Host-Guest Technique for Designing Highly Energetic Compounds with the Nitroamino Group

A new energetic material, 2-azido-4,7-nitroamino-1H-imidazo[4,5-d]pyridazine (ANIP) with a highly sensitive azido group and its host-guest compounds (ANIP/H2O and ANIP/H2O2), and energetic salts were obtained. With the guest and protons in host molecules, an abundant hydrogen bond system can be formed. This results in high crystal density and good sensitivity, which suggests that the host-guest strategy is a promising way to balance the contradiction between energy and sensitivity and provides a new path to obtain a new generation of high energetic materials.

Deep Potential Molecular Dynamics Study of Chapman-Jouguet Detonation Events of Energetic Materials

Detonation of energetic materials (EMs) is of great importance for military applications, while the understanding of detailed events and mechanisms for the detonation process is scarce. In this study, the first deep neural network potential NNP_Shock for molecular dynamics (MD) simulation of shock-induced detonation of EMs was generated based on a deep potential model, providing DFT accuracy but 106 times the computational efficiency. On this basis, we employ our deep potential to perform MD simulations of shock-induced detonation of high-performance EM material 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20, C6H6N12O12) and obtain the theoretical Chapman-Jouguet (C-J) detonation velocities and pressures directly by multiscale shock technique (MSST) for the first time, which are in good agreement with experiment. In addition, the Hugoniot curves and initial reaction mechanisms were successfully obtained. Therefore, the NNP_Shock potential is competent in research of the detonation performance and shock sensitivity of CL-20, and the method can also be transplanted to studies of other EMs.

Effects of Different Guests on Pyrolysis Mechanism of α-CL-20/Guest at High Temperatures by Reactive Molecular Dynamics Simulations at High Temperatures

The host-guest inclusion strategy has the potential to surpass the limitations of energy density and suboptimal performances of single explosives. The guest molecules can not only enhance the detonation performance of host explosives but also can enhance their stability. Therefore, a deep analysis of the role of guest influence on the pyrolysis decomposition of the host-guest explosive is necessary. The whole decomposition reaction stage of CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH was calculated by ReaxFF-MD. The incorporation of CO₂, N₂O and NH₂OH significantly increase the energy levels of CL-20. However, different guests have little influence on the initial decomposition paths of CL-20. The E_a1 and E_a2 values of CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are higher than the CL-20/H₂O system. Clearly, incorporation of CO₂, N₂O, NH₂OH can inhibit the initial decomposition and intermediate decomposition stage of CL-20/H₂O. Guest molecules become heavily involved in the reaction and influence on the reaction rates. k₁ of CL-20/N₂O and CL-20/NH₂OH systems are significantly larger than that of CL-20/H₂O at high temperatures. k₁ of CL-20/CO₂ system is very complex, which can be affected deeply by temperatures. k₂ of the CL-20/CO₂, CL-20/N₂O systems is significantly smaller than that of CL-20/H₂O at high temperatures. k₂ of CL-20/NH₂OH system shows little difference at high temperatures. For the CL-20/CO₂ system, the k₃ value of CO₂ is slightly higher than that for CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems, while the k₃ values of N₂ and H₂O are slightly smaller than that for the CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems. For the CL-20/N₂O system, the k₃ value of CO₂ is slightly smaller than that for CL-20/H₂O, CL-20/CO₂, CL-20/NH₂OH systems. For the CL-20/NH₂OH system, the k₃ value of H₂O is slightly larger than that for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O systems. These mechanisms revealed that CO₂, N₂O and NH₂OH molecules inhibit the early stages of the initial decomposition of CL-20 and play an important role for the decomposition subsequently.

