Energetic Salts with π-Stacking and Hydrogen-Bonding Interactions Lead the Way to Future Energetic Materials

A high-density energetic ammonium salt via a polynitropyrazolate counteranion

The nitroamino group as a powerful explophore is widely employed in the design of energetic compounds. However, its strong electron-withdrawing effect often introduces acidic sites in the molecules, resulting in hygroscopicity. In this study, we successfully synthesized a high-density energetic ammonium salt (2a) by employing large 2,2'-dinitroamino-4,4',5,5'-tetranitro-2H,2'H-3,3'-bipyrazolate as the counteranion. This salt (2a) is non-hygroscopic and exhibits the highest density among nitrogen-rich ammonium energetic salts, positioning it as a highly promising candidate for high-energy-density compounds. In addition, an aminoguanidium salt (2b) was also prepared. These salts were comprehensively characterized using X-ray diffraction, nuclear magnetic resonance, and infrared spectroscopy, with their physicochemical properties thoroughly examined.

Intramolecular Cyclization and Energetic Group Modifications for Thermally Stable and Low-Sensitivity Monocyclic Dinitromethyl Zwitterionic Pyrazoles

Zwitterionic energetic materials offer a unique combination of high performance and stability, yet their synthesis and stability enhancement remain key challenges. In this study, we report the synthesis of a highly stable (dinitromethyl-functionalized zwitterionic compound, 1-(amino(iminio)methyl)-4,5-dihydro-1H-pyrazol-5-yl)dinitromethanide (4), with a thermal decomposition temperature of 215 °C, surpassing that of most previously reported energetic monocyclic zwitterions (Td < 150 °C). This compound was synthesized via intramolecular cyclization of a trinitromethyl-functionalized hydrazone precursor. Further chemical modifications, including nitration and fluorination, enabled zwitterion-to-zwitterion transformations, resulting in the formation of nitramines 10 and 12. Additionally, the perchlorate salt (8) of 4 was synthesized, along with ammonium (13), guanidinium (14), and potassium (15) salts derived from 10, all retaining zwitterionic properties. Physicochemical evaluations reveal that zwitterion 12 exhibits excellent thermal stability (Td = 181 °C) and an optimal balance between high energy output (detonation velocity: 8329 m s-1, detonation pressure: 29.4 GPa) and reduced sensitivity (impact sensitivity: 35 J, friction sensitivity: 320 N). Notably, potassium salt 15 demonstrates superior thermal stability (Td = 233 °C), exceeding that of RDX. These results expand the design framework for energetic zwitterions and contribute to the development of high-energy, low-sensitivity energetic materials.

Combination of Tetrazole and 4-Azido-pyrazolotriazine Oxide: Balance of High Nitrogen, Energy, and Safety

Energetic materials containing a pyrazolotriazine oxide skeleton have the potential for high performance. However, research on the pyrazolotriazine oxide skeleton is very limited due to the inherent ring system instability and limited synthetic approaches. In this paper, APTO and OPTO with a combination of high-nitrogen tetrazole and a promising azido-pyrazolotriazine oxide skeleton have been synthesized. Of these, OPTO and its energetic salts exhibit excellent detonation properties (Dv > 8220 m s-1; P > 26.50 GPa). Surprisingly, APTO exhibits a detonation performance (Dv = 8913 m s-1; P = 32.43 GPa) comparable to that of RDX.

Synthesis and Properties of Thermostable Energetic Benzotriazines

In an effort to balance energy and molecular stability effectively, several energetic compounds (3-6) based on benzotriazine were designed and synthesized. These structures were comprehensively characterized using NMR, IR, and elemental analysis, with compounds 3, 5, and 6 further confirmed by single-crystal X-ray diffraction. Notably, 3-amino-5,7-dinitrobenzo[e][1,2,4]triazine 1-oxide (5), which features a face-to-face crystal stacking arrangement, exhibits good detonation velocity (Dv = 8050 m/s), a high thermal decomposition temperature (Td = 290 °C), and low sensitivities (impact sensitivity >40 J, friction sensitivity >360 N). The preparation of compound 5 was further optimized by using a commercially available flow microreactor system, achieving an improved yield of 62%. Overall, the comprehensive properties of compound 5 make it a promising candidate for heat-resistant explosive applications.

