CL-20 Cocrystals Research Articles

Two Novel CL-20 Cocrystals with Different Performances Obtained by Molecular Similarity Combined with Hydrogen Bonding Pairing Energy: An Effective Strategy to Design and Screen Energetic Cocrystals

Two Novel CL-20 Cocrystals with Different Performances Obtained by Molecular Similarity Combined with Hydrogen Bonding Pairing Energy: An Effective Strategy to Design and Screen Energetic Cocrystals

Molecular Dynamics Simulation of Shock Response of CL-20 Co-crystals Containing Void Defects

To investigate the effect of void defects on the shock response of hexanitrohexaazaisowurtzitane (CL-20) co-crystals, shock responses of CL-20 co-crystals with energetic materials ligands trinitrotoluene (TNT), 1,3-dinitrobenzene (DNB), solvents ligands dimethyl carbonate (DMC) and gamma-butyrolactone (GBL) with void were simulated, using molecular dynamics method and reactive force field. It is found that the CL-20 co-crystals with void defects will form hot spots when impacted, significantly affecting the decomposition of molecules around the void. The degree of molecular fragmentation is relatively low under the reflection velocity of 2 km/s, and the main reactions are the formation of dimer and the shedding of nitro groups. The existence of voids reduces the safety of CL-20 co-crystals, which induced the sensitivity of energetic co-crystals CL-20/TNT and CL-20/DNB to increase more significantly. Detonation has occurred under the reflection velocity of 4 km/s, energetic co-crystals are easier to polymerize than solvent co-crystals, and are not obviously affected by voids. The results show that the energy of the wave decreases after sweeping over the void, which reduces the chemical reaction frequency downstream of the void and affects the detonation performance, especially the solvent co-crystals.

Chemical interplay between components in overall thermolysis of CL-20/N2O revealed by ReaxFF molecular dynamics simulations

Understanding of the interplay reactions between components in CL-20 bicomponent crystals is fundamental for synthesis and applications of new CL-20 cocrystals and host-guest materials. This paper reports the thermal decomposition of CL-20/N2O host-guest crystal investigated using ReaxFF MD simulations under both isothermal and adiabatic conditions. The isothermal thermolysis simulation at low temperature of 800 ​K and adiabatic decomposition simulation initiated at 800 ​K were performed to reveal as much as the real and overall scenario of CL-20/N2O thermolysis. Two reference systems of ε-CL-20 and CL-20/H2O were employed for clear depiction of reaction mechanism. Permitted by VARxMD for reaction details and facilitated by the three-stage classification, the overall scenario of CL-20/N2O thermolysis and deep insight about the interplay reaction details between host CL-20 and guest N2O were obtained. The dominating of host CL-20 decomposition during entire thermolysis of CL-20/N2O is similar with that of CL-20/HMX and CL-20/TNT. However, the branching ratios of major reaction pathways of CL-20 initial decomposition were altered. The decomposition kinetics in initial thermolysis of CL-20/N2O was found significantly slowed when compared with ε-CL-20, which is caused by the guest N2O molecules that capture the NO2 intermediates generated from host CL-20 decomposition and prevent NO2 from their active participation in further decomposition of the system. It should be noted that the early formation of N2 from guest N2O in CL-20/N2O decomposition accelerates self-heating at certain extent and provides extra oxidative NO3, which slightly compensates to the slowed kinetics in the initial stage. The acceleration effect of guest oxidant N2O on CL-20/N2O decomposition will sustain through the ​oxygen migration of guest N2O in later secondary reactions to form more NOx than that of CL-20/H2O, consequently reducing the reaction zone of CL-20/N2O thermolysis. What was obtained in this work demonstrates that ReaxFF MD simulations combined with the analysis scheme of three-stage classification can facilitate the depiction of thermolysis mechanism of CL-20 bicomponent crystals and be useful in searching for leading candidates of guest molecules for CL-20 host-guest materials.

