Theoretical insight into the binding energy and detonation performance of ε-, γ-, β-CL-20 cocrystals with β-HMX, FOX-7, and DMF in different molar ratios, as well as electrostatic potential

Theoretical study of potential energetic material CL-20/DNAN eutectic explosive based on molecular dynamics method

CONTEXT

The exploration of CL-20 eutectic has been a subject of fervent interest within the realm of high-energy material modification. Through the utilization of density functional and molecular dynamics methods, an investigation into the characteristics of hexanitrohexaazaisowurtzitane (CL-20)/2,4-dinitroanisole (DNAN) within the molar ratio range of 9:1-1:9 was conducted. This inquiry encompassed the scrutiny of molecular interaction pathway, attachment force, initiating molecular distance, unified energy concentration, and physical characteristics. Furthermore, EXPLO-5 was harnessed to prognosticate the explosion features and byproducts of unadulterated CL-20, DNAN, and CL-20/DNAN frameworks. The findings delineate a substantial differentiation in the electrostatic charge distribution on the surface between CL-20 and DNAN particles, signifying the preeminence of intermolecular interactions between disparate entities over those within similar entities, thus intimating the plausibility of eutectic constitution. Remarkably, the identification of maximal attachment force at a molar ratio of 4:6 suggests the heightened likelihood of eutectic formation, propelled primarily by electrostatic and van der Waals forces. The resultant eutectic explosive evinces intermediate reactivity and exemplary mechanical attributes. Moreover, the detonation achievement of the eutectic with a molar proportion of 4:6 straddles that of CL-20 and DNAN, representing a new type of insensitive high-energy material.

METHODS

The testing method employs the Materials Studio software and utilizes the molecular dynamics (MD) method to predict the properties of CL-20/DNAN co-crystals with different ratios and crystal faces. The MD simulation time step is set to 1 fs, and the total MD simulation time is 2 ns. An isothermal-isobaric (NPT) ensemble is used for the 2-ns MD simulation. The COMPASS force field is employed, with the temperature set to 295 K. The prediction of detonation characteristics and products is conducted using the EXPLO-5 software.

Theoretical investigations on CL-20/ANTA co-crystal explosive via molecular dynamics method

CONTEXT

The study of CL-20 co-crystal has always been a focal point within the field of energetic material modification. In this study, we employed a combination of density functional theory and molecular dynamics simulations to investigate the properties of hexanitrohexaazaisowurtzitane (CL-20)/3-amino-5-nitro-1,2,4-triazole (ANTA) with different molar ratios ranging from 4:1 to 1:4. Additionally, EXPLO-5 software utilized to predict the detonation properties and products of pure CL-20, ANTA, and CL-20/ANTA systems. The results revealed that there was an interaction between CL-20 and ANTA molecules, which had the potential to form a co-crystal. The most likely molar ratio for co-crystal formation was 1:1, and the main driving forces for co-crystal formation were electrostatic force, dispersion force, and van der Waals force. The co-crystal explosive exhibited moderate sensitivity and excellent mechanical properties. Furthermore, the co-crystal detonation performance at a molar ratio of 1:1 was between that of CL-20 and ANTA, representing a new type of insensitive high-energy material.

METHODS

The properties of CL-20/ANTA co-crystal were predicted by molecular dynamics (MD) method under Materials Studio software. For the whole MD simulations, set the temperature at 298 K, and the pressure was 0.0001 GPa. Conducted MD simulation under the NPT ensemble for a total simulation duration of 1 ns. The first 0.5 ns was used for thermodynamic equilibrium, and the last 0.5 ns was used for statistical calculation and analysis. Sampling was recorded every 10 fs during the calculation.

Structural evolution of CL-20/DNB cocrystals at high temperature: Phase transition and kinetics of thermal decomposition

As a typical new energetic material, CL-20/DNB cocrystals have been recognized as a promising explosive owing to their excellent comprehensive performance. The thermal decomposition behavior, structural evolution and dynamic process of CL-20/DNB cocrystals under high temperature were studied by means of thermogravimetric differential heating, X-ray diffraction, Raman spectroscopy to gain insight into the cocrystal materials. The study found that the decomposition of CL-20/DNB cocrystal is a heterogeneous process accompanied by the sublimation of DNB and structural change of CL-20. The phase transition of β → γ-CL-20 was observed at 120 °C. The kinetics of decomposition and the mechanism of micro structural evolution on CL-20/DNB cocrystals with heating were revealed. The primary NO⋯H hydrogen bonds of the cocrystal are broken, accompanied by the melting of DNB in the temperature range of 100-120 °C. Subsequently, the DNB single component decomposes completely, leading to lattice collapse of cocrystal; simultaneously, CL-20 undergoes a transition process from β phase to γ phase. Ultimately, γ-CL-20 gradually decomposes with increasing temperature. The activation energy of cocrystal is also obtained as 129 ± 10 kJ/mol. The understanding of cocrystal explosive was deepened and the further application was promoted.