Analytical Techniques in Molecular Simulation and Its Application in Energetic Materials

Efficient design on the molecular and crystal levels is an urgent need to accelerate the development of energetic materials (EMs), and the performance analysis of microstructures is the most important thing in the research and design of EMs. Although molecular simulation methods are widely used in various research fields, there are few comprehensive reviews on analytical techniques. It is urgent to understand the basic principles of various analytical methods in the research of EMs. In this article, the characterization/analysis methods in quantum mechanics and molecular mechanics simulation technology are summarized, and their applications in the field of EMs are listed. At the molecular level, energy, composition, geometric structure, and electronic structure are all related to macroscopic properties, and most of them have been widely used as variables in numerical models to predict and compare the properties of EMs. In addition, this paper emphasizes that the correlation between theoretical calculation and confirmatory experiment needs to be further verified because the current experimental characterization can also be dealt with by molecular simulation, which is helpful to the popularization and application of theoretical research.

A Method for the Preparation of Fused Dinitromethyl High-Energy-Density Materials

This work investigates a simple synthetic method for producing fused dinitromethyl energetic compounds. Fused compound 2 has a high detonation velocity (9358 m s-1) close to that of the well-known high-energy-density explosive CL-20 (9455 m s-1), an extremely high density (1.97 cm-1), and acceptable sensitivity (IS = 8 J). The good thermal stability of 2 (Td = 181 °C) is rarely reported in the field of dinitromethyl-based high explosives. The results show that compound 2 is a promising candidate for high explosives and provide new insight into the synthesis of dinitromethyl compounds.

Engaging Two Anions with Single Cation in Energetic Salts: Approach for Optimization of Oxygen Balance in Energetic Materials

The field of high energy density materials faces a long-standing challenge to achieve an optimum balance between energy and stability. While energetic salt formation via a combination of oxygen- and nitrogen-rich anions (providing energy) with nitrogen-containing cations (providing stability) has been a proven approach for improving physical stability, constraints such as lowering density and energetic performance remain unresolved. This can be addressed by utilizing oxygen-containing cations for salt formation. However, this approach is rarely explored because its synthesis is challenging. In this work, we have designed an oxygen-rich cationic precursor 2 by incorporating 4-amino-3,5-dinitropyrazole with 3,4-diaminotriazole via an N-methylene-C bridge. Further combination with energetic acids resulted in the formation of high-performing, physically stable energetic salts 3-10. The salts were found to be generally more energetic than the salts of respective energetic acids with previously reported cations and showed prominent improvement in physical stability with respect to their anionic precursors. All the compounds were thoroughly characterized through IR, NMR spectroscopy, HRMS, and elemental analysis. Salt 9 was confirmed through 15N NMR analysis, and salts 2 and 8 were confirmed through single-crystal X-ray diffraction. Additional analyses such as Hirshfeld surface analysis, noncovalent interaction (NCI) analysis, and electrostatic potential studies were also carried out to correlate the structure-property relationship.

Energetic derivatives substituted with trinitrophenyl: improving the sensitivity of explosives

The incorporation of trinitrophenyl-modified 1,3,4-oxadiazole fragments is commonly observed in high-energy molecules with heat-resistant properties. This study explores the strategy of developing heat-resistant energetic materials by incorporating trinitrophenyl and an azo group into 1,3,4-oxadiazole, which involved the synthesis and characterization of (E)-1,2-bis(5-(2,4,6-trinitrophenyl)-1,3,4-oxadiazol-2-yl)diazene (2), N-(5-(2,4,6-trinitrophenyl)-1,3,4-oxadiazol-2-yl)nitramide (3), and the energetic salts of 3. Characterization techniques employed included 1H and 13C NMR, IR and elemental analysis. Additionally, the structures of 2 and 3 were validated using single crystal X-ray analysis. To further understand the physical and chemical characteristics of these novel energetic compounds, various calculations and measurements were performed. Compound 2 exhibits excellent thermostability (Td = 294 °C), which is comparable to that of traditional heat-resistant explosive HNS (Td = 318 °C). But 2 is insensitive towards impact (>40 J) and friction (>360 N), surpassing HNS (5 J, 240 N), suggesting that compound 2 deserves further investigation as a potential heat-resistant explosive.

