2₁, rather than the orthorhombic space group C222₁ suggested by diffraction symmetry) using a dual space recycling method. The significant differences in the room-temperature densities of the three crystalline forms allow examination of molecular differences due to packing arrangements. An interesting relationship with the stilbene analog, HNS, is discussed. Interatomic separations are compared with other explosives and/or nitro-containing compounds.