Structural and Functional Insights into Metal-Free Perovskites

In the past three years, metal-free perovskites have garnered significant interest as promising candidates for utilization in next-generation wearable electronics. A variety of different molecular structures for these perovskites have been designed for different applications. However, there is still no systematic understanding that can elucidate the relationship between the structural details and properties of perovskites. This would provide a helpful guide for designing a metal-free perovskite with the desired packing structure and properties. Herein, we summarize recently reported structural and functional insights into metal-free perovskites. The underlying design of the molecular structure and its role in the packing structure and resulting properties are explained. In addition, important factors and challenges in the design of a molecular structure that will be useful for future applications are discussed. This information will help enrich the library of potential structures and future applications of metal-free perovskites, which is believed to be much larger than is currently known.

Theoretical Studies on the Role of Guest in α-CL-20/Guest Crystals

The contradiction between energy and safety of explosives is better balanced by the host-guest inclusion strategy. To deeply analyze the role of small guest molecules in the host-guest system, we first investigated the intermolecular contacts of host and guest molecules through Hirshfeld surfaces, 2-D fingerprint plots and electrostatic interaction energy. We then examined the strength and nature of the intermolecular interactions between CL-20 and various small molecules in detail, using state-of-the-art quantum chemistry calculations and elaborate wavefunction analyses. Finally, we studied the effect of the small molecules on the properties of CL-20, using density functional theory (DFT). The results showed that the spatial arrangement of host and guest molecules and the interaction between host and guest molecules, such as repulsion or attraction, may depend on the properties of the guest molecules, such as polarity, oxidation, hydrogen content, etc. The insertion of H2O2, H2O, N2O, and CO2 had significant influence on the electrostatic potential (ESP), van der Waals (vdW) potential and chemical bonding of CL-20. The intermolecular interactions, electric density and crystal orbital Hamilton population (COHP) clarified and quantified the stabilization effect of different small molecules on CL-20. The insertion of the guest molecules improved the stability of CL-20 to different extents, of which H2O2 worked best.

Roles of Small Molecules in the Stability and Sensitivity of CL-20-Based Host-Guest Explosives under Electric Fields: A Reactive Molecular Dynamics Study

Host-guest inclusion, constructed by inserting small molecules into voids of energetic crystals, is a novel strategy for creating new energetic materials (EMs) with desired energy and safety. To provide an atomistic-level insight into the fact that small guest molecules can effectively regulate the stability and sensitivity of CL-20, we conducted ReaxFF-lg reactive molecular dynamics simulations on electric-field (EF)-induced decomposition of two typical host-guest EMs, CL-20/H₂O₂ and CL-20/N₂O, and compared it to that of α-CL-20 and ε-CL-20. Our findings show that the sensitivity order of the CL-20-based EMs under EFs, α-CL-20/H₂O₂ > ε-CL-20 > α-CL-20 > α-CL-20/N₂O, agrees well with the sensitivity obtained from the experiment (ε-CL-20 > α-CL-20 > α-CL-20/N₂O). Different effects of H₂O₂ and N₂O molecules were found responsible for the distinct stability and sensitivity of these materials toward EFs. On the one hand, H₂O₂ accelerate(s) the structural transformation of CL-20 and thus increases the sensitivity, because the wobbling NO₂ group reduces the stability of CL-20 by weakening its adjacent C-N bonds, whereas N₂O makes this transition less likely, resulting in low sensitivity of α-CL-20/N₂O. On the other hand, H₂O₂ and its decomposition intermediate OH radical can promote destruction of CL-20's cage structure and produce a large amount of water molecules to release heat, making CL-20/H₂O₂ to decompose faster than ε-CL-20. N₂O molecules rarely react with CL-20 molecules but absorb heat from the surrounding decomposed CL-20 and thus slow down CL-20's decomposition, resulting in low sensitivity of α-CL-20/N₂O, as confirmed by transition-state calculations. The results provide a comprehensive understanding of the stability and sensitivity of CL-20-based host-guest explosives under EFs.

Energetic host–guest inclusion compounds: an effective design paradigm for high-energy materials

Due to the stringent requirements of performance, safety, and cost for the development of new energetic materials (EMs), the synthesis of host–guest inclusion compounds is an attractive way to fully exploit the application potential of existing EMs.

