Advances in science and technology of modern energetic materials: an overview

Construction and Modification of Nitrogen-Rich Polycyclic Frameworks: A Promising Fused Tricyclic Host-Guest Energetic Material with Heat Resistance, High Energy, and Low Sensitivity

The construction and modification of novel energetic frameworks to achieve an ideal balance between high energy density and good stability are a continuous pursuit for researchers. In this work, a fused [5,6,5]-tricyclic framework was utilized as the energetic host to encapsulate the oxidant molecules for the first time. A series of new pyridazine-based [5,6] and [5,6,5] fused polycyclic nitrogen-rich skeletons and their derivatives were designed and synthesized. Two strategies, amino oxidation and host-guest inclusion, were used to modify the skeleton in only one step. All compounds exhibit good comprehensive properties (Td (onset) > 200 °C, ρ > 1.85 g cm-3, Dv > 8400 m s-1, IS > 20 J, FS > 360 N). Benefiting from the pyridazine-based fused tricyclic structure with more hydrogen bonding units and larger conjugated systems, the first example of [5,6,5]-tricyclic host-guest energetic material triamino-9H-pyrazolo[3,4-d][1,2,4]triazolo[4,3-b]pyridazine-diperchloric acid (10), shows high decomposition temperature (Td (onset) = 336 °C), high density and heats of formation (ρ = 1.94 g cm-3, ΔHf = 733.4 kJ mol-1), high detonation performance (Dv = 8820 m s-1, P = 36.2 GPa), high specific impulse (Isp = 269 s), and low sensitivity (IS = 30 J, FS > 360 N). The comprehensive performance of 10 is superior to that of high-energy explosive RDX and heat-resistant explosives such as HNS and LLM-105. 10 has the potential to become a comprehensive advanced energetic material that simultaneously satisfies the requirements of high-energy and low-sensitivity explosives, heat-resistant explosives, and solid propellants. This work may give new insights into the construction and modification of a nitrogen-rich polycyclic framework and broaden the applications of fused polycyclic framework for the development of host-guest energetic materials.

Thermal decomposition mechanism of HMX/HTPB hybrid explosives studied by reactive molecular dynamics

CONTEXT

The thermal decomposition process of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine/hydroxyl-terminated polybutadiene (HMX/HTPB) hybrid explosives and pure HMX explosives at different temperatures (2000 ~ 3500 K) was investigated using the reactive molecular dynamics method. This study aimed to analyze the effect of binders on the thermal decomposition of HMX at the atomic scale and reveal the thermal decomposition mechanism of HMX/HTPB. The results showed that the thermal decomposition process of the HMX molecule in the HMX/HTPB hybrid system involves a continuous denitration followed by the disintegration of the main ring. The HTPB chain will experience dehydrogenation, dehydroxylation, and chain fragmentation. Including HTPB in the hybrid system significantly increased the presence of H and OH radicals. These radicals then interacted with HMX and its decomposition products and produced more of the final products H2O and H2 in the HMX/HTPB hybrid system compared to pure HMX. Additionally, it was observed that the HTPB chain fragments attached to the HMX decomposition products prevented the formation of N2 and CO2. Furthermore, the activation energies (Ea) of the initial and intermediate decomposition stages of the HMX/HTPB hybrid system were 98.45 kJ mol-1 and 90.69 kJ mol-1, respectively. The results showed that the activation energies of the HMX/HTPB hybrid system are lower than the pure HMX system in these two stages. Consequently, HTPB will enhance HMX's thermal decomposition and decreased the system's insensitivity to heat stimuli.

METHODS

The molecular dynamics simulation of the HMX/HTPB hybrid system was performed using the ReaxFF module in the LAMMPS software, and the ReaxFF/lg force field was used to describe the interatomic interactions as well as the chemical reactions.

Theoretical study of a series of 1,2-diazete based trinitromethyl derivatives as potential energetic compounds

CONTEXT

Explosive properties of novel potential high energy density materials of a series of 1,2-diazete-based molecules with trinitromethyl functional group were investigated computationally. All the sixty seven molecules were optimised to obtain their molecular geometries and electronic structures. Electrostatic potential analysis was also carried out in the determination of different parameters. The calculations indicate that the majority of the compounds have high positive heat of formations, high density and good detonation performance greater than that of traditional energetic materials like RDX and HMX. They are also having comparable values of impact sensitivity. These features promise their potential to be used as energetic materials for future applications. Most of the designed molecules are having high positive oxygen balance values so that the study of these molecules can also be extended as potential candidates for oxidisers in solid propellants.

METHODS

Optimisation and vibrational frequency analysis of the studied molecules were done with density functional theory using B3LYP/aug-cc-pVDZ as the basis set to zero imaginary frequencies using Gaussian 09. Electrostatic potential analysis were carried using the Multiwfn program.

Exploring an accurate machine learning model to quickly estimate stability of diverse energetic materials

High energy and low sensitivity have been the focus of developing new energetic materials (EMs). However, there has been a lack of a quick and accurate method for evaluating the stability of diverse EMs. Here, we develop a machine learning prediction model with high accuracy for bond dissociation energy (BDE) of EMs. A reliable and representative BDE dataset of EMs is constructed by collecting 778 experimental energetic compounds and quantum mechanics calculation. To sufficiently characterize the BDE of EMs, a hybrid feature representation is proposed by coupling the local target bond into the global structure characteristics. To alleviate the limitation of the low dataset, pairwise difference regression is utilized as a data augmentation with the advantage of reducing systematic errors and improving diversity. Benefiting from these improvements, the XGBoost model achieves the best prediction accuracy with R2 of 0.98 and MAE of 8.8 kJ mol-1, significantly outperforming competitive models.

