CL-20 Molecules Research Articles

Reactive molecular dynamics simulation of thermal decomposition for nano-AlH3/TNT and nano-AlH3/CL-20 composites

Using reactive molecular dynamics method with the reactive force field framework, the thermal decomposition of nano-AlH3/TNT and nano-AlH3/CL-20 composites was investigated to develop new high explosives. The binding energies and relaxed densities of the composites show that the compatibility of TNT/AlH3 is better than CL-20/AlH3, which is confirmed by the DFT calculations. The pyrolysis simulations showed that the potential barrier of TNT/AlH3 (1.14 kJ g−1) and CL-20/AlH3 (0.72 kJ g−1) are smaller than pure TNT (1.99 kJ g−1) and pure CL-20 (1.01 kJ g−1), respectively. This indicates the catalytic effect of AlH3 nanoparticle on the decomposition of TNT and CL-20. The catalytic effect was confirmed by the DFT calculations of the R–NO2 rupture of TNT and CL-20 molecules on AlH3 surfaces. The slower heating rate (rh) of 16 K ps−1 led to similar evolution results as rh = 32 K ps−1, which implies that the dependence of the simulation results on the heating rate is small. Moreover, TNT/AlH3 and CL-20/AlH3 release 1.07 and 1.97 kJ g−1 more energies and generate 33.8% and 14.0% more total gas products than pure TNT and CL-20, respectively. Therefore, AlH3 nanoparticles can improve the detonation performance and specific impulse for TNT and CL-20. These are in accord with the effect of AlH3 on nitromethane and ammonium perchlorate/hydroxyl-terminated polybutadiene propellants in the experiments. As a result, the nano-AlH3/TNT and nano-AlH3/CL-20 composites can be promising candidates of new high explosives.

Theoretical insight into the effect of solvent polarity on the formation and morphology of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)/2,4,6-trinitro-toluene(TNT) cocrystal explosive

Abstract Molecular dynamics simulations were employed to study the effect of solvents of varying polarity on the formation and morphology of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20)/2,4,6-trinitro-toluene(TNT) cocrystal explosive. Six solvents, i.e. n-heptane, toluene, ethanol, acetonitrile, methanol and dimethyl sulfoxide with a wide range of polarity were used. The cocrystal morphology was predicted by Growth Morphology Model. The surface conformation and polarity were used to estimate the molecular parking of cocrystal surface. The interaction energies and binding energies were used to determine the influence of solvent properties (i.e. polarity, permittivity, molecule weight and vapor pressure) on the formation and morphology. The results indicate that the predicted morphology of CL-20/TNT cocrystal is dominated by the (0 0 2), (1 1 1), (0 2 0) and (0 2 1) in vacuum. CL-20/TNT cocrystal is easier to form in moderately polar solvents, that is, ethanol and acetonitrile. CL-20 molecules are more inclined to interact with TNT in (1 1 1) and (0 2 1) CL-20/TNT-solvent interfaces than in (0 0 2) and (0 2 0) interfaces. The predicted cocrystal morphology is in good agreement with the experimental morphology in ethonal solution. In addition, radial distribution function (RDF) analyses further proved the existence of interactions between the cocrystal faces and solvents.

Towards the low-sensitive and high-energetic co-crystal explosive CL-20/TNT: from intermolecular interactions to structures and properties.

Employing molecular dynamic (MD) simulations and solid-state density functional theory (DFT), we carried out thorough studies to understand the interaction-structure-property interrelationship of the co-crystal explosive 1 : 1 CL-20 : TNT. Our results revealed that the co-crystallization of CL-20 and TNT molecules enhances the intermolecular binding forces, where the main driving force for the formation of the co-crystal CL-20/TNT comes from HO and CO interactions, while OO contributes to the co-crystal stabilization. Furthermore, we also used the concept of atoms in molecule (AIM) and the reduced density gradient (RDG) to describe the spatial arrangements and interactions of co-crystal compositions, which showed that although the adjoining TNT molecules possess two symmetry groups and the adjoining CL-20 molecules possess the same symmetry group, their interactions are not identical. These spatial arrangements provide a good reference to the formation of other co-crystals. When the obtained structural and detonation properties of the three crystals were compared, it was observed that the CL-20/TNT co-crystal achieved the desirable properties of explosives, i.e., low-sensitivity and high-energy, possessing the advantages of both CL-20 and TNT explosives.

Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations

The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K. The VARxMD code previously developed by our group is used for detailed reaction analysis. Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained. Nitrogen oxides are the major initial decomposition products, generated in a sequence of NO2, NO3, NO, and N2O. NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides. NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis. However, it is unstable and quickly decomposes at high temperatures, while other nitrogen oxides remain. At all temperatures, the unimolecular pathways of N-NO2 bond cleavage and ring-opening C-N bond scission dominate the initial decomposition of condensed CL-20. The cleavage of the N-NO2 bond is greatly enhanced at high temperatures, but scission of the C-N bond is not as favorable. A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20, which should be considered as one of the major pathways for CL-20 decomposition at low temperatures. After the initiation of CL-20 decomposition, fragments with different ring structures are formed from a series of bond-breaking reactions. Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process, which further decompose to single ring pyrazine. Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis, supporting the proposed existence of pyrazine in Py-GC/MS experiments. The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis. Many C-4 and C-2 intermediates are observed after the initial fragmentation, but eventually convert into stable products. The distribution of the final products (N-2, H2O, CO2, and H-2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment. The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation, combined with the reaction analysis capability of VARxMD, would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.

Ultrasonic-assisted emulsion synthesis of well-distributed spherical composite CL-20@PNA with enhanced high sensitivity

A novel spherical composite explosive of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane and p-nitroaniline (CL-20@PNA) in a molar ratio of 1:1 has been prepared by ultrasonic-assisted emulsion method. Then, the shape, structure, and thermodynamic behaviours of the spherical composite CL-20@PNA was characterized by Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Differential scanning calorimetery (DSC). The SEM results indicate that the composite CL-20@PNA are relatively uniform with an average particle size of micron, and the morphology is spherical. XRD analyses confirm that the spherical composite CL-20@PNA has some peak patterns with large differences from CL-20 and PNA. FT-IR spectra suggest that hydrogen-bonding interactions may exist between CL-20 and PNA molecules. Thermal analysis shows that the spherical composite CL-20@PNA has excellent thermal stability. Moreover, an impact sensitivity test indicates that the sensitivity of the spherical composite CL-20@PNA is sufficiently reduced relative to CL-20 and CL-20/PNA physical mixing, which data is up to 60.1cm.

CL-20/DNB co-crystal based PBX with PEG: molecular dynamics simulation

Molecular dynamics simulation was carried out for CL-20/DNB co-crystal based PBX (polymer-bonded explosive) blended with polymer PEG (polyethylene glycol). In this paper, the miscibility of the PBX models is investigated through the calculated binding energy. Pair correlation function (PCF) analysis is applied to study the interaction of the interface structures in the PBX models. The mechanical properties of PBXs are also discussed to understand the change of the mechanical properties after adding the polymer. Moreover, the calculated diffusion coefficients of the interfacial explosive molecules are used to discuss the dispersal ability of CL-20 and DNB molecules in the interface layer.

Pyrolysis of CL20-BTF Co-crystal via ReaxFF-lg Reactive Force Field Molecular Dynamics Simulations

To obtain detailed information on the potential energy, the evolution of species, the initial reaction paths, and thermal decomposition products, we conducted simulations on pyrolysis process of CL20/BTF co-crystal using the ReaxFF/lg reaction force field, with temperature set at 2000 K to 3000 K. With the analysis of evolution curves of potential energy based on exponential function, we obtain the overall characteristic time. Via a description of the total package reaction with classical Arrhenius law, we obtain the activation energy of CL20/BTF co-crystal: Ea=60.8 kcal/mol. Based on the initial path of CL20/BTF co-crystal thermal decomposition we studied, we conclude that N−NO2 bond of CL20 molecules breaks first, working as a dominant role in the initial stage of thermal decomposition under the condition of different temperatures, and that all CL20 molecules completely decompose before BTF molecular regardless of different temperatures. We also find that the main products of CL20/BTF co-crystal are NO2, NO, NO3, HNO, O2, N2, H2O, CO2, N2O, and HONO, etc., on which the temperature forms certain influence.

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.