Host–guest energetic materials: a promising strategy of incorporating small insensitive molecule into the lattice cavities of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane to enhance the safety on the premise of maintaining the excellent energy density

A novel HNIW-MA host–guest explosive was constructed by embedding the mall molecules into the lattice cavities of HNIW, and it enhances the safety on the premise of maintaining its energy density.

Smart Host-Guest Energetic Material Constructed by Stabilizing Energetic Fuel Hydroxylamine in Lattice Cavity of 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane Significantly Enhanced the Detonation, Safety, Propulsion, and Combustion Performances

The host-guest inclusion strategy has become a promising method for developing novel high-energy density materials (HEDMs). The selection of functional guest molecules was a strategic project, as it can not only enhance the detonation performance of host explosives but can also modify some of their suboptimal performances. Here, to improve the propulsion and combustion performances of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), a novel energetic-energetic host-guest inclusion explosive was obtained by incorporating energetic rocket fuel, hydroxylamine (HA), into the lattice cavities of HNIW. Based on their perfect space matching, the crystallographic density of HNIW-HA was determined to be 2.00 g/cm³ at 296 K, which has reached the gold standard regarding the density of HEDMs. HNIW-HA also showed higher thermal stability (T_d = 245.9 °C) and safety (H₅₀ = 16.8 cm) and superior detonation velocity (D_V = 9674 m/s) than the ε-HNIW. Additionally, because of the excellent combustion performance of HA, HNIW-HA possessed higher propulsion performances, including combustion speed (S_C = 39.5 mg/s), combustion heat (Q_C = 8661 J/g), and specific impulse (I_sp = 276.4 s), than ε-HNIW. Thus, the host-guest inclusion strategy has potential to surpass the limitations of energy density and suboptimal performances of single explosives and become a strategy for developing multipurpose intermolecular explosives.

Energetic Co-Crystal of a Primary Metal-Free Explosive with BTF. Ideal Pair for Co-Crystallization

Co-crystallization is an elegant technique to tune the physical properties of crystalline solids. In the field of energetic materials, co-crystallization is currently playing an important role in the engineering of crystals with improved performance. Here, based on an analysis of the structural features of the green primary explosive, tetramethylammonium salt of 7-oxo-5-(trinitromethyl)-4,5,6,7-tetrahydrotetrazolo[1,5-a][1,3,5]triazin-5-ide (1), a co-former such as the powerful secondary explosive, benzotrifuroxan (BTF, 2), has been proposed to improve it. Compared to the original 1, its co-crystal with BTF has a higher detonation pressure and velocity, as well as an initiating ability, while the impact sensitivity and thermal stability remained at about the same level. Both co-formers, 1 and 2, and co-crystal 3 were characterized by single-crystal X-ray diffraction and their crystal packing was analyzed in detail by the set of approaches, including periodic calculations. In the co-crystal 3, all intermolecular interactions were significantly redistributed. However, no new types of intermolecular interactions were formed during co-crystallization. Moreover, the interaction energies of structural units in crystals before and after co-crystallization were approximately the same. A similar trend was observed for the volumes occupied by structural units and their densifications. The similar nature of the organization of the crystals of the co-formers and the co-crystal gives grounds to assert that the selected co-formers are an ideal pair for co-crystallization, and the invariability of the organization of the crystals was probably responsible for the preservation of some of their properties.

The role of electric field on decomposition of CL-20/HMX cocrystal: A reactive molecular dynamics study

Electric field can initiate decomposition or detonation of explosives, but underlying mechanism is unclear. Here, we performed ReaxFF molecular dynamics simulation for decomposition of a cocrystal, formed by 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), solely induced by electric field. A new analytical method was proposed to obtain detailed decomposition mechanism. Results show that electric fields play important roles in decomposition of CL-20/HMX cocrystal, such as heating the system and causing the explosive to decompose. Strong constant field makes CL-20 molecules in the cocrystal decompose at significantly lower temperature, which greatly increases sensitivity. This is ascribed to the distinct decomposition mechanism that CN bond rupture dominates the initial step of CL-20's decomposition. Contrarily, oscillating field has a stronger heating effect but weaker influence on sensitivity. Moreover, HMX exhibits desensitizing effect in CL-20/HMX cocrystal under electric field. These results enhance our understanding of sensitivity mechanism beyond mechanical stimuli in explosives.