Initial decomposition pathways of 1,1-diamino-2,2-dinitroethylene (α-FOX-7) in the condensed phase

The initial decomposition pathways of α-FOX-7 in the condensed phase (crystal) were investigated via density functional theory. Calculations were carried out using three FOX-7 systems with increasing complexity from 1-layer (sheet) via 2-layer (surface) to 3-layer (bulk). The encapsulated environment of the central α-FOX-7 molecule, where decomposition takes place, is reconstructed by neighbouring molecules following a crystal structure. A minimal number of neighbouring molecules that have an impact on the energetics of decomposition are identified among all surrounding molecules. The results show that the presence of intermolecular hydrogen bonds due to the encapsulated environment in the condensed phase decreases the sensitivity of α-FOX-7, i.e. it increases the barrier of decomposition, but it does not alter the initial decomposition pathways of the reaction compared to the gas phase. Moreover, increasing the complexity of the system from a single gas phase molecule via sheet and surface to bulk increases the decomposition barriers. The calculations reveal a remarkable agreement with experimental data [A. M. Turner, Y. Luo, J. H. Marks, R. Sun, J. T. Lechner, T. M. Klapötke and R. I. Kaiser, Exploring the Photochemistry of Solid 1, 1-Diamino-2, 2-Dinitroethylene (FOX-7) Spanning Simple Bon Ruptures, Nitro-to-Nitrite Isomerization, and Nonadiabatic Dynamics, J. Phys. Chem. A, 2022, 126, 29, 4747-4761] and suggest that the initial decomposition of α-FOX-7 likely takes place at the surface of the crystal.

Determining the mechanical and decomposition properties of high energetic materials (α-RDX, β-HMX, and ε-CL-20) using a neural network potential

Molecular simulations of high energetic materials (HEMs) are limited by efficiency and accuracy. Recently, neural network potential (NNP) models have achieved molecular simulations of millions of atoms while maintaining the accuracy of density functional theory (DFT) levels. Herein, an NNP model covering typical HEMs containing C, H, N, and O elements is developed. The mechanical and decomposition properties of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), hexahydro-1,3,5-trinitro-1,3,5-triazine (HMX), and 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20) are determined by employing the molecular dynamics (MD) simulations based on the NNP model. The calculated results show that the mechanical properties of α-RDX, β-HMX, and ε-CL-20 agree with previous experiments and theoretical results, including cell parameters, equations of state, and elastic constants. In the thermal decomposition simulations, it is also found that the initial decomposition reactions of the three crystals are N-NO2 homolysis, corresponding radical intermediates formation, and NO2-induced reactions. This decomposition trajectory is mainly divided into two stages separating from the peak of NO2: pyrolysis and oxidation. Overall, the NNP model for C/H/N/O elements in this work is an alternative reactive force field for RDX, HMX, and CL-20 HEMs, and it opens up new potential for future kinetic study of nitramine explosives.

Infrared spectra and electronic structural changes of DNTF under high pressure: experimental and theoretical studies

3,4-Bis(3-nitrofurazan-4-yl)furoxan (DNTF) is a novel energetic material with an excellent performance and has attracted considerable attention. Motivated by recent theories and experiments, we had carried out experimental and theoretical studies on the high-pressure responses of vibrational characteristics, in conjunction with structural and electronic characteristics. It is found that all observed infrared spectra peaks seem to shift towards higher frequencies. And the peaks attributed to N-Oc (coordinated oxygen atom) stretching vibrations become broader due to the decrease of lattice constants and the free region of DNTF crystals with the increase of pressure, where the a-direction is more sensitive to pressure. In addition, the non-covalent interaction between adjacent DNTF molecules in the same layer changes from the van der Waals interaction to the steric effect with the increase of pressure, and that between layers also changes from the van der Waals interaction to the π-π stacking interaction. More importantly, these results highlight that the increase of pressure may lead to the stability decrease and impact the sensitivity increase of DNTF. This study can deepen the understanding of the energetic material DNTF under high pressure and is of great significance for blasting and detonation applications of DNTF.

In Situ Electrochemical Construction of CuN3@CuCl Hybrids for Controllable Energy Release and Self-Passivation Ability

Advanced energetic materials (EMs) play a crucial role in the advancement of microenergetic systems as actuation parts, igniters, propulsion units, and power. The sustainable electrosynthesis of EMs has gained momentum and achieved substantial improvements in the past decade. This study presents the facile synthesis of a new type of high-performance CuN3@CuCl hybrids via a co-electrodeposition methodology utilizing porous Cu as the sacrificial template. The composition, morphology, and energetic characteristics of the CuN3@CuCl hybrids can be easily tuned by adjusting the deposition times. The resulting hybrids demonstrate remarkable energy output (1120 J·g-1) and good laser-induced initiating ability. As compared with porous CuN3, the uniform doping of inert CuCl enhances the electrostatic safety of the hybridized material without compromising its overall energetic characteristics. Notably, the special oxidizing behavior of CuCl gradually lowers the susceptibility of the hybrid material to laser and electrostatic stimulation. This has significant implications for the passivation or self-destruction of highly sensitive EMs. Overall, this study pioneers a new path for the development of MEMS-compatible EMs, facilitating further microenergetic applications.