Burning rate and flame structure of cocrystals of CL-20 and a polycrystalline composite crystal of HMX/AP

The synthesis and development of novel energetic materials is a costly, challenging process. Rather than synthesizing new materials, cocrystallization provides the potential opportunity to achieve improved properties of existing energetic materials. This work examined the effects of cocrystallization on the deflagration of a 2:1 molar cocrystal of CL-20 and HMX as well as a 1:1 molar cocrystal of CL-20 and TNT. A hydrogen peroxide (HP) solvate of CL-20 as well as a polycrystalline composite of HMX and ammonium perchlorate (AP) were also studied. A physical mixture of each material was also tested for comparison. The burning rate of each material was measured as a function of pressure. Flame structure during self-deflagration was examined using planar laser-induced fluorescence (PLIF) of CN and OH. The burning rate of the HMX/CL-20 cocrystal and the CL-20/HP solvate closely matched that of CL-20, but the burning rate of the TNT/CL-20 cocrystal was between the burning rate of its coformers. All HMX/AP materials had a higher burning rate than either HMX or AP individually and the burning rate of a physical mixture was found to be a function of particle size. The differences in the burning rate of the physical mixtures and composite crystal of HMX/AP can be explained by changes in the flame structure observed using PLIF. Burning rates and flame structure of the cocrystals were found to closely match those of their respective physical mixtures when smaller particle sizes were used (approx. less than 100 µm). The results obtained demonstrate that the deflagration behavior of the coformers is not indicative of the deflagration behavior of the resulting physical mixture or cocrystal. However, changes in the resulting flame structure greatly affect the burning rate.

Analysis of the structures and interactions between CL-20 and its formers

The CL-20-based cocrystals are now the most attractive materials in the field of energetic cocrystals because of the probable performance superior to the CL-20. In order to search the method of screening the formers of the CL-20, analysis of the structures and interactions between CL-20 and its formers reported up to present was performed in present work. The structures of the heterodimers between CL-20 and its formers were investigated, and six interaction modes were found. The intermolecular interactions of the heterodimers between CL-20 and its formers were calculated and analyzed, and the distances and strengths of various intermolecular interactions were discussed, and it was found that the O⋯H intermolecular interaction plays an important role. The relative hydrogen strength between the homodimer and the heterodimer was also estimated by the strongest hydrogen bond interaction method, and it was found that the hydrogen strength in heterodimer was stronger than that in homodimer for CL-20 cocrystal. The compatibilities between CL-20 and its formers were evaluated by the estimation of the Flory-Huggins interaction parameter, and the mixing energies of CL-20 and its formers were generally negative or little positive. The relations of CL-20 and its formers in the structures and polarity were evaluated according to the molecular complementary function, and the relationships of the dipole and the shape parameter M/L between CL-20 and its former had an effect on the formation of the CL-20 cocrystal. The methods and protocols of rough and quick screening the formers of new CL-20 cocrystals were suggested, and this work would contribute to the screening of the formers of new CL-20 cocrystals.

Decomposition mechanism scenarios of CL-20 co-crystals revealed by ReaxFF molecular dynamics: similarities and differences.

Understanding the similarities and differences of decomposition mechanisms of CL-20 and its cocrystals is of great interest for practical applications of CL-20 cocrystals. The responses of CL-20 cocrystals to thermal stimulus were investigated using ReaxFF molecular dynamics simulations of two representative cocrystals, CL-20/HMX and CL-20/TNT, under adiabatic conditions and comparing to the baseline system of pure CL-20. The comprehensive chemical details were revealed with the aid of the unique code of VARxMD. The three CL-20-involved reactive systems all exhibit a distinct three-stage character during adiabatic decomposition when using the double peaks of the major intermediate NO2 amount as the boundary. By taking advantage of the three-stage classification, a clear scenario for the similar stimulus-response of the CL-20 cocrystals can be elucidated for the dominant primary decomposition of CL-20 in stage I and the transition of favored chemical mechanisms from the generation of intermediates/radicals in stage II into their consumption to form stable products in stage III. The similar chemical behaviors are rooted in the dominance of CL-20 chemistry in the initial response of its cocrystals to thermal stimulus. The prolonged reaction zone uncovers the slowed decomposition kinetics of CL-20/HMX and CL-20/TNT, which is associated with the altered kinetics of CL-20 decomposition specifically by N-NO2 bond scission and CL-20 skeleton decay. The retarded CL-20 decomposition in its cocrystals consequently results in more moderate self-heating and less violent exothermic reactions that agrees with the experimental observations of improved stability and damaged detonation performance of CL-20 cocrystals, particularly for CL-20/TNT. The results obtained in this work suggest that ReaxFF MD simulations can provide useful insight for the modulated chemical properties of varied CL-20 cocrystals.

A Study of the Shock Sensitivity of Energetic Single Crystals by Large-Scale Ab Initio Molecular Dynamics Simulations.

