Packing Structures of CL-20-Based Cocrystals

Abstract

The CL-20-based cocrystals (CCCs) are now the most active in the field of energetic cocrystals, due to an advantage of high energy density while a disadvantage of low stability of CL-20, which may be tuned with desired structures and properties by cocrystallization. This work presents a comprehensive insight into the packing structures of 27 CCCs observed since 2017. First, it shows a high multiplicity of the coformer molecules of CL-20, with various shapes and sizes. Regarding the conformers, the β-, γ-, η-, ε-, and ζ-forms appear in the CCCs, with a total above that observed in the CL-20 polymorphs; that two forms can exist in the same CCC highlights a difference in conformers between single component crystals and cocrystals; and the γ- and β-forms govern the CCCs with a total population of 87%. This high conformational diversity serves as a reason for the abundance of CCCs. Meanwhile, various stoichiometric ratios from 1:1 to 1:6 except from 1:5 are observed, and the lower ones predominate the CCCs, with populations of 48 and 40% for 1:1 and 1:2, respectively. Moreover, it exhibits wavelike, sandwich, channel, and caged molecular stacking in the 27 CCCs. Among these stacking, O···H, O···N, and O···O contacts dominate the weak intermolecular interactions, which feature the hydrogen bonding between the H atoms of CL-20 and the acyl/ether O atoms of the coformer molecules, and the p (of O atoms on the NO2 of CL-20)−π (of the big π-bonds of coformer molecules) interactions. The weak intermolecular interactions contribute to the small molecular volume variations of CL-20 after cocrystallization, with a maximum relative error of ∼3%. Besides, each CCC mediates the packing density between those of the two related pure components; and no CCC outperforms ε-CL-20 in packing density. Finally, we find that the high contents of O and N facilitate an increase in packing coefficients and packing densities. All these findings are expected to enrich the knowledge of both energetic materials and cocrystals, and enhance the rationalization of crystal design.