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Comparative Study of Experiments and Calculations on the Polymorphisms of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) Precipitated by Solvent/Antisolvent Method

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journal contribution
posted on 10.02.2016, 00:00 by Xianfeng Wei, Jinjiang Xu, Hongzhen Li, Xinping Long, Chaoyang Zhang
2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is the most powerful explosive. However, the application of this compound is limited by its high sensitivity and serious polymorphic transformations. Thus, elucidating the mechanism of crystallization and polymorphic transformation of CL-20 is crucial. This work presents a comparative study of experiments and calculations to clarify the mechanism of CL-20 precipitation using an solvent/antisolvent method. Calculations show that the β-formed CL-20 conformations are always the most energetically favored. These conformations have generally the highest content in solutions, and the intermolecular conformational transformations in solutions have low energy barriers. In addition, it is predicted that the β-CL-20 crystal possesses the lowest lattice energy among all polymorphs. The calculated results are successfully applied to explain the experimental observations, as β-CL-20 crystal is initially precipitated from most of the highly supersaturated solutions and then converted into ε-CL-20 crystal. This precipitation is kinetically controlled by the dominance of β-CL-20 molecules in a metastable phase and rapid crystallization. The final conversion into ε-CL-20 crystal is attributed to its low energy barrier for polymorphic transformation and stability, that is, the conversion is dynamically dominated. Furthermore, calculated coherent energy densities (CEDs) of various CL-20 polymorphs, including hydrates with different hydration degrees, agree well with the thermal stabilities, as the higher CED corresponds to the higher thermal stability. Therefore, the complex crystallization of CL-20 is elucidated by combining experimental observations with theoretical calculations and simulations.