Study on the effect of solvent on cocrystallization of CL-20 and HMX through theoretical calculations and experiments

Cocrystallization is a helpful method for explosives design. However, lack of understanding of the cocrystallization mechanism leads to inefficiency in cocrystal preparation. Therefore, studying the effects of solvent on cocrystal is of great importance for the efficient application of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20). In this paper, the effect of solvent on cocrystallization is investigated by the CL-20/HMX cocrystal/solvent cluster model, the CL-20/HMX/solvent mixture model, the CL-20/HMX cocrystal/solvent interface model combined with quantum chemistry and molecular dynamic methods. The authors find that the hydrogen bond between CL-20 and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) is the strongest and the binding energy of cocrystal and solvent molecules is the weakest in ethyl acetate (EA) solvent, indicating that CL-20 and HMX tend to be combined together and there is less hindrance by solvent molecules. Analysis of the CL-20/HMX/solvent mixture and mass density distribution studies show that the solvent effect has a great influence on the crystal faces and the cocrystallization rate of CL-20 and HMX is the highest in EA solvent. The XRD and SEM characterization results are consistent with the theoretical calculations. The present work on the effects of solvent on CL-20/HMX cocrystals is beneficial for understanding the mechanism of the growth of energetic cocrystal materials. It is helpful in selecting more suitable theoretical and experimental conditions and makes access to excellent cocrystals more efficient.

Cocrystallization is a helpful method for explosives design. Studying the effects of solvent on cocrystal is of great importance for the efficient application of CL-20/HMX cocrystal.

Study on the Cocrystallization Mechanism of CL-20/HMX in a Propellant Aging Process through Theoretical Calculations and Experiments

Energetic materials undergo physical and chemical aging due to environmental effects, resulting in the degradation of safety and detonation performances. Therefore, studying the aging performance of energetic materials is of great importance for the efficient application of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20)-based solid propellants. In this paper, XRD and FTIR of the CL-20-based propellant and CL-20/1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)-based propellant samples showed CL-20/HMX cocrystal formation according to appearance of new peaks. SEM and EDS analyses showed that pores and dehumidification in the propellant occurred with the cocrystallization of CL-20 and HMX during the aging process. Furthermore, molecular dynamics simulation was used to predict the crystal transformation of the CL-20- and HMX-based propellant under a long-term storage process. The stability of ε-CL-20 was obtained by analyzing the crystal transformation rate. The binding energy, radial distribution function between CL-20 and HMX, as well as mechanical properties of the CL-20/HMX cocrystal and the mixture were calculated to reveal the stronger binding between CL-20 and HMX in the cocrystal. Meanwhile, the inducer effect of a nitrate ester during the cocrystallization process was analyzed. The theoretical calculation shows that during aging, ε-CL-20 tends to exist stably, while CL-20/HMX tends to form cocrystals because of the strong bond. The present work on the transformation and cocrystallization of CL-20 and HMX during long-term storage is beneficial for understanding the degradation mechanism of the propellant performances, facilitating safe storage and life evaluation of propellants.

Theoretical investigations on stability, sensitivity, energetic performance, and mechanical properties of CL-20/TNAD cocrystal explosive by molecular dynamics method