Comparative study on structure, energetic and mechanical properties of a ε-CL-20/HMX cocrystal and its composite with molecular dynamics simulation

Molecular dynamics (MD) simulation was conducted for a ε-CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexazisowurtzitane) crystal, a β-HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) crystal, a ε-CL-20/HMX cocrystal and its composite with the same molar ratio as in the cocrystal using a COMPASS force field with NPT ensemble at different temperatures. The maximum bond length (Lmax) of the N–NO2 trigger bond, cohesive energy density (CED) and binding energy (Ebind) between HMX and CL-20 molecules as well as elastic properties were calculated. Lmax increases with rising temperature and is found to be in the order of ε-CL-20/HMX cocrystal < CL-20/HMX composite < ε-CL-20 crystal at the same temperature. CED and Ebind of the cocrystal decrease with increasing temperature and are all greater than those of the composite at the same temperature. These indicate that the cocrystal is the most insensitive and its thermal stability is better than that of the composite. Furthermore, the pair correlation function g(r) analysis reveals that hydrogen bonds exist. The tensile modulus (E), bulk modulus (K) and shear modulus (G) of the ε-CL-20/HMX cocrystal and the composite are smaller than those of ε-CL-20 and β-HMX crystals and decrease with increasing temperature. However, the K/G values of the cocrystal and the composite are larger than those of the other two crystals, implying that they have better ductility.

New conformer of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20). Crystal and molecular structures of the CL-20 solvate with glyceryl triacetate

Crystallization of CL-20 in different plasticizers (gyceryl triacetate and diethyleneglycol dinitrate) was studied. IR spectra of the crystals obtained under various conditions, as a rule, do not correspond to the spectra of known modifications. Crystal and molecular structures of the CL-20 solvate with glyceryl triacetate were studied. A new stable conformer of the CL-20 molecule was discovered.

Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane or HNIW) is a high-energy nitramine explosive. To improve atomistic understanding of the thermal decomposition of CL-20 gas and solid phases, we performed a series of ab initio molecular dynamics simulations. We found that during unimolecular decomposition, unlike other nitramines (e.g., RDX, HMX), CL-20 has only one distinct initial reaction channelhomolysis of the N-NO2 bond. We did not observe any HONO elimination reaction during unimolecular decomposition, whereas the ring-breaking reaction was followed by NO 2 fission. Therefore, in spite of limited sampling, that provides a mostly qualitative picture, we proposed here a scheme of unimolecular decomposition of CL-20. The averaged product population over all trajectories was estimated at four HCN, two to four NO2, two to four NO, one CO, and one OH molecule per one CL-20 molecule. Our simulations provide a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20, allowing elucidation of key features of such processes as composition of primary reaction products, reaction timing, and Arrhenius behavior of the system. The primary reactions leading to NO2, NO, N 2O, and N2 occur at very early stages. We also estimated potential activation barriers for the formation of NO2, which essentially determines overall decomposition kinetics and effective rate constants for NO2 and N2. The calculated solid-phase decomposition pathways correlate with available condensed-phase experimental data.

Nitroreductase catalyzed biotransformation of CL-20

Previously, we reported that a salicylate 1-monooxygenase from Pseudomonas sp. ATCC 29352 biotransformed CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane) (C 6H 6N 12O 12) and produced a key metabolite with mol. wt. 346 Da corresponding to an empirical formula of C 6H 6N 10O 8 which spontaneously decomposed in aqueous medium to produce N 2O, NH 4 + , and HCOOH [Appl. Environ. Microbiol. (2004)]. In the present study, we found that nitroreductase from Escherichia coli catalyzed a one-electron transfer to CL-20 to form a radical anion (CL-20 − ) which upon initial N-denitration also produced metabolite C 6H 6N 10O 8. The latter was tentatively identified as 1,4,5,8-tetranitro-1,3a,4,4a,5,7a,8,8a-octahydro-diimidazo[4,5- b:4′,5′- e]pyrazine [IUPAC] which decomposed spontaneously in water to produce glyoxal (OHC CHO) and formic acid (HCOOH). The rates of CL-20 biotransformation under anaerobic and aerobic conditions were 3.4 ± 0.2 and 0.25 ± 0.01 nmol min −1 mg of protein −1, respectively. The product stoichiometry showed that each reacted CL-20 molecule produced about 1.8 nitrite ions, 3.3 molecules of nitrous oxide, 1.6 molecules of formic acid, 1.0 molecule of glyoxal, and 1.3 ammonium ions. Carbon and nitrogen products gave mass-balances of 60% and 81%, respectively. A comparative study between native-, deflavo-, and reconstituted-nitroreductase showed that FMN-site was possibly involved in the biotransformation of CL-20.

Initial reaction(s) in biotransformation of CL-20 is catalyzed by salicylate 1-monooxygenase from Pseudomonas sp. strain ATCC 29352.