Spectroscopic and imaging considerations of THz-TDS and ULF-Raman techniques towards practical security applications

Nitrogen-containing high-energy organic compounds represent a class of materials with critical implications in various fields, including military, aerospace, and chemical industries. The precise characterization and analysis of these compounds are essential for both safety and performance considerations. Spectroscopic characterization in the far-infrared region has great potential for non-destructive investigation of high energetic and related compounds. This research article presents a comprehensive study of common organic energetic materials in the far-infrared region (5-200 cm-1), aiming to enhance security measures through the utilization of cutting-edge spectroscopic techniques. Broadband terahertz time-domain spectroscopy and ultra-low frequency Raman spectroscopy are employed as powerful tools to probe the vibrational and rotational modes of various explosive materials. One of the key objectives of this present work is unveiling the characteristic spectral features and optical parameters of five common nitrogen based high energy organic compounds towards rapid and accurate identification. Further, we have explored the potential of terahertz reflection imaging for non-contact through barrier sensing, a critical requirement in security applications. Based on the spectral features obtained from the spectroscopic studies and using advanced imaging algorithms we have been able to detect these compounds under various barriers including paper, cloth, backpack, etc. Subsequently, this study highlights the capabilities of the two techniques offering a pathway to enhance their utility over a wide range of practical security applications.

Azo-Bridged Triazole Macrocycles: Computational Design, Energy Content, Performance, and Stability Assessment

Oxadiazole and triazole are extensively investigated heterocyclic scaffolds in the development of energetic materials. New energetic molecules were designed by replacing 1,2,5-oxadiazole with 2H-1,2,3-triazole in the reported conjugated macrocyclic systems to assess the influence on the energetic properties and stability. In addition, nitro groups were introduced in triazole units (N-functionalization) to improve the energetic performance. Energetic properties, including heat of formation, oxygen balance, density, detonation pressure and velocity, and impact sensitivity, were estimated for these triazole-based macrocycles. The replacement of 1,2,5-oxadiazole with 2H-1,2,3-triazole and 2-nitro-1,2,3-triazole significantly enhances the energy content, detonation performance, and noncovalent interactions. The theoretically computed energetic properties of triazole-based macrocycles reveal high positive heats of formation (1507-2761 kJ/mol), oxygen balance (-88.8 to -22.8%), high densities (1.87-1.90 g/cm3), superior detonation velocities (8.41-9.52 km/s), pressures (26.64-40.55 GPa), acceptable impact sensitivity (27-40 cm), and safety factor (51-290). The overall energetic assessment highlights triazole-based macrocycles as a potential framework that will be useful for developing advanced energetic materials.

Quantum dots: a tool for the detection of explosives/nitro derivatives

Nitro derivatives are considered as major environmental pollutants and issues of health concern. In current times, a variety of methods and techniques have been utilized for the sensing of these nitro derivatives. In view of this, the remarkable fluorescence properties of quantum dots (QDs) provide a great opportunity to detect these nitro derivatives. This review highlighted the recent reports of QDs as the sensing material for these nitro derivative explosives. Different modifications in QDs using physical and chemical approaches can be used to improve their sensing output. Various interaction mechanisms have been discussed between QDs and nitro derivatives to change their fluorescence properties. Finally, the current challenges and the perspective for the forthcoming future are provided in the concluding section. We hope this review will be beneficial in guiding the utilization of QDs in sensing applications.

Design of insensitive high explosives based on FOX-7: a theoretical prospectives

CONTEXT

This article presents a theoretical study of three insensitive high explosives based on the FOX-7 moiety. A few heterocyclic five- and six-member nitrogen-rich compounds have been created in an effort to better serve as a potential insensitive high explosive. It has been addressed how these molecules should be optimised in terms of stability, sensitivity, detonation properties, IR frequency computations, formal charge calculations, and more. Comprehensive research has been done on these compounds' molecular density and energy of activation associated with the conversion from nitro (C-NO2) to nitrito (C-ONO) during the initial phase of their decomposition. The bond dissociation energy along with BSSE correction for the most reactive C-NO2 bond is examined. The two designed molecules have intra-molecular hydrogen-bonding while other does not have any intra-molecular hydrogen-bonding. The newly designed compounds exhibit higher detonation values compared to TNT, which suggests that they ought to be prepared in a laboratory by skilled experimenters.

METHODS

The stability of the C-NO2 link and the covalent character of the bonds have both been calculated using the atoms in molecule (AIM) method. The electronic structure calculations have been recovered at DFT method with aug-cc-pVDZ basis set using the Gaussian-16 quantum chemistry programme.

Explosives removal and quantification using porous adsorbents based on poly(2-oxazoline)s with various degree of functionalization

Polymeric porous adsorbents are reported for removal of explosives, namely picric acid, 1,3,5-trinitro-1,3,5-triazinane (RDX), and pentaerythritol tetranitrate (PETN) and their subsequent quantification using direct analysis with ambient plasma mass spectrometry. The adsorbents are obtained by functionalization of short-chain poly(2-oxazoline)s with methyl ester side chains using 4-(aminomethyl)pyridine with a degree of functionalization equal to 0, 5, 10, and 20%. The subsequent step consist of cross-linking using a high internal phase emulsion procedure by further side-chain amidation with diethylenetriamine as crosslinker. Picric acid, RDX, and PETN were chosen as the model compounds as they belong to three different groups of explosives, in particular nitroaromatics, nitroamines, and nitrate esters, respectively. The adsorption isotherms, kinetics, as well as the influence of pH and temperature on the adsorption process was investigated. The porous adsorbents showed the highest maximum adsorption capacity towards picric acid, reaching 334 mg g-1, while PETN (80 mg g-1) and RDX (17.4 mg g-1) were less efficiently adsorbed. Subsequent quantification of the adsorbed explosives is performed by a specially designed ambient mass spectrometry setup equipped with a thermal heater. The obtained limits of detection were found to be 20-times improved compared to direct analysis of analyte solutions. The effectiveness of the proposed analytical setup is confirmed by successful quantification of the explosives in river water samples. The research clearly shows that functional porous adsorbents coupled directly with ambient mass spectrometry can be used for rapid quantification of explosives, which can be, e.g., used for tracking illegal manufacturing sites of these compounds.