Understanding the reaction initiation of energetic single crystals under external stimuli is a long-term challenge in the field of high energy density materials. Herewith, we developed an ab initio molecular dynamics method based on the multiscale shock technique (MSST) and reported the reaction initiation mechanism by performing large-scale simulations for the sensitive explosive benzotrifuroxan (BTF), insensitive explosive triaminotrinitrobenzene (TATB), four polymorphs of hexanitrohexaazaisowurtzitane (CL-20) pristine crystals and five novel CL-20 cocrystals. A theoretical indicator, tinitiation, the delay of decomposition reaction under shock, was proposed to characterize the shock sensitivity of energetic single crystal, which was proved to be reliable and satisfactorily consistent with experiments. We found that it was the coupling of heat and pressure that drove the shock reaction, wherein the vibrational spectra, the specific heat capacity, as well as the strength of the trigger bonds were the determinants of the shock sensitivity. The intermolecular hydrogen bonds were found to effectively buffer the system from heating, thereby delaying the decomposition reaction and reducing the shock sensitivity of the energetic single crystal. Theoretical rules for synthesizing novel energetic materials with low shock sensitivity were given. Our work is expected to provide a useful reference for the understanding, certifying and adjusting of the shock sensitivity of novel energetic materials.

Combustion of CL-20 cocrystals

Combustion behavior, flame structure, and thermal decomposition of bimolecular crystals of hexanitrohexaazaisowurtzitane (CL-20) with glycerol triacetate (GTA), tris[1,2,5]oxadiazolo[3,4-b:3′,4′-d:3″,4″-f]azepine-7-amine (ATFAz), 4,4′′-dinitro-ter-furazan (BNTF), oxepino[2,3-c:4,5-c′:6,7-c′′]trisfurazan (OTF), oxepino[2,3-c:4,5-c′:6,7-c′′]trisfurazan-1-oxide (OTFO) have been studied by using a constant-pressure bomb, microthermocouple technique, and TG/DSC analysis. It has been found that the introduction of volatile and thermally stable compounds into the composition with CL-20 brought about unexpected results: first, the thermally stable component decreased the thermal stability of CL-20, and second, even twofold dilution of rapidly-burning CL-20 with slow-burning compound may practically does not change the burning rate of the former. It was suggested that a reason for the observed phenomena might be amorphous CL-20 which remains after evaporation of the volatile component in the combustion wave. The above assumption was confirmed by combustion modeling of bimolecular crystals in a wide pressure interval. It has been found that the combustion mechanism of the CL-20 cocrystals depends both on the burning rate of the second component and its volatility. Depending on these parameters, several combustion models of cocrystals are implemented: the burning rate is determined by (1) the CL-20 heat release kinetics at its boiling temperature, (2) the CL-20 heat release kinetics at the boiling point of the second component and (3) by the heat flow from the gas phase. It is abundantly clear that all these combustion mechanisms can be realized in real CL-20-based propellants.

Design and Synthesis of a Series of CL-20 Cocrystals: Six-Membered Symmetrical N-Heterocyclic Compounds as Effective Coformers

Cocrystallization of energetic compounds has recently been considered as an effective method to adjust the properties through the selection of coformers, particularly to improve the detonation properties and reduce the mechanical sensitivity of CL-20. In this research, a reliable and promising design strategy to synthesize CL-20-based cocrystals was presented, where (i) the coformer should show local structural similarity to CL-20, i.e., five- or six-membered symmetric heterocycles, and (ii) the molecular structure of the coformer should contain electron-donating groups. In order to verify the adaptability of this design strategy, pyrazine, 2,6-dimethoxy-3,5-dinitropyrazine, and 2,5-dimethylpyrazine were selected as the templates of six-membered symmetrical N-heterocyclic compounds for cocrystallization with CL-20. Fortunately, on the basis of the results of the experiments and the crystal analysis, three compounds were successfully cocrystallized with CL-20. Moreover, two cocrystals with the same compone...

Isomeric Cocrystals of CL-20: A Promising Strategy for Development of High-Performance Explosives

Two novel isomeric cocrystals based on CL-20 (2,4,6,8,10,12-hexanitrohexaazaisowurtzitane) and MDNI (1-methyl-dinitroimidazole) isomers including a 1:1 CL-20/2,4-MDNI (1) and a 1:1 CL-20/4,5-MDNI (2) were obtained. These cocrystals have high densities, high predicted detonation properties, and low impact sensitivities with excellent thermal stabilities, for example, 2 (ρ: 1.882 g cm–3; D: 8915 m s–1, P: 35.88 GPa; IS: 11 J) exhibits excellent comprehensive performances. Notably, adopting the isomer as a coformer for constructing energetic cocrystals or even new cocrystal forms and further tuning properties represents a study orientation of explosives that is not yet exploited in the energetic material field.

