Positional isomerism for strengthening intermolecular interactions: Toward monocyclic nitramino oxadiazoles with enhanced densities and energies

Development of high-density energetic materials has drawn considerable attention because the densities significantly affect their detonation performance. Herein, we propose an effective strategy based on positional isomerism for enhancing the densities of pre-existing energetic materials by optimizing their structures to reinforce intermolecular interactions. By applying this strategy, we design and synthesize 2-amino-5-nitramino-1,3,4-oxadiazole (2), a suitable isomer of 3-amino-5-nitramino-1,2,4-oxadiazole (1). This isomer is the first example of a monocyclic nitramino 1,3,4-oxadiazole reported to date. Single-crystal X-ray diffraction reveals that the isomer has a high crystal density (1.938 g cm(-3) at 110 K), which is 0.083 g cm(-3) greater than that of the original material (1, 1.855 g cm(-3) at 110 K). Mechanistic studies confirmed that the isomer possesses stronger intermolecular hydrogen-bonding and pi - pi interactions, resulting in denser stacking, smaller cell volume, and thus, higher density. Remarkably, the isomer has a very short intermolecular hydrogen bond (1.956 angstrom), which is significantly shorter than that of 1 (2.133 angstrom) and other representative strongly hydrogen-bonded energetic materials such as 2,4,6-triamino-1,3,5-trinitrobenzene (TATB, 2.239 A) and 1,1-diamino-2,2-dinitroethylene (FOX-7, 2.143 angstrom). Moreover, this strategy can be applied to its energetic salts. The higher densities of the isomer and its salts endow better detonation performance. Particularly, the detonation velocity of the isomer is more than 400 m s(-1) higher than that of the original material (8668 m s(-1) for 2 vs 8250 m s(-1) for 1). Meanwhile, the hydroxylammonium salt 2b exhibits a high detonation velocity of 9087 m s(-1), which is superior to that of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX).

Automated summary

The effective strategy based on positional isomerism has been proposed to enhance the density of energetic materials and led to the synthesis of a high-density nitramino compound 2 with improved detonation performance compared to the original material 1.