Application of machine learning in developing a quantitative structure-property relationship model for predicting the thermal decomposition temperature of nitrogen-rich energetic ionic salts

While thermal decomposition temperature (T d) is one of the most important indexes for energetic materials, the most common way of determining and evaluating T d requires laboratory experiments that are complicated, time-consuming and expensive. In the present study, the quantitative structure-property relationship (QSPR) model of T d for 21 nitrogen-rich energetic ionic salts was built and used for T d prediction through 13 descriptors and principal component analysis. The relatively small dataset of 21 samples may lead to overfitting. In the case of small datasets, possible overfitting was reduced by the support vector machine to derive the non-linear QSPR model. The obtained correlation coefficient (R 2) of 96.31% and root-mean-square error (RMSE) of 15.72 indicate the relative reliability of the QSPR model developed in this work. Moreover, T d values of 6 newly designed nitrogen-rich energetic ionic salts were predicted using the new QSPR model. The predicted T d values range from 194 to 225 °C, which are better than that of 4-amino-3,5-dinitro-1H-pyrazole (LLM-116: 178 °C), and no. 1, 2 and 5 are comparable to that of the traditional explosive 1,3,5-trinitro-1,3,5-triazinane (RDX: 230 °C), indicating the excellent properties of the designed energetic ionic salts, which can be used for the preparation of potential energetic materials.

An advanced furoxan-bridged heat-resistant explosive

Nowadays, thousands of energetic materials have been synthesized, but only a few compounds meet all the high standards of detonation performance comparable to that of the widely used military explosive RDX, thermal stability comparable to that of the most widely used heat-resistant explosive HNS, and impact sensitivity comparable to that of the traditional explosive TNT. Also, as a goal, a novel and unexpected one-step method for constructing the furoxan-bridged energetic compound 3,4-bis(3,8-dinitropyrazolo[5,1-c][1,2,4]triazin-4-amino-7-yl)-1,2,5-oxadiazole 2-oxide (OTF) has been achieved under the conventional TFA/100% HNO3 nitration reaction system from the acetic acid intermediate. In this work, OTF with a high density of 1.90 g cm-3, the highest decomposition temperature of 310 °C (onset) among furoxan-based high explosives to date, superior detonation velocity (DV: 9109 m s-1), and low sensitivity (IS: 25 J) is reported. This work is a giant step forward in the development of advanced high-energy heat-resistant explosives and could improve future possibilities for the design of furoxan-based energetic materials.

N-Methylene-C-linked nitropyrazoles and 1,2,4-triazol-3-one: thermally stable energetic materials with reduced sensitivity

Recently, there has been a surge in research focusing on triazolone-based energetic materials, propelled by their remarkable properties such as good detonation performance as well as acceptable thermal and physical stability. In this work, a novel combination of the triazolone framework with dinitropyrazoles has been attained using the N-methylene-C-linked approach. Different substituents (NH2, NO2, N3, OH) were utilized on the dinitropyrazole moiety to obtain neutral energetic compounds 3-5 and 8. Furthermore, the hydroxy derivative (compound 8) facilitates the formation of energetic salts 9-13 to fine-tune the overall properties further. All the novel compounds 3-13 were thoroughly characterized by IR, multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), and elemental analysis. Compounds 3, 4, 8, and 10 were further confirmed via15N NMR spectroscopy. The structure of compounds 3 and 8 was also confirmed through single-crystal X-ray diffraction studies. The majority of synthesized compounds showed good thermal stability as well as insensitivity toward external stimuli. Computational studies, including analyses such as Hirshfeld surface, non-covalent interaction, electrostatic potential surface, and HOMO-LUMO analysis, were conducted to examine the influence of substitution at the 4th position on the overall stability of compounds 3, 4, and 8.