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) (C(6)H(6)N(12)O(12)), a future-generation high-energy explosive, is biodegradable by Pseudomonas sp. strain FA1 and Agrobacterium sp. strain JS71; however, the nature of the enzyme(s) involved in the process was not understood. In the present study, salicylate 1-monooxygenase, a flavin adenine dinucleotide (FAD)-containing purified enzyme from Pseudomonas sp. strain ATCC 29352, biotransformed CL-20 at rates of 0.256 +/- 0.011 and 0.043 +/- 0.003 nmol min(-1) mg of protein(-1) under anaerobic and aerobic conditions, respectively. The disappearance of CL-20 was accompanied by the release of nitrite ions. Using liquid chromatography/mass spectrometry in the negative electrospray ionization mode, we detected a metabolite with a deprotonated mass ion [M - H](-) at 345 Da, corresponding to an empirical formula of C(6)H(6)N(10)O(8), produced as a result of two sequential N denitration steps on the CL- 20 molecule. We also detected two isomeric metabolites with [M - H](-) at 381 Da corresponding to an empirical formula of C(6)H(10)N(10)O(10). The latter was a hydrated product of the metabolite C(6)H(6)N(10)O(8) with addition of two H(2)O molecules, as confirmed by tests using (18)O-labeled water. The product stoichiometry showed that each reacted CL-20 molecule produced about 1.7 nitrite ions, 3.2 molecules of nitrous oxide, 1.5 molecules of formic acid, and 0.6 ammonium ion. Diphenyliodonium-mediated inhibition of salicylate 1-monooxygenase and a comparative study between native, deflavo, and reconstituted enzyme(s) showed that FAD site of the enzyme was involved in the biotransformation of CL-20 catalyzed by salicylate 1-monooxygenase. The data suggested that salicylate 1-monooxygenase catalyzed two oxygen-sensitive single-electron transfer steps necessary to release two nitrite ions from CL-20 and that this was followed by the secondary decomposition of this energetic chemical.

Biotransformation of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) by denitrifying Pseudomonas sp. strain FA1.

The microbial and enzymatic degradation of a new energetic compound, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), is not well understood. Fundamental knowledge about the mechanism of microbial degradation of CL-20 is essential to allow the prediction of its fate in the environment. In the present study, a CL-20-degrading denitrifying strain capable of utilizing CL-20 as the sole nitrogen source, Pseudomonas sp. strain FA1, was isolated from a garden soil. Studies with intact cells showed that aerobic conditions were required for bacterial growth and that anaerobic conditions enhanced CL-20 biotransformation. An enzyme(s) involved in the initial biotransformation of CL-20 was shown to be membrane associated and NADH dependent, and its expression was up-regulated about 2.2-fold in CL-20-induced cells. The rates of CL-20 biotransformation by the resting cells and the membrane-enzyme preparation were 3.2 +/- 0.1 nmol h(-1) mg of cell biomass(-1) and 11.5 +/- 0.4 nmol h(-1) mg of protein(-1), respectively, under anaerobic conditions. In the membrane-enzyme-catalyzed reactions, 2.3 nitrite ions (NO(2)(-)), 1.5 molecules of nitrous oxide (N(2)O), and 1.7 molecules of formic acid (HCOOH) were produced per reacted CL-20 molecule. The membrane-enzyme preparation reduced nitrite to nitrous oxide under anaerobic conditions. A comparative study of native enzymes, deflavoenzymes, and a reconstituted enzyme(s) and their subsequent inhibition by diphenyliodonium revealed that biotransformation of CL-20 is catalyzed by a membrane-associated flavoenzyme. The latter catalyzed an oxygen-sensitive one-electron transfer reaction that caused initial N denitration of CL-20.

Molecular Dynamics Studies of Nanoparticles of Energetic Materials

ABSTRACTThe structural properties of several nanoparticles of 2,4,6,8,10,12-hexanitrohexaazaiso-wurtzitane, HNIW or CL-20, are studied by using molecular dynamics simulations. The internal structure of the CL-20 molecule is held rigid and the intermolecular interactions in the nanoparticles are taken from a previously developed force field. [Sorescu et al., J. Phys. Chem. B, 102, 948 (1998)] Molecular dynamics simulations of solid-like and annealed nanoparticles with 48 and 88 CL-20 molecules have been carried out in the solid-state range of temperatures from 50 to 500 K. The center-of-mass to center-of-mass radial distribution functions, dipole-dipole correlation function, the orientations of the surface dipoles, and the density of the nanoparticles were calculated at fixed temperatures for the nanoparticles.