An experimental and theoretical study on the growth of plate-like β-HMX crystals in the hydroxylated interlayer space

Plate-like β-HMX crystals are grown in the hydroxylated interlayer space using a crystallization technique combining the cooling crystallization and solvent-antisolvent methods. The obtained crystals have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The experimental results indicate that the most morphologically important face of the plate-like β-HMX crystals is the (011) face adopting a layer-by-layer growth mode. Meanwhile, molecular dynamics (MD) simulations were performed to study the crystal morphology in HMX crystal growth in the hydroxylated interlayer space based on a modified attachment energy (MAE) model. The calculated results show that the major face is the (011) face and the interaction energies between the crystal face and the hydroxylated interlayer are in the order of (011) > (110) > (020), which agree well with the experimental results above.

Mechanism of the improvement of the energy of host-guest explosives by incorporation of small guest molecules: HNO3 and H2O2 promoted C-N bond cleavage of the ring of ICM-102

Host–guest materials exhibit great potential applications as an insensitive high-energy–density explosive and low characteristic signal solid propellant. To investigate the mechanism of the improvement of the energy of host–guest explosives by guest molecules, ReaxFF-lg reactive molecular dynamics simulations were performed to calculate the thermal decomposition reactions of the host–guest explosives systems ICM-102/HNO₃, ICM-102/H₂O₂, and pure ICM-102 under different constant high temperatures and different heating rates. Incorporation of guest molecules significantly increased the energy level of the host–guest system. However, the initial reaction path of the ICM-102 molecule was not changed by the guest molecules. The guest molecules did not initially participate in the host molecule reaction. After a period of time, the H₂O₂ and HNO₃ guest molecules promoted cleavage of the C–N bond of the ICM-102 ring. Stronger oxidation and higher oxygen content resulted in the guest molecules more obviously accelerating destruction of the ICM-102 ring structure. The guest molecules accelerated the initial endothermic reaction of ICM-102, but they played a more important role in the intermediate exothermic reaction stage: incorporation of guest molecules (HNO₃ and H₂O₂) greatly improved the heat release and exothermic reaction rate. Although the energies of the host–guest systems were clearly improved by incorporation of guest molecules, the guest molecules had little effect on the thermal stabilities of the systems.

Pressure-Induced In Situ Construction of P-CO/HNIW Explosive Composites with Excellent Laser Initiation and Detonation Performance

Laser initiation is a popular research topic in the energetic community. Particularly, the direct ignition of secondary explosives by laser ignitors is considered to be an advanced strategy for enhancing safety and promoting the miniaturization of weapons. Here, to improve the laser sensitivity of secondary explosives, P-CO synthesized under high pressure was employed as a coating for HNIW owing to its laser sensitivity and excellent energetic properties. In this operation, HNIW underwent an obvious isostructural phase transition from the ε-phase to the γ'-phase in the pressure range of 1.0-4.8 GPa. Subsequently, sub-nanoscale HNIW-based composites were formed when CO in situ polymerized to P-CO on the surfaces of HNIW at 5.1 GPa. This HNIW-based composite could be ignited at a much lower laser power (0.49-0.65 W) compared with pure HNIW (2.75-2.98 W) when excited by an Nd:YAG laser with a wavelength of 1064 nm. Additionally, the DFT calculations demonstrated that the arrangement density between HNIW and P-CO was significantly enhanced as the pressure increased. Thus, the introduction of advanced materials into explosive formulations through high-pressure technology is a novel and feasible strategy for developing multipurpose energetic materials.