The crystal models of trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin (TNAD), hexanitrohexaazaisowurtzitane (CL-20), and CL-20/TNAD cocrystal explosive with different component ratios were established. Molecular dynamics (MD) method was applied to predict the stability, sensitivity, energetic properties, and mechanical properties. The effect of component ratio on properties of CL-20/TNAD cocrystal explosive was investigated and estimated. Results show that the cocrystal model with component ratio in 1:1 exhibits the highest binding energy and it is more stable. In CL-20/TNAD cocrystal explosive, the interaction energy of trigger bond is increased by 0.8 ~ 15.0 kJ/mol, implying that the mechanical sensitivity of CL-20/TNAD cocrystal explosive is lower than CL-20 and the safety is effectively improved. Compared with raw CL-20, the crystal density of cocrystal explosive is declined by 0.014 ~ 0.193 g/cm³, detonation velocity is declined by 39 ~ 755 m/s, and detonation pressure is declined by 0.95 ~ 11.40 GPa; namely the energy density of CL-20/TNAD cocrystal explosive is lower than CL-20. The cocrystal explosives with component ratio in 10:1 ~ 1:1 still exhibit desirable detonation performance and can be regarded as high energy density explosives. The values of tensile modulus, shear modulus, and bulk modulus of CL-20/TNAD cocrystal explosive are decreased by 0.448 ~ 10.285 GPa, 0.195 ~ 4.189 GPa, and 0.194 ~ 6.292 GPa, respectively, Cauchy pressure is increased by 0.990 ~ 5.704 GPa, meaning that the rigidity, fracture strength, and hardness of cocrystal explosive are declined, while the plastic property and ductility are increased and the mechanical properties are improved. The cocrystal model with component ratio in 1:1 has the best mechanical properties. Consequently, the CL-20/TNAD cocrystal explosive with component ratio in 1:1 is more stable and insensitive; it also has high energy density and the best mechanical properties and may be an attractive candidate for high energy explosives.

Co-agglomerated crystals of cyclic nitramines with sterically crowded molecules

Co-crystallization of cyclic nitramines with sterically crowded molecules, i.e., ε-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (ε-CL-20) with cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole, is difficult, but co-agglomeration given good results.

Multilevel strategies for the composition and formation of DAAF/HNIW composite crystals

Insensitive energetic materials are still a great challenge for potential applications.

CL-20-Based Cocrystal Energetic Materials: Simulation, Preparation and Performance

The cocrystallization of high-energy explosives has attracted great interests since it can alleviate to a certain extent the power-safety contradiction. 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane (CL-20), one of the most powerful explosives, has attracted much attention for researchers worldwide. However, the disadvantage of CL-20 has increased sensitivity to mechanical stimuli and cocrystallization of CL-20 with other compounds may provide a way to decrease its sensitivity. The intermolecular interaction of five types of CL-20-based cocrystal (CL-20/TNT, CL-20/HMX, CL-20/FOX-7, CL-20/TKX-50 and CL-20/DNB) by using molecular dynamic simulation was reviewed. The preparation methods and thermal decomposition properties of CL-20-based cocrystal are emphatically analyzed. Special emphasis is focused on the improved mechanical performances of CL-20-based cocrystal, which are compared with those of CL-20. The existing problems and challenges for the future work on CL-20-based cocrystal are discussed.

Multimolecular Complexes of CL-20 with Nitropyrazole Derivatives: Geometric, Electronic Structure, and Stability

The multimolecular complexes formed between 2,4,6,8,10,12-hexanitro-2,4,6,6,8,10,12-hexaazaisowurtzitane (CL-20) and nitropyrazole compounds were investigated using B3LYP-D3/6-311G(d,p) and B97-3c methods. CL-20 in these complexes was surrounded by methyl, nitro, and amino derivatives of 4-nitropyrazole. The influence of substituents on the molecular electrostatic potential distribution of nitropyrazoles was investigated to figure out the potential electrostatic interaction sites. For the complex, the O···H hydrogen bond was popular in the intermolecular interactions, and dispersion interaction played an essential role, especially in Cx/CL-20 multimolecular complexes. Trigger bond analysis showed that their strength increased upon the formation of intermolecular weak interactions. Nitro group charge calculations stated that the negative charge on almost all nitro groups showed a significant increase. Therefore, the sensitivity of CL-20 seemed to be lower than the original. In addition, the transfer of electron density between CL-20 and nitropyrzoles in complexes was investigated, revealing the influence of weak interactions on the electron density of CL-20.

Charge Distributions of Nitro Groups Within Organic Explosive Crystals: Effects on Sensitivity and Modeling

The charge distribution of NO₂ groups within the crystalline polymorphs of energetic materials strongly affects their explosive properties. We use the recently introduced basis-space iterated stockholder atom partitioning of high-quality charge distributions to examine the approximations that can be made in modeling polymorphs and their physical properties, using 1,3,5-trinitroperhydro-1,3,5-triazine, trinitrotoluene, 1-3-5-trinitrobenzene, and hexanitrobenzene as exemplars. The NO₂ charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO₂ torsion angle within the possible variations found in observed crystal structures. Thus, the proposed correlations between the molecular electrostatic properties, such as trigger-bond potential or maxima in the electrostatic potential, and impact sensitivity will be affected by the changes in conformation that occur on crystallization. We establish the relationship between the NO₂ torsion angle and the likelihood of occurrence in observed crystal structures, the conformational energy, and the charge and dipole magnitude on each atom, and how this varies with the neighboring groups. We examine the effect of analytically rotating the atomic multipole moments to model changes in torsion angle and establish that this is a viable approach for crystal structures but is not accurate enough to model the relative lattice energies. This establishes the basis of transferability of the NO₂ charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials.

