Toward low-sensitive and high-energetic co-crystal II: structural, electronic and energetic features of CL-20 polymorphs and the observed CL-20-based energetic--energetic co-crystals.

2,4,6,8-Hexanitro-2,4,6,8,10,12-hexaazatetracyclododecane (CL-20) is the most powerful explosive applied, and CL-20-based energetic--energetic co-crystals are promising new alternative explosives with tunable power and safety, resulting in much interest in them. This work discusses the structural, electronic and energetic features of three CL-20 polymorphs, β, γ and ε forms, and three CL-20-based energetic--energetic co-crystals, CL-20/TNT, CL-20/HMX and CL-20/BTF. As a result, we find that, relative to the pure CL-20 polymorphs, the co-crystallization of CL-20 with HMX, TNT and BTF cause little molecular deformation except from some torsion of its nitro groups, and the narrower band gaps. And dominantly, the O⋯O, O⋯H and O⋯N interactions hold all the crystal packing. There is possibly thermodynamic and kinetic dominance in the CL-20/TNT and CL-20/HMX, and CL-20/BTF co-crystallization, respectively, in terms of their formation energy. Further, a rough criterion for predicting energetic co-crystal formation is obtained, as the solubility parameter difference of two coformers of a binary energetic co-crystal is less than 8 MPa0.5. [ABSTRACT FROM AUTHOR]