Iron-Mediated Photochemical Anti-Markovnikov Hydroazidation of Unactivated Olefins

Unactivated olefins are converted to alkyl azides with bench-stable NaN3 in the presence of FeCl3·6H2O under blue-light irradiation. The products are obtained with anti-Markovnikov selectivity, and the reaction can be performed under mild ambient conditions in the presence of air and moisture. The transformation displays broad functional group tolerance, which renders it suitable for functionalization of complex molecules. Mechanistic investigations are conducted to provide insight into the hydroazidation reaction and reveal the role of water from the iron hydrate as the H atom source.

Ultraviolet-Initiated Decomposition of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7)

FOX-7 (1,1-diamino-2,2-dinitroethylene) was photolyzed with 202 nm photons to probe reaction energies, leading to the decomposition of this energetic material and to compare results from irradiations using lower-energy 532 and 355 nm photons as well as higher-energy electrons. The photolysis occurred at 5 K to suppress thermal reactions, and the solid samples were monitored using Fourier transform infrared spectroscopy (FTIR), which observed carbon dioxide (CO2), carbon monoxide (CO), cyanide (CN-), and cyanate (OCN-) after irradiation. During warming to 300 K, subliming products were detected using electron-impact quadrupole mass spectrometry (EI-QMS) and photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). Five products were observed in QMS: water (H2O), carbon monoxide (CO), nitric oxide (NO), carbon dioxide (CO2), and cyanogen (NCCN). The ReTOF-MS results showed overlap with electron irradiation products but also included three intermediates for the oxidation of ammonia and nitric oxide: hydroxylamine (NH2OH), nitrosamine (NH2NO), and the largest product at 76 amu with the proposed assignment of hydroxyurea (NH2C(O)NHOH). These results highlight the role of reactive oxygen intermediates and nitro-to-nitrite isomerization as key early reactions that lead to a diverse array of decomposition products.

Ab Initio Crystal Structure Prediction of the Energetic Materials LLM-105, RDX, and HMX

Crystal structure prediction (CSP) is performed for the energetic materials (EMs) LLM-105 and α-RDX, as well as the α and β conformational polymorphs of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), using the genetic algorithm (GA) code, GAtor, and its associated random structure generator, Genarris. Genarris and GAtor successfully generate the experimental structures of all targets. GAtor's symmetric crossover scheme, where the space group symmetries of parent structures are treated as genes inherited by offspring, is found to be particularly effective. However, conducting several GA runs with different settings is still important for achieving diverse samplings of the potential energy surface. For LLM-105 and α-RDX, the experimental structure is ranked as the most stable, with all of the dispersion-inclusive density functional theory (DFT) methods used here. For HMX, the α form was persistently ranked as more stable than the β form, in contrast to experimental observations, even when correcting for vibrational contributions and thermal expansion. This may be attributed to insufficient accuracy of dispersion-inclusive DFT methods or to kinetic effects not considered here. In general, the ranking of some putative structures is found to be sensitive to the choice of the DFT functional and the dispersion method. For LLM-105, GAtor generates a putative structure with a layered packing motif, which is desirable thanks to its correlation with low sensitivity. Our results demonstrate that CSP is a useful tool for studying the ubiquitous polymorphism of EMs and shows promise of becoming an integral part of the EM development pipeline.

Fluorine-Containing Functional Group-Based Energetic Materials

Molecules featuring fluorine-containing functional groups exhibit outstanding properties with high density, low sensitivity, excellent thermal stability, and good energetic performance due to the strong electron-withdrawing ability and high density of fluorine. Hence, they play a pivotal role in the field of energetic materials. In light of current theoretical and experimental reports, this review systematically focuses on three types of energetic materials possessing fluorine-containing functional groups F- and NF2 - substituted trinitromethyl groups (C(NO2 )2 F, C(NO2 )2 NF2 ), trifluoromethyl group (CF3 ), and difluoroamino and pentafluorosulfone groups (NF2 , SF5 ) and investigates the synthetic methods, physicochemical parameters, and energetic properties of each. The incorporation of fluorine-containing functional moieties is critical for the development of novel high energy density materials, and is rapidly being adopted in the design of energetic materials.

Design and Synthesis of Energetic Melt-Castable Materials by Substituent-Specific Modification

With the increase in the demand for high-performance composite explosives, the search for advanced energetic melt-castable compounds has attracted increasing attention in the field of energetic materials. Herein, two new energetic materials with nitromethyl and azidomethyl substituents (1-(nitromethyl)-3,4-dinitro-1H-pyrazole (NMDNP) and 1-(azidomethyl)-3,4-dinitro-1H-pyrazole (AMDNP) were prepared by the substituent modification of a potential melt-castable molecule ((3,4-dinitro-1H-pyrazol-1-yl) methyl nitrate, MC-4), respectively. NMDNP exhibited a suitable melting point (90 °C), good thermal stability (Td : 185 °C) and excellent detonation performance (8484 m s-1 ) and impact sensitivity (25 J), thereby demonstrating promise as an energetic melt-castable material. Simultaneously, compared with the nitrato-methyl and azidomethyl substituents, the nitromethyl substituent exhibited greater advantages in regulating performance.