Direct insight into the formation driving force, sensitivity and detonation performance of the observed CL-20-based energetic cocrystals

This work elucidated the underlying mechanism of the dramatic and divergent physicochemical properties of CL-20-based energetic cocrystals.

Molecular dynamics calculation on structures, stabilities, mechanical properties, and energy density of CL-20/FOX-7 cocrystal explosives

In this article, different CL-20/FOX-7 cocrystal models were established by the substitution method based on the molar ratios of CL-20:FOX-7. The structures and comprehensive properties, including mechanical properties, stabilities, and energy density, of different cocrystal models were obtained and compared with each other. The main aim was to estimate the influence of molar ratios on properties of cocrystal explosives. The molecular dynamics (MD) simulation results show that the cocrystal model with molar ratio 1:1 has the best mechanical properties and highest binding energy, so the CL-20/FOX-7 cocrystal model is more likely to form in 1:1 M ratio. The detonation parameters show that the cocrystal explosive exhibited preferable energy density and excellent detonation performance. In a word, the 1:1 cocrystal model has the best comprehensive properties, is very promising, and worth more theoretical investigations and experimental tests. This paper gives some original theories to better understand the cocrystal mechanism and provides some helpful guidance and useful instructions to help design CL-20 cocrystal explosives.

Theoretical investigation of the effects of the molar ratio and solvent on the formation of the pyrazole-nitroamine cocrystal explosive 3,4-dinitropyrazole (DNP)/2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)

The effects of the molar ratio, temperature, and solvent on the formation of the cocrystal explosive DNP/CL-20 were investigated using molecular dynamics (MD) simulation. The cocrystal structure was predicted through Monte Carlo (MC) simulation and using first-principles methods. The results showed that the DNP/CL-20 cocrystal might be more stable in the molar ratio 1:1 near to 318 K, and the most probable cocrystal crystallizes in the triclinic crystal system with the space group P[Formula: see text]. Cocrystallization was more likely to occur in methanol and ethanol at 308 K as a result of solvent effects. The optimized structure and the reduced density gradient (RDG) of the DNP/CL-20 complex confirmed that the main driving forces for cocrystallization were a series of hydrogen bonds and van der Waals forces. Analyses of the trigger bonds, the charges on the nitro groups, the electrostatic surface potential (ESP), and the free space per molecule in the cocrystal lattice were carried out to further explore their influences on the sensitivity of CL-20. The results indicated that the DNP/CL-20 complex tended to be more stable and insensitive than pure CL-20. Moreover, an investigation of the detonation performance of the DNP/CL-20 cocrystal indicated that it possesses high power. Graphical abstract DNP/CL-20 cocrystal models with different molar ratios were investigated at different temperatures using molecular dynamics (MD) simulation methods. Binding energies and mechanical properties were probed to determine the stability and performance of each cocrystal model. Solvated DNP/CL-20 models were established by adding solvent molecules to the cocrystal surface. The binding energies of the models in various solvents were calculated in order to identify the most suitable solvent and temperature for preparing the cocrystal explosive DNP/CL-20.

Theoretical insight into the cocrystal explosive of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)/1-Methyl-4,5-dinitro-1H-imidazole (MDNI)

Cocrystal explosive is getting more and more attention in high energy density material field. Different molar ratios of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)/1-Methyl-4,5-dinitro-1H-imidazole (MDNI) cocrystal were studied by molecular dynamics (MD) simulation and quantum-chemical density functional theory (DFT) calculation. Binding energy of CL-20/MDNI cocrystal and radial distribution function (RDF) were used to estimate the interaction. Mechanical properties were calculated to predict the elasticity and ductility. The length and bond dissociation energy of trigger bond, surface electrostatic potentials (ESP) of CL-20/MDNI framework were calculated at B3LYP/6-311[Formula: see text]G(d,p) level. The results indicate that CL-20/MDNI cocrystal explosive might have better mechanical properties and stability in a molar ratio 3:2. The N–NO2 bond becomes stronger upon the formation of intermolecular H-bonding interaction. The surface electrostatic potential further confirms that the sensitivity decreases in cocrystal explosive in comparison with that in isolated CL-20. The oxygen balance (OB), heat of detonation ([Formula: see text], detonation velocity ([Formula: see text] and detonation pressure ([Formula: see text] of CL-20/MDNI suggest that the CL-20/MDNI cocrystal possesses excellent detonation performance and low sensitivity.

