α- and β-FOX-7, Polymorphs of a High Energy Density Material, Studied by X-ray Single Crystal and Powder Investigations in the Temperature Range from 200 to 423 K

The α−β phase transition in the novel energetic material 1,1-diamino-2,2-dinitroethylene, C₂H₄N₄O₄ (FOX-7), has been studied by single-crystal X-ray investigations at five different temperatures over the 200−393 K range. In these investigations, the positions of the hydrogen atoms were experimentally determined without any geometric constraints. In addition, X-ray powder investigations using the Guinier technique have been performed to characterize the β-phase up to 423 K. The α−β phase transition at 389 K is first order, shows a discontinuous increase of the molar volume and entropy (ΔV = 1.75 cm³/mol, X-ray investigation; ΔS = 1.5 cal/K mol, DSC analysis), and can be classified as displacive. The hitherto unknown structure of β-FOX-7 was solved at 393 K and showed simple structural relations to the α-polymorph. The characteristic bonding in wave-shaped layers is now found for β-FOX-7 (P2₁2₁2₁, z = 4, a= 6.9738(7) Å, b = 6.635(1) Å, c = 11.648(2) Å, 393 K), as well as for α-FOX-7 (P2₁/n, z = 4, a = 6.9467(7) Å, b = 6.6887(9) Å, c = 11.350(1) Å, β = 90.143(13)°, 373 K). Interestingly, whereas the intramolecular C−C, C−N, N−O, and N−H bond distances remain nearly unchanged for both polymorphs over the whole temperature range from 200 to 393 K, the two nitro groups deviate strongly from the molecular plane formed by the two carbon and two amino nitrogen atoms. In α-FOX-7 at 373 K, the nitro groups are twisted −47 and +6° with respect to the carbon−carbon bond, but in β-FOX-7 at 393 K, these twist angles are changed to −36 and +20°. Within the layers, the FOX-7 molecules show strong π-conjugation and extensive intra- and intermolecular hydrogen bonding. In this investigation, we have been able to show that α- and β-FOX-7 build up different nets of intermolecular hydrogen bonds. In α-FOX-7, each oxygen atom of the nitro groups is involved in two hydrogen bonds resulting in two intramolecular and six intermolecular hydrogen bonds. But in β-FOX-7 this coordination changes, and half of the oxygen atoms build up two and the other half build up three hydrogen bonds leading to two intramolecular and eight intermolecular hydrogen bonds. The average intermolecular hydrogen bond distance increases slightly from 2.31 Å in α-FOX-7 to 2.52 Å in β-FOX-7. The C−NO₂ bonds are of particular interest because they are referred to as the detonation trigger. It has been suggested that these bonds could be strengthened by the extensive intermolecular hydrogen bonding within the layers in both polymorphs. Such bond strengthening via cooperative effects was proposed in earlier DFT calculations on FOX-7 and may be one key to understanding its low sensitivity and high activation energy to impact.