Multi-aspect simulation insight on thermolysis mechanism and interaction of NTO/HMX-based plastic-bonded explosives: a new conception of the mixed explosive model

Reactive molecular dynamics (RMDs) calculations were used to determine, for the first time, the process of thermolysis of the mixed explosives, including 3-nitro-1,2,4-triazol-5-one (NTO) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazoline (HMX). Significantly, this is the first time that a layered model for mixed explosives, which is an extreme innovation of mixed explosive models was adopted. It is shown that a large amount of NO2 in the HMX and OH groups generated by the decomposition of HNO2 has a favorable effect on the thermolysis of NTO, as further validated by a reduction in the activation energy of NTO/HMX. The amount of H2O and N2 in the resulting products increased significantly, but the amount of NH3 changed slightly. The analysis results correspond to the change in chemical bonds. Whenever there is an increase in temperature, the time for the maximum number of clusters to appear is shortened accordingly. In addition, the acidity of NTO has been considered. An independent gradient model based on Hirshfeld partition (IGMH) and atoms in molecule (AIM) analysis of NTO/HMX was implemented. The relatively strong hydrogen bonds indicate that HMX can inhibit the acidity of NTO and is beneficial for the wide application of NTO/HMX-based plastic-bonded explosives (PBXs).

Structural, vibrational and electronic properties of nitrogen-rich 2,4,6-triazide-1,3,5-triazine under high pressure

CONTEXT AND RESULTS

2,4,6-triazide-1,3,5-triazine (TAT) has received widespread attention for its great potential to synthesize or convert to nitrogen-rich high energy density materials (HEDMs). The TAT structure alteration in the compression process up to 30 GPa has characteristics as follows: (a) [N₃] groups straighten; (b) [N₃] groups gather toward the six-membered C-N heterocycles. At about 5 GPa, Raman peak split at 700 cm^-1 was observed both in calculation and in-situ Raman experiment, which is caused by pressure-induced intramolecular stress. Besides, the broad band of the amorphous two-dimensional C=N network (centered at 1630 cm^-1) occurred at about 12 GPa. Meantime, the study on electronic features suggests the pressure-induced deformation in TAT molecular structure cause the discontinuous change of band gap at about 4.5 GPa and 8.0 GPa, respectively.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The static compression process of TAT was explored in the range of 0-30 GPa by using dispersion corrected density functional theory (DFT-D) calculations combined with in-situ Raman experiment. The GGA/PBE+G06 method that has less errors than other calculation methods was used to predict the geometry structure, vibrational properties and electronic structure of TAT under pressure.

Initial Decomposition of DATB Induced by an External Electric Field

1,3-Diamino-2,4,6-trinitrobenzene (DATB), a nitro aromatic explosive with excellent properties, can be detonated by an electric field. Using first-principles calculation, we have investigated the initial decomposition of DATB under an electric field. In the realm of electric fields, the rotation of the nitro group around the benzene ring will cause deformation of the DATB structure. Furthermore, when an electric field is applied along the [100] or [001] direction, the C4-N10/C2-N8 bonds initiate decomposition due to electron excitation. On the contrary, the electric field along the [010] direction has a weak influence on DATB. These, together with electronic structures and infrared spectroscopy, give us a visual perspective of the energy transfer and the decomposition caused by C-N bond breaking.

Constructing Highly Reliable and Adaptive Primary Explosive Composites for Micro-Initiator Assisting by a Hybrid Template of Metal-Organic Frameworks and Cross-Linked Polymers

Primary explosive, as a reliable initiator for secondary explosives, is the central component of micro-initiators for modern aerospace systems and military operations. However, they are typically prepared as powders, posing potential safety risks because of the inevitable particles scattering issues in the actual working environments. Here, the fabrication of a highly adaptive bulk material of copper azide (CA)-based safe primary explosive for micro-initiators is demonstrated. This bulk material, as derived by a complete azidation reaction of the carbonized metal-organic framework/cross-linked polymer hybrid template, enables the firm embedding of active CA species in a cross-linked carbon network (denoted as CA-C). Interestingly, this CA-C bulk material demonstrates multifarious mechanical stabilities (e.g., good shock and vibration resistance, and anti-overload capacity) in the simulated working conditions. Meanwhile, the CA contents in the CA-C bulk material reached as high as 70.3%, ensuring its detonation power. As a proof of concept, CA-C bulk material assembling in a micro-detonator can efficiently detonate the secondary explosive of CL-20 under laser irradiation. This work hereby advances the fabrication of safe and powerful primary explosives for the fulfillment of safe micro-initiator in a broad range of applications in aerospace systems.

Pressure-dependent structure and electronic properties of energetic NTO crystals dominated by hydrogen-bonding interactions

3-Nitro-1,2,4-trihydroxy-5-one (NTO), a highly potential high-performance explosive with good thermal stability and low sensitivity, has attracted much attention for its physicochemical properties in recent years. In this work, the pressure effect of the vibrational and electronic properties is investigated to understand the intermolecular interaction of NTO under hydrostatic compression. From the pressure-dependent Raman and infrared spectra, we found that the red-shifts of high-wavenumber N-H stretching modes and the discontinuous shifts of all Raman modes occur at 3 and 6 GPa, indicating an evident change of molecular configuration and intermolecular interaction upon compression. Based on structural analysis, the changes of intra- and intermolecular hydrogen bonds (HBs) are closely relevant to the anomalous rotation of the nitro group and the lengthening of N-H bonds, which can be treated as an important step of a potential structural transformation of NTO. Moreover, intermolecular hydrogen-bonding interaction leads to the shrinkage of the band gap at 6 GPa, caused by the fast charge transfer of 0.07 e from the nitrogen heterocycle to the nitro group. These results manifest a non-covalent interaction mechanism for modulating the molecular configuration of EMs under pressure loading and provide vital insights into understanding the pressure effects for energetic molecular crystals.