Theoretical advances in understanding and enhancing the thermostability of energetic materials

The quest for thermally stable energetic materials is pivotal in advancing the safety of applications ranging from munitions to aerospace. This perspective delves into the role of theoretical methodologies in interpreting and advancing the thermal stability of energetic materials. Quantum chemical calculations offer an in-depth understanding of the molecular and electronic structure properties of energetic compounds related to thermal stability. It is also essential to incorporate the surrounding interactions and their impact on molecular stability. Ab initio molecular dynamics (AIMD) simulations provide detailed theoretical insights into the reaction pathways and the key intermediates during thermal decomposition in the condensed phase. Analyzing the kinetic barrier of rate-determining steps under various temperature and pressure conditions allows for a comprehensive assessment of thermal stability. Recent advances in machine learning have demonstrated their utility in constructing potential energy surfaces and predicting thermal stability for newly designed energetic materials. The machine learning-assisted high-throughput virtual screening (HTVS) methodology can accelerate the discovery of novel energetic materials with improved properties. As a result, the newly identified and synthesized energetic molecule ICM-104 revealed excellence in performance and thermostability. Theoretical approaches are pivotal in elucidating the mechanisms underlying thermal stability, enabling the prediction and design of enhanced thermal stability for emerging EMs. These insights are instrumental in accelerating the development of novel energetic materials that optimally balance performance and thermal stability.

Tuning Green Explosives through SNAr Chemistry

Reactivity and regioselectivity of SNAr-type fluorine substitution with azide in polyfluorosubstituted nitrobenzenes was studied both theoretically and experimentally. The obtained polyazido-substituted nitrobenzene derivatives were extensively characterized by NMR, IR, HPLC, X-ray, and DFT methods. It was found that the substitution with the azide nucleophile occurs first at the para- and the ortho-positions to the NO2-group and that transazidation reactions also occur here. Thermal decomposition of prepared azidonitrobenzenes was studied both in controlled (kinetic decay) and uncontrolled (explosion) modes. In case of the controlled thermal decomposition of ortho-azidonitrobenzenes, benzofuroxans were found as major products of the reaction unless another azido group was adjacent to the furoxan moiety. The bursting power of azidonitrobenzenes was found to rise gradually with the number of the azide substituents in the aromatic ring.

Single Step Synthesis of gem-Dinitro Methyl-1,2,4-triazole and Its Hydroxylamine Salt: An Alternative to the FOX-7 and Other Benchmark Explosives

gem-Dinitro methyl based high-energy-density material 5-(dinitromethylene)-4,5-dihydro-1H-1,2,4-triazole (2) and its hydroxylamine salt (4) were synthesized for the first time in a single step and characterized. Further, the structure of 2 was confirmed by single-crystal X-ray diffraction (SCXRD) studies. Interestengly, both the compounds show excellent density (> 1.83 g cm-3), detonation velocity (> 8700 m s-1), pressure (> 30 GPa) and are insensitive toward mechanical stimuli such as impact and friction sensitivity. Considering their synthetic fesibility and balanced energetic performance, compounds 2 and 4 show future prospects as potential next-generation energetic materials for the replacenent of many presently used benchmark high energy density materials such as RDX, FOX-7 and highly insensitive H-FOX.