Comparative studies on structures, mechanical properties, sensitivity, stabilities and detonation performance of CL-20/TNT cocrystal and composite explosives by molecular dynamics simulation

To investigate and compare the differences of structures and properties of CL-20/TNT cocrystal and composite explosives, the CL-20/TNT cocrystal and composite models were established. Molecular dynamics simulations were performed to investigate the structures, mechanical properties, sensitivity, stabilities and detonation performance of cocrystal and composite models with COMPASS force field in NPT ensemble. The lattice parameters, mechanical properties, binding energies, interaction energy of trigger bond, cohesive energy density and detonation parameters were determined and compared. The results show that, compared with pure CL-20, the rigidity and stiffness of cocrystal and composite models decreased, while plastic properties and ductility increased, so mechanical properties can be effectively improved by adding TNT into CL-20 and the cocrystal model has better mechanical properties. The interaction energy of the trigger bond and the cohesive energy density is in the order of CL-20/TNT cocrystal > CL-20/TNT composite > pure CL-20, i.e., cocrystal model is less sensitive than CL-20 and the composite model, and has the best safety parameters. Binding energies show that the cocrystal model has higher intermolecular interaction energy values than the composite model, thus illustrating the better stability of the cocrystal model. Detonation parameters vary as CL-20 > cocrystal > composite, namely, the energy density and power of cocrystal and composite model are weakened; however, the CL-20/TNT cocrystal explosive still has desirable energy density and detonation performance. This results presented in this paper help offer some helpful guidance to better understand the mechanism of CL-20/TNT cocrystal explosives and provide some theoretical assistance for cocrystal explosive design.

Theoretical insights into the effects of molar ratios on stabilities, mechanical properties, and detonation performance of CL-20/HMX cocrystal explosives by molecular dynamics simulation

To research and estimate the effects of molar ratios on structures, stabilities, mechanical properties, and detonation properties of CL-20/HMX cocrystal explosive, the CL-20/HMX cocrystal explosive models with different molar ratios were established in Materials Studio (MS). The crystal parameters, structures, stabilities, mechanical properties, and some detonation parameters of different cocrystal explosives were obtained and compared. The molecular dynamics (MD) simulation results illustrate that the molar ratios of CL-20/HMX have a direct influence on the comprehensive performance of cocrystal explosive. The hardness and rigidity of the 1:1 cocrystal explosive was the poorest, while the plastic property and ductibility were the best, thus implying that the explosive has the best mechanical properties. Besides, it has the highest binding energy, so the stability and compatibility is the best. The cocrystal explosive has better detonation performance than HMX. In a word, the 1:1 cocrystal explosive is worth more attention and further research. This paper could offer some theoretical instructions and technological support, which could help in the design of the CL-20 cocrystal explosive.

Theoretical insights into the stabilities, detonation performance, and electrostatic potentials of cocrystals containing α- or β-HMX and TATB, FOX-7, NTO, or DMF in various molar ratios

A molecular dynamics method was employed to study the binding energies associated with the cocrystallization (at selected crystal planes) of either 1,3,5-triamino-2,4,6-trinitro-benzene (TATB), 1,1-diamino-2,2-dinitroethylene, 3-nitro-1,2,4-triazol-5-one (TATB, FOX-7, and NTO, respectively, all of which are explosives), or N,N-dimethylformamide (DMF, a nonenergetic solvent) in various molar ratios with 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane in its α and β conformations (α-HMX and β-HMX, respectively). The results showed that the cocrystals with low molar ratios (2:1, 1:1, 1:2, and 1:3) were the most stable. The binding energies of HMX/NTO and HMX/DMF were larger than those of HMX/TATB and HMX/FOX-7. According to the calculated stabilities, HMX prefers to adopt its α form in HMX/TATB and its β form in HMX/NTO, whereas the two forms coexist in HMX/FOX-7. For HMX/TATB, HMX/NTO, and α-HMX/FOX-7, increasing the proportion of the cocrystal component with the highest detonation heat (HMX in the first two cases, FOX-7 in the latter) increases the detonation heat, velocity, and pressure of the cocrystal. However, increasing the proportion of the component with the highest detonation heat in β-HMX/FOX-7 and γ-CL-20/FOX-7 increases the detonation heat of the cocrystal but decreases its detonation velocity. An investigation of the surface electrostatic potential revealed how the sensitivity changes upon cocrystal formation. Graphical Abstract Surface electrostatic potential of HMX/TATB.