Electron-Induced Decomposition of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7) at Cryogenic Temperatures

Solid FOX-7 (1,1-diamino-2,2-dinitroethylene), an energetic material of interest due to its high stability and low shock/thermal sensitivity, was exposed to energetic electrons at 5 K to explore the fundamental mechanisms leading to decomposition products and provide a better understanding of the reaction pathways involved. As a result of the radiation exposure, infrared spectroscopy revealed carbon dioxide (CO₂) and carbon monoxide (CO) trapped in the FOX-7 matrix, while these compounds along with water (H₂O), nitrogen monoxide (NO), and cyanogen (C₂N₂) were detected exploiting quadrupole mass spectrometry both during irradiation and during the warming phase from 5 to 300 K. Photoionization reflectron time-of-flight mass spectrometry detected small molecules such as ammonia (NH₃), nitrogen monoxide (NO), and nitrogen dioxide (NO₂) as well as more complex molecules up to 96 amu. Potential reaction pathways are presented and assignments are discussed. Among the reaction mechanisms, the importance of an initial nitro-to-nitrite isomerization is highlighted by the observed decomposition products.

Preparation of Mesoporous Si Nanoparticles by Magnesiothermic Reduction for the Enhanced Reactivity

In this study, mesoporous silicon nanoparticles (M-Si) were successfully prepared by a magnesiothermic reduction of mesoporous silica nanoparticles, which were synthesized by a templated sol-gel method and used as the precursors. M-Si exhibited a uniform size distribution with an average diameter of about 160 nm. The measured BET surface area was 93.0 m² g^-1, and the average pore size calculated by the BJH method was 16 nm. The large internal surface area provides rich reaction sites, resulting in unique interfacial properties and reduced mass diffusion limitations. The mechanism of the magnesiothermic reduction process was discussed. The reactivity of prepared M-Si was compared with that of commercially available non-porous Si nanopowder (with the average diameter of about 30 nm) by performing simultaneous thermogravimetry and differential scanning calorimetry in the air. The results showed that the reaction onset temperature indicated by weight gain was advanced from 772 °C to 468 °C, indicating the promising potential of M-Si as fuel for metastable intermolecular composites.

Which molecular properties determine the impact sensitivity of an explosive? A machine learning quantitative investigation of nitroaromatic explosives

We decomposed density functional theory charge densities of 53 nitroaromatic molecules into atom-centered electric multipoles using the distributed multipole analysis that provides a detailed picture of the molecular electronic structure. Three electric multipoles, (the charge of the nitro groups), (the total dipole, i.e., polarization, of the nitro groups), (the total electron delocalization of the C ring atoms), and the number of explosophore groups (#NO₂) were selected as features for a comprehensive machine learning (ML) investigation. The target property was the impact sensitivity h₅₀ (cm) values quantified by drop-weight measurements, with a large h₅₀ (e.g., 150 cm) indicating that an explosive is insensitive and vice versa. After a preliminary screening of 42 ML algorithms, four were selected based on the lowest root mean square errors: Extra Trees, Random Forests, Gradient Boosting, and AdaBoost. Compared to experimental data, the predicted h₅₀ values of molecules having very different sensitivities for the four algorithms have differences in the range 19-28%. The most important properties for predicting h₅₀ are the electron delocalization in the ring atoms and the polarization of the nitro groups with averaged weights of 39% and 35%, followed by the charge (16%) and number (10%) of nitro groups. A significant result is how the contribution of these properties to h₅₀ depends on their actual sensitivities: for the most sensitive explosives (h₅₀ up to ∼50 cm), the four properties contribute to reducing h₅₀, and for intermediate ones (∼50 cm ≲ h₅₀ ≲ 100 cm) #NO₂ and contribute to increasing it and the other two properties to reducing it. For highly insensitive explosives (h₅₀ ≳ 200 cm), all four properties essentially contribute to increasing it. These results furnish a consistent molecular basis of the sensitivities of known explosives that also can be used for developing safer new ones.

Insensitive High-Energy Density Materials Based on Azazole-Rich Rings: 1,2,4-Triazole N-Oxide Derivatives Containing Isomerized Nitro and Amino Groups

It is an arduous and meaningful challenge to design and develop new energetic materials with lower sensitivity and higher energy. How to skillfully combine the characteristics of low sensitivity and high energy is the key problem in designing new insensitive high-energy materials. Taking a triazole ring as a framework, a strategy of N-oxide derivatives containing isomerized nitro and amino groups was proposed to answer this question. Based on this strategy, some 1,2,4-triazole N-oxide derivatives (NATNOs) were designed and explored. The electronic structure calculation showed that the stable existence of these triazole derivatives was due to the intramolecular hydrogen bond and other interactions. The impact sensitivity and the dissociation enthalpy of trigger bonds directly indicated that some compounds could exist stably. The crystal densities of all NATNOs were larger than 1.80 g/cm³, which met the requirement of high-energetic materials for crystal density. Some NATNOs (9748 m/s for NATNO, 9841 m/s for NATNO-1, 9818 m/s for NATNO-2, 9906 m/s for NATNO-3, and 9592 m/s for NATNO-4) were potential high detonation velocity energy materials. These study results not only indicate that the NATNOs have relatively stable properties and excellent detonation properties but also prove that the strategy of nitro amino position isomerization coupled with N-oxide is an effective means to develop new energetic materials.

Molecular conformation evolutions of trans HNS under high pressure

The molecular conformation evolution of Hexanitrostilbene (HNS) under high pressure was systematically investigated using Raman and Fourier transform infrared (FTIR) spectroscopy. The vibration modes of HNS associated with C-H, nitro groups, CC and the ring have been analyzed and clarified in detail under ambient conditions. trans-HNS is symmetrically distributed about -CHCH-, and six nitro groups are symmetrically distributed under ambient conditions. Two molecular conformation changes of HNS were observed at 1.4 GPa and 5 GPa due to the variations of hydrogen-bond interaction between C-H (in the ring) and N-O and the distortion of trans olefin, respectively. The hydrogen-bond interaction between C-H (in the ring) and N-O strengthened at 1.4 GPa. It induced the degenerated symmetry of the nitro groups and the Raman changes of ν_as (NO₂), ν(CC), ν(C-C) and ν(C-H). In addition, the nonplanarity property of HNS and the sensitivity of trans olefin to pressure promoted the deformation of trans olefin, as well as the hydrogen bond interaction between C-H (in trans olefin) and N-O at about 5 GPa. When further loading pressure on HNS, the variations in the hydrogen-bond interaction between C-H and N-O restricted the vibrations of C-H, NO₂ and the ring. It blocked the nonradiative pathway and activated the strong fluorescent background in the Raman spectra as the pressure increased above 5.7 GPa. These current results reveal that there is no structural transformation and only conformational changes under high pressure for HNS.