Synthesis, Characterization, and Properties of Heat-Resistant Energetic Materials Based on C-C Bridged Dinitropyrazole Energetic Materials

Polycyclic energetic materials make up a distinctive class of conjugated structures that consist of two or more rings. In this work, 1,3-bis(3,5-dinitro-1H-pyrazol-4-yl)-4,6-dinitrobenzene (BDPD) was synthesized and investigated in detail as a polycyclic heat-resistant energetic molecule that can be deprotonated by bases to obtain its anionic (3-5) salts. All compounds were thoroughly characterized by 1H and 13C NMR, infrared spectroscopy, high-resolution mass spectrometry, and elemental analysis. The structural features of BDPD and its salts were investigated by single-crystal X-ray diffraction and analyzed by different kinds of computing software, like Multiwfn, Gaussian 09W, and so on. In addition, their thermal decomposition temperatures were evaluated by differential scanning calorimetry to be 319.8-329.0 °C, revealing that they possessed high thermal stabilities. The results of impact sensitivity and friction sensitivity analysis confirm that these energetic compounds were insensitive. The detonation properties of neutral compound BDPD and all its nonmetallic salts were calculated by the EXPLO5 v6.05.04 program. The results revealed that their detonation performances were higher than those of the widely used heat-resistant explosive 2,2',4,4',6,6'-hexanitrostilbene (HNS). Combining the above results, it is reasonable to suggest that these compounds have the potential to be heat-resistant energetic materials.

Two Energetic Framework Materials Based on DNM-TNBI as Host Molecule: Effectively Coordinated by Different Cations

With the demand of develop outstanding-performance energetic materials, 1-(dinitromethyl)-4,4',5,5'-tetranitro-1H,1'H-2,2'-biimidazole (DNM-TNBI) emerged as a great contender (D: 9102 m ⋅ s-1; P: 37.6 GPa). However, the relatively poor thermal stability (Td: 142 °C) limits its practical application. In this study, DNM-TNBI as a host molecule to synthesize two new energetic open-framework materials by effectively coordinated with different cations. Their supramolecular structures were investigated and indicated that [DNM-TNBI2 -][2NH4 +] and [DNM-TNBI2 -][2K+] can be classified as a new energetic hydrogen-bonded ammonium framework (EHAF) and an energetic metal organic framework (EMOF). Meanwhile, their thermal stabilities are higher than that of DNM-TNBI and have satisfactory detonation performance ([DNM-TNBI2 -][2NH4 +], D: 8050 m ⋅ s-1, P: 26.4 GPa; [DNM-TNBI2 -][2K+], D: 8301 m ⋅ s-1, P: 30.8 GPa).

Construction of Adaptive Deformation Block: Rational Molecular Editing of the N-Rich Host Molecule to Remove Water from the Energetic Hydrogen-Bonded Organic Frameworks

Energetic hydrogen-bonded organic frameworks (E-HOFs), as a type of energetic material, spark fresh vitality to the creation of high energy density materials (HEDMs). However, E-HOFs containing cations and anions face challenges such as reduced energy density due to the inclusion of crystal water. In this work, the modification of amino groups in N-rich organic units could form a smart building block of hydrogen-bonded frameworks capable of changing the volume of the void space in the molecule through adaptive deformation of E-MOF blocks, thus enabling the replacement of water. Based on the above strategy, we report an interesting example of a series of hydrogen-bonded organic frameworks (E-HOF 2a and 3a) synthesized using a facile method. The crystal structure data of all of the compounds were also obtained in this work. Anhydrous 2a and 3a exhibit higher density, good thermal stability, and low mechanical sensitivity. The strategy of covalent bond modification for the host molecules of energetic frameworks shows enormous potential in eliminating the crystalline H2O of hydration and exploring high energy density materials.