Theoretical investigations on stabilities, sensitivity, energetic performance and mechanical properties of CL-20/NTO cocrystal explosives by molecular dynamics simulation

In this paper, a novel energetic cocrystal explosive consisted of CL-20 and NTO with different molar ratios was established through substitution method. Molecular dynamics method was chosen to optimize the geometric structures and predict the properties of different cocrystal models. The binding energies, trigger bond energies, cohesive energy density, detonation parameters and mechanical properties of each substituted model were made and compared. The effects of cocrystallization and molar ratios on stabilities, sensitivity, energetic performance and mechanical properties of cocrystal explosives were evaluated. The results show that the CL-20/NTO cocrystal explosive has better stability and is more probably to be formed with molar ratio in 2:1, 1:1 or 1:2. Besides, these cocrystal models also exhibit better mechanical properties than other substituted patterns. The cocrystal model has higher trigger bond strength and cohesive energy density than CL-20, indicating that CL-20/NTO cocrystal model has lower mechanical sensitivity and better safety. The detonation performance and energetic property of cocrystal models are declined. However, the cocrystal explosive still exhibits excellent energy density. In a word, the CL-20/NTO cocrystal model has desirable properties and can be regarded as a new kind of energetic compounds. This paper could provide some helpful instructions and novel guidance for CL-20 cocrystals designing.

Theoretical analysis of the formation driving force and decreased sensitivity for CL-20 cocrystals

Methods that analyze the driving force in the formation of the new energetic cocrystal are proposed in this paper. Various intermolecular interactions in the 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo [5.5.0.05,9.03,11]dodecane (CL-20) cocrystals are compared with those in pure CL-20 and coformer crystals by atom in molecule (AIM) and Hirshfeld surface methods under the supramolecular cluster model. The driving force in the formation of the CL-20 cocrystals is analyzed. The main driving force in the formation of the cocrystal CL-20/HMX comes from the O···H interactions, that in the formation of the cocrystal CL-20/TNT from the O···H and C···O interactions, and that in the formation of the cocrystal CL-20/BTF from the N···H and N···O interactions. Other interactions in the CL-20 cocrystals only contribute to their stabilization. At the same time, the reasons for the decreased impact sensitivity of the CL-20 cocrystals are also analyzed. They are the strengthening of the intermolecular interactions, the reducing of the free space, and the changing of the surrounding of CL-20 molecule in the CL-20 cocrystals in comparison with those in the pure CL-20 crystal.

Thermal decomposition and combustion of cocrystals of CL-20 and linear nitramines

Cocrystals of CL-20/nitramine decompose into components at the melting point. Subsequent evaporation of nitramine leaves CL-20 in the amorphous state. The lack of the crystal lattice results in an increased decomposition rate of CL-20.

Smart Energetic Nanosized Co-Crystals: Exploring Fast Structure Formation and Decomposition

The interest in co-crystals of energetic materials is explained by the fact that they can offer better thermodynamic stability and tunable sensitivity and detonation performance. In the present work, a combination of DSC, ultrafast chip calorimetry, high-resolution X-ray powder diffraction, and nanofocus X-ray diffraction was employed to investigate the thermal behavior and structure formation in nanosized co-crystals of CL-20 with HMX and TNT prepared using Spray Flash Evaporation (SFE). The CL-20/HMX co-crystal does not reveal any thermal transitions up to the thermal decomposition. In contrast, CL-20/TNT exhibits an irreversible melting transition. Upon melting, it can rapidly crystallize on heating or, at a slower pace, at room temperature to form homocrystals of γCL-20, the polymorph stable at high temperature. These observations constitute the first evidence of a CL-20 crystallization process, which occurs from the melt and not from solution. The solid–liquid phase separation occurring during heatin...

Crystal engineering of energetic materials: Co-crystals of CL-20

Co-crystallisation is proposed as an effective method to adapt the physico-chemical properties of energetic materials, thus presenting the opportunity to fine-tune performance characteristics at the molecular level. This is illustrated by the characterisation of four co-crystals of the high explosive CL-20.