Salidroside protect Chinese hamster V79 cells from genotoxicity and oxidative stress induced by CL-20

Hexanitrohexaazaisowurtzitane (CL-20) is a high-energy elemental explosive widely used in chemical and military fields. CL-20 harms environmental fate, biosafety, and occupational health. However, there is little known about the genotoxicity of CL-20, in particular its molecular mechanisms. Therefore, this study was framed to investigate the genotoxic mechanisms of CL-20 in V79 cells and evaluate whether the genotoxicity could be diminished by pretreating the cells with salidroside. The results showed that CL-20-induced genotoxicity in V79 cells primarily through oxidative damage to DNA and mitochondrial DNA (mtDNA) mutation. Salidroside could significantly reduce the inhibitory effect of CL-20 on the growth of V79 cells and reduce the levels of reactive oxygen species (ROS), 8-hydroxy-2 deoxyguanosine (8-OHdG), and malondialdehyde (MDA). Salidroside also restored CL-20-induced superoxide dismutase (SOD) and glutathione (GSH) in V79 cells. As a result, salidroside attenuated the DNA damage and mutations induced by CL-20. In conclusion, oxidative stress may be involved in CL-20-induced genotoxicity in V79 cells. Salidroside could protect V79 cells from oxidative damage induced by CL-20, mechanism of which may be related to scavenging intracellular ROS and increasing the expression of proteins that can promote the activity of intracellular antioxidant enzymes. The present study for the mechanisms and protection of CL-20-mediated genotoxicity will help further to understand the toxic effects of CL-20 and provide information on the therapeutic effect of salidroside in CL-20-induced genotoxicity.

Effects of pressure on structural, electronic, optical, and mechanical properties of nitrogen-rich energetic material: 6-azido-8-nitrotetrazolo[1,5-b]pyridazine-7-amine (3at)

CONTEXT AND RESULTS

6-Azido-8-nitrotetrazolo[1,5-b]pyridazine-7-amine (3at) is a promising green energetic material, which meets the development requirements of environment-friendly explosives. By discussing the relationship between lattice parameters and pressure, it is found that the compression ratio indicates anisotropy of compressibility. And bond lengths get shorter under pressure, resulting in stronger intermolecular bonds. The N₃ group rotates under pressure. And then, the optical properties basically change regularly with the change of pressure. As the pressure increases, the absorption range widens. In the low energy interval, it shows transparency, and then with the increase of energy and pressure, it shows better optical activity. With the increase of pressure and energy, the absorption coefficient increases, representing that the optical activity becomes high. Finally, according to the analysis of mechanical properties, 3at exhibited brittle behavior at 0 GPa and 100 GPa, while at 10 to 90 GPa, the values of ν and B/G are malleable.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

Based on density functional theory, the crystal parameters, electronic properties, optical properties, and elastic and mechanical properties of 3at under different pressures were studied theoretically. The GGA-PW91+OBS method was used to calculate the physical parameters under pressure, such as lattice parameters, energy band structures, dielectric function, refractive index, absorption coefficient, and elastic constants. Physical properties under (3at) pressure are predicted.

High pressure Raman study of LiClO4

Lithium perchlorate (LiClO₄), as one of the new high-energy oxidizers, is chosen for high pressure Raman research to gain a better understanding of the structure and stability, which is very important for the performance of an explosive. Raman spectra of LiClO₄ crystal have been measured from ambient to 25.07 GPa with diamond anvil cells (DACs) at room temperature to investigate the structural stability of this system. Raman vibrational modes of LiClO₄ crystal at ambient pressure were resolved comprehensively on the basis of our experimental and calculated results. Upon increase of pressure on LiClO₄ crystal sample to 1.96 GPa, it was found that the LiClO₄ crystal exhibited a pressure-induced first-order phase transformation behavior. The occurrence of a second phase transformation of LiClO₄ crystal induced by pressure was observed at about 5.09 GPa. Both phase transformations were demonstrated based on the detailed spectroscopic analysis of the variations in the number of lattice modes, splitting of Raman bands and frequency jumps of the Raman vibrational modes of LiClO₄ crystal.

Coupling Effect of Non-Ignition Impact and Heat on the Decay of FOX-7

Non-ignition impact and heat stimuli are the most common external stimuli loaded on energetic materials. Nevertheless, there is thereby an urgent need, but it is still a significant challenge to comprehend their coupling effects on the decay and safety mechanisms of energetic materials. Then, reactive molecular dynamics simulation was employed to mimic practical situations and reveal the impact heat coupling effect on the decay mechanism of FOX-7. The temperature and the degree of compression of the crystal caused by the impact are considered variables in the simulation. Both increasing the degree of compression and elevating the temperature promotes the decay of FOX-7. However, their underlying response mechanism is not the same. The acceleration of decomposition is due to the elevated potential energy of the FOX-7 molecules because of elevating the temperature. In addition to the elevated potential energy of the molecule, the main contribution to the decomposition from the compression is to change the decomposition path. The results of the analysis show that compression reduces the stability of the C=C bond, so that chemical reactions related to the double bond occur. In addition, interestingly, the compression along the c direction has an almost equal effect on the final product as the compression along the b direction. Finally, the decay reaction networks are proposed to provide insights into the decomposition mechanism on atomic level. All these findings are expected to pave a way to understand the underlying response mechanism for the FOX-7 against external stimuli.