Probing into the theory of impact sensitivity: propelling the understanding of phonon-vibron coupling coefficients

The determination of impact sensitivity of energetic materials traditionally relies on expensive and safety-challenged experimental means. This has instigated a shift towards scientific computations to gain insights into and predict the impact response of energetic materials. In this study, we refine the phonon-vibron coupling coefficients ζ in energetic materials subjected to impact loading, building upon the foundation of the phonon up-pumping model. Considering the full range of interactions between high-order phonon overtones and molecular vibrational frequencies, this is a pivotal element for accurately determining phonon-vibron coupling coefficients ζ. This new coupling coefficient ζ relies exclusively on phonon and molecular vibrational frequencies within the range of 0-700 cm-1. Following a regression analysis involving ζ and impact sensitivity (H50) of 45 molecular nitroexplosives, we reassessed the numerical values of damping factors, establishing a = 2.5 and b = 35. This coefficient is found to be a secondary factor in determining sensitivity, secondary to the rate of decomposition propagation and thermodynamic factor (heat of explosion). Furthermore, the relationship between phonon-vibron coupling coefficients ζ and impact sensitivity was studied in 16 energetic crystalline materials and eight nitrogen-rich energetic salts. It was observed that as the phonon-vibron coupling coefficient increases, the tendency for reduced impact sensitivity H50 still exists.

Synthetic manifestation of trinitro-pyrazolo-2H-1,2,3-triazoles (TNPT) as insensitive energetic materials

This study showcases the design and development of a facile method for synthesizing trinitro-pyrazolo-triazole (TNPT) and its derivatives. The synthesized compounds are analysed using multinuclear NMR [1H, 13C, and 15N] and HRMS analyses. Furthermore, X-ray diffraction studies confirm the structure of some TNPT derivatives. Notably, compounds 8, 9, 11, and 12 exhibit good thermal stability with a decomposition threshold above 250 °C, and show a high level of insensitivity towards impact and friction [impact sensitivity (IS) is more than 25 J and friction sensitivity (FS) is above 180 N]. Compound 12, in particular, displays excellent performance characteristics [density 1.76 g cc-1 (at 298 K), a high detonation velocity (Dv = 8550 m s-1), and good thermal stability (Td = 280 °C), with high insensitivity towards impact and friction (IS = 35 J; FS = 180 N)]. The Hirshfeld surface analysis study provides further insight into the sensitivity of the TNPT derivatives.

A three-dimensional energetic coordination compound (BLG-1) with excellent initiating ability for lead-free primary explosives

Exploration of advanced lead-free primary explosives is a challenging issue in the field of energetic materials. Herein, we designed and synthesized a novel N-rich copper bromate energetic coordination compound (ECC) [Cu(ATRZ)(BrO3)2]n (BLG-1, ATRZ: 4,4'-azo-1,2,4-triazole) by a simple one-step reaction. BLG-1 is the first reported three-dimensional (3D) N-rich copper bromate ECC. Its interesting 3D reticular architecture contributed to its highest thermal decomposition temperature (Td: 226 °C) and crystal density (ρ: 2.69 g cm-3) among N-rich copper bromate ECCs. More importantly, a primary charge of BLG-1 as little as 3 mg could reliably detonate compressed RDX, and 1 mg could detonate CL-20. These incredible values indicated that BLG-1 had an ultra-powerful initiating ability far superior to that of previously reported primary explosives. BLG-1 had improved mechanical sensitivities (IS: 13 J; FS: 1 N) and electrostatic sensitivity (EDS: 240 mJ) compared with those of the typical lead-based primary explosive, lead azide (IS: 4J; FS: 0.75N; EDS: 5 mJ). In particular, BLG-1 had a low laser-initiation threshold of 13 mJ at 808 nm, suggesting that it could serve as a laser-ignitable primary explosive. This work suggests that BLG-1 is a promising candidate with engreat practical application prospects for lead-free primary explosives.

