Prediction of the crystal packing of di-tetrazine-tetroxide (DTTO) energetic material

Previous calculations suggested that di-tetrazine-tetroxide (DTTO), aka tetrazino-tetrazine-tetraoxide, might have a particularly large density (2.3 g/cm³) and high energy release (8.8 kJ/kg), but it has not yet been synthesized successfully. We report here density functional theory (DFT) (M06, B3LYP, and PBE-ulg) on 20 possible isomers of DTTO. For the two most stable isomers, c1 and c2 we predict the best packings (i.e., polymorphs) among the 10 most common space groups for organic molecular crystal using the Universal force field and Dreiding force field with Monte Carlo sampling. This was followed by DFT calculations at the PBE-ulg level to optimize the crystal packing. We conclude that the c1 isomer has the P2₁2₁2₁ space group with a density of 1.96 g/cm³, while the c2 isomer has the Pbca space group with a density of 1.98 g/cm³. These densities are among the highest of current energetic materials, RDX (1.81 g/cm³) and CL20 (2.01 g/cm³). We observe that the stability of the polymorphs increases with the density while the planarity decreases.