A quantum-based molecular dynamics study of the ICM-102/HNO3 host-guest reaction at high temperatures

The contradiction between energy and safety of explosives is better balanced by the host-guest inclusion strategy. Understanding the reaction mechanism of the host-guest explosive is necessary. To deeply analyze the role of the small guest molecules in the host-guest system, a quantum-based molecular dynamics method was used to calculate the initial decomposition reaction of the new host-guest explosive ICM-102/HNO3 against the pure ICM-102 at several high temperatures. The incorporation of HNO3 had no significant influence on the initial decomposition step of ICM-102. Conversely, the earliest intramolecular hydrogen transfer reaction is delayed partly because the H and O atoms of HNO3 connect with the O and H atoms of ICM-102, respectively. As the reaction proceeds, guest molecules get heavily involved in the reaction and increase the reaction rate. The generation rate and quantity of the small oxidizing molecules in the final product were increased significantly in the ICM-102/HNO3 system. These mechanisms revealed that HNO3 molecules inhibit the early stages of the initial decomposition of ICM-102 to some extent, and play an important role in accelerating the decomposition subsequently.

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

CL-20-based host-guest complexes are promising energetic materials, which are prepared by embedding small molecules into the crystal lattice cavity of anhydrous CL-20. The structure, interaction, stability, and detonation performance of a series of host-guest complexes were investigated by the combination method of density functional theory and experiment. Both the crystal structure of α-CL-20/H₂O and α-CL-20/N₂O revealed by powder X-ray diffraction and the thermal stability order of α-CL-20/N₂O, α-CL-20/CO₂, α-CL-20/H₂O, and α-CL-20/H₂O₂ measured using a differential scanning calorimeter show excellent accordance between experimental results and simulative predication. Thus, the reliability of the calculation method can be judged by the result of this comparison. The stability of different host-guest structures was compared under vacuum, and the influence of intermolecular interactions on the structural stability was discussed. In view of the various factors affecting the performance of high-energy explosives, such as detonation performance, thermal stability, and density, we conclude that α-CL-20/O₃ could be regarded as a potential target high-energetic compound. On the basis of the above results, this calculation method can provide a theoretical basis for the preparation of CL-20-based host-guest energetic compounds.

Superior thermally robust energetic materials featuring Z–E isomeric bis(3,4-diamino-1,2,4-triazol-5-yl)-1H-pyrazole: self-assembly nitrogen-rich tubes and templates with Hofmeister anion capture architecture

A super thermally robust nitrogen-rich framework was synthesized, and Z → E isomerization as well as supramolecular assembly inclusion strategy gave rise to two different nitrogen-rich tubes and templates with Hofmeister anions capture architecture.

Micronization of CL-20 using supercritical and liquefied gases

Liquid and supercritical CO₂ and TFE have been systematically studied as media for CL-20 recrystallization to estimate their potential for the selective preparation of ultrafine particles.

Insight into the roles of small molecules in CL-20 based host–guest crystals: a comparative DFT-D study

The host–guest inclusion strategy has emerged as a promising method for developing advanced energetic materials and has been successfully applied to a CL-20 crystal.

The competition between cocrystallization and separated crystallization based on crystallization from solution

The competition between cocrystallization and separated crystallization in a solvent was explored via X-ray diffraction and high-performance liquid chromatography methods in different solvents and by considering the solvent evaporation rate. The results revealed that the solvent system and solvent evaporation rate can affect the nucleation order of the cocrystal and coformers in the solution. In fact, solubility tests in different solvents confirmed that the solubility plays a key role in the cocrystal formation process. Furthermore, the width of the metastable zone influenced the solute nucleation order and was a decisive factor in the cocrystal formation process when the solvent evaporation rate was varied. Cocrystals could therefore be obtained by adjusting the solvents and solvent evaporation rate. The preparation of kinetic 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane–2,4,6-trinitrophenol cocrystals via rapid solvent evaporation proves the practicability of this theory.

From BTO2− to HBTO− insensitive energetic salt: a route to boost energy

A promising strategy was utilized to boost the detonation performance of insensitive energetic salts.