Condensation of 4-Tert-butyl-2,6-dimethylbenzenesulfonamide with Glyoxal and Reaction Features: A New Process for Symmetric and Asymmetric Aromatic Sulfones

The synthesis of substituted aza- and oxaazaisowurtzitanes via direct condensation is challenging. The selection of starting ammonia derivatives is very limited. The important step in developing alternative synthetic routes to these compounds is a detailed study on their formation process. Here, we explored an acid-catalyzed condensation between 4-tert-butyl-2,6-dimethylbenzenesulfonamide and glyoxal in aqueous H₂SO₄, aqueous acetonitrile and acetone, and established some new processes hindering the condensation. In particular, an irreversible rearrangement of the condensation intermediate was found to proceed and be accompanied by the 1,2-hydride shift and by the formation of symmetric disulfanes and sulfanes. It has been shown for the first time that aldehydes may act as a reducing agent when disulfanes are generated from aromatic sulfonamides, as is experimentally proved. The condensation between 4-tert-butyl-2,6-dimethylbenzenesulfonamide and formaldehyde resulted in 1,3,5-tris((4-(tert-butyl)-2,6-dimethylphenyl)sulfonyl)-1,3,5-triazinane. It was examined if diimine could be synthesized from 4-tert-butyl-2,6-dimethylbenzenesulfonamide and glyoxal by the most common synthetic procedures for structurally similar imines. It has been discovered for the first time that the Friedel-Crafts reaction takes place between sulfonamide and the aromatic compound. A new synthetic strategy has been suggested herein that can reduce the stages in the synthesis of in-demand organic compounds of symmetric and asymmetric aromatic sulfones via the Brønsted acid-catalyzed Friedel-Crafts reaction, starting from aromatic sulfonamides and arenes activated towards an electrophilic attack.

Theoretical investigation of vibrational and electronic properties of HMX crystal under uniaxial compression

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is a typical energetic molecular crystal with excellent detonation performance and good thermal stability, has been widely used in military and civilian purposes. In this work, the vibrational properties of HMX combined with structure and electronic properties are studied to understand its pressure response against uniaxial compressions. The calculated eigenvalues of stress tensors show significant anisotropy of intermolecular interactions. Especially, the direction of shear stressτ_xyandτ_xzin [100] compression have an abrupt change nearV/V₀= 0.84. Further, Raman spectra under each uniaxial compression are simulated to inspect the molecular configuration of HMX. Compared to the blue shifts of [010] and [001] orientations, the discontinuous Raman shifts of atV/V₀= 0.86-0.84 in [100] orientation suggest that HMX would undergoes a possible structural transformation at the pressure of 6.82-9.15 GPa. Structural analysis implies that the subtle rotation of NO₂group is changed by intermolecular interactions of HMX. Moreover, the abnormal evolution of band gap is observed atV/V₀= 0.84 in [100] orientation, which is associated with the structure modification of HMX. Overall, the compression behaviors of HMX under uniaxial compressions would provide a useful insight for the actual shock compression conditions.

Tri-explosophoric groups driven fused energetic heterocycles featuring superior energetic and safety performances outperforms HMX

The design and synthesis of novel energetic compounds with integrated properties of high density, high energy, good thermal stability and sensitivities is particularly challenging due to the inherent contradiction between energy and safety for energetic compounds. In this study, a novel structure of 4-amino-7,8-dinitropyrazolo-[5,1-d] [1,2,3,5]-tetrazine 2-oxide (BITE-101) is designed and synthesized in three steps. With the help of the complementary advantages of different explosophoric groups and diverse weak interactions, BITE-101 is superior to the benchmark explosive HMX in all respects, including higher density of 1.957 g·cm^-3, highest decomposition temperature of 295 °C (onset) among CHON-based high explosives to date and superior detonation velocity and pressure (D: 9314 m·s^-1, P: 39.3 GPa), impact and friction sensitivities (IS: 18 J, FS: 128 N), thereby showing great potential for practical application as replacement for HMX, the most powerful military explosive in current use.

Single-Crystal Diffraction, Raman Spectroscopy, and Density Functional Theory of DTO [N-(1,7-Dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide]

A combined experimental and modeling study of energetic compound N-(1,7-dinitro-1,2,6,7-tetrahydro-[1,3,5]triazino[1,2-c][1,3,5]oxadiazin-8(4H)-ylidene)nitramide [C₅H₆N₈O₇, (DTO)] has been performed. We report its crystal structure, solid-phase heat of formation, and its vibrational and electronic structure obtained by single-crystal X-ray diffractometry, Raman spectroscopy, and density functional theory (DFT). DTO exhibits two adjoining six-membered rings, a triazine ring (C₃N₃) and an oxadiazine ring (C₃N₂O) ring containing two nitro functional groups and one nitroamino group. DTO crystallizes with four molecules in its unit cell and presents a density of 1.862 kg/m³ at 298 K, in excellent agreement with both DFT calculations performed both at the molecular level using the B3LYP with the 6-311+G** basis set and the solid-state level using the hybrid functional HSE6 optimized with norm-conserving pseudopotentials. The calculated vibrational structure allows for the symmetry assignment of key Raman modes in terms of atomic movements, and the calculated frequency values are in good agreement with experiment. The solid-phase DFT calculations reveal that the N atoms of the triazine ring contribute mostly to the density of states at the Fermi level. In addition, we present and discuss the computed solid-phase heat of formation (215.9 kJ/mol) and molecular electrostatic potential surface of DTO and compare them to complementary materials.