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

Abstract

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.

Experimental and Theoretical Study on the Stability of CL-20-Based Host–Guest Energetic Materials

“…The design and preparation of high-energy density materials (HEDMs) with desired properties is one of the hotspots in the research of energetic materials. , Many experimental and theoretical studies have been devoted to improving the performance of energetic materials by exploring a variety of new strategies. − However, it remains a challenge to design and construct energetic crystals with the highest possible energy density and the maximum possible chemical stability owing to the contradictory relationship between power and safety.…”

Phase Retransformation and Void Evolution of Previously Heated HMX-Based Plastic-Bonded Explosive in Wet Air

“…33 Calculation results showed that solvents lowered the energy barrier (ΔE) of the phase transformation, and the solvents with the higher polarity, especially water, more significantly decreased ΔE. 34 A similar effect might exist in the system of HMX with highpolarity water molecules because water molecules strongly interacted even with neutral β-HMX through negatively charged NO 2 groups. 35 This interaction could sufficiently lower the ΔE and permit the equilibrium within a shorter period (e.g., several hours).…”

Molecular Forcefield Methods for Describing Energetic Molecular Crystals: A Review

“…Thermodynamic properties are crucial to energetic crystals. For example, COMPASS-MD simulations were used to compare the total energies for α/γ, β, and ε-formed conformations of CL-20 with or without solvents; as a result, β-CL-20 is the most energetically favored and easily converted into ε-CL-20 crystal [ 85 ]. COMPASS was also employed to calculate lattice energy for a series of co-crystals involving CL-20, HMX, TNT, BTF, TNB, TNA, MATNB, or TNAZ molecules [ 86 ].…”