Metal-Free Hybrid Energetic Composites Based on Donor-Acceptor π-Conjugated Organic Energetic Catalysts with Enlightening the Laser Ignition Performance of Multi-Scale Ammonium Perchlorate

Photosensitive materials, such as energetic complexes, usually have high sensitivity and cause heavy-metal pollution, whereas others, like carbon black and dye, do not contain energy, which affects energy output and mechanical properties. In this work, donor-acceptor π-conjugated energetic catalysts, denoted as D-n, are designed and synthesized. Nonmetallic hybrid energetic composites are prepared by assembling the as-synthesized catalysts into multiscale ammonium perchlorate (AP). Composites containing catalysts and APs can be successfully ignited without the involvement of metals. The new ignition mechanism is further analyzed using experimental and theoretical analyses such as UV-vis-near-infrared (NIR) spectra, electron-spin resonance spectroscopy, and energy-gap analysis. The shortest ignition delay time is 56 ms under the experimental condition of a NIR wavelength of 1064 nm and a laser power of 10 W. At the voltage of 1 kV and the electric field of 500 V mm-1 , the laser-ignition delay time of D-2/AP hybrid composite decreases from 56 to 35 ms because D-2 also exhibits organic semiconductor-like properties. D-2/AP and D-12/AP can also be used to successfully laser ignite other common energetic materials. This study can guide the development of advanced metal-free laser-ignitable energetic composites to address challenges in the field of aerospace engineering.

Trinitromethyl-triazolone (TNMTO): a highly dense oxidizer

A scalable synthesis of 5-(trinitromethyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (TNMTO) is possible from commercially available 2-methylpyrimidine-4,6-diol. It exhibits high density (1.90 g cm-3) with comparably low thermal stability (Td = 80 °C) and positive oxygen balance (OBco = 20.51%, OBCO2 = 0.0%). TNMTO has an attractive combination of detonation properties (P = 35.01 GPa, D = 8997 ms-1) and propulsive properties (Isp(neat) = 251.85 s, ρIsp(neat) = 478.52 gs cm-3, ). These are superior to ammonium dinitroamide (ADN), 2,2,2-tetranitroacetimidic acid (TNAA) and ammonium perchlorate (AP), making it a potential green oxidizer in solid rocket propulsion.

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.

Thermal stability of azole-rich energetic compounds: their structure, density, enthalpy of formation and energetic properties

Energetic compounds, as a type of special material, are widely used in the fields of national defense, aerospace and exploration. Their research and production have received growing attention. Thermal stability is a crucial factor for the safety of energetic materials. Azole-rich energetic compounds have emerged as a research hotspot in recent years owing to their excellent properties. Due to the aromaticity of unsaturated azoles, many azole-rich energetic compounds have significant thermal stability, which is one of the properties that researchers focus on. This review presents a comprehensive summary of the physicochemical and energetic properties of various energetic materials, highlighting the relationship between thermal stability and the structural, physicochemical, and energetic properties of azole-rich energetic compounds. To improve the thermal stability of compounds, five aspects can be considered, including functional group modification, bridging, preparation of energetic salts, energetic metal-organic frameworks (EMOFs) and co-crystals. It was demonstrated that increasing the strength and number of hydrogen bonds of azoles and expanding the π-π stacking area are the key factors to improve thermal stability, which provides a valuable way to develop energetic materials with higher energy and thermal stability.

A Physical Organic Approach towards Statistical Modeling of Tetrazole and Azide Decomposition

Nitrogen atom-rich heterocycles and organic azides have found extensive use in many sectors of modern chemistry from drug discovery to energetic materials. The prediction and understanding of their energetic properties are thus key to the safe and effective application of these compounds. In this work, we disclose the use of multivariate linear regression modeling for the prediction of the decomposition temperature and impact sensitivity of structurally diverse tetrazoles and organic azides. We report a data-driven approach for property prediction featuring a collection of quantum mechanical parameters and computational workflows. The statistical models reported herein carry predictive accuracy as well as chemical interpretability. Model validation was successfully accomplished via tetrazole test sets with parameters generated exclusively in silico. Mechanistic analysis of the statistical models indicated distinct divergent pathways of thermal and impact-initiated decomposition.

The design and synthesis of new advanced energetic materials based on pyrazole-triazole backbones

A series of derivatives of the nitropyrazole-triazole backbone were designed through units' screening of 219 N-heterocycle compounds and were synthesized. Among them, the thermal stability of DNPAT (T_dec = 314 °C) is close to that of traditional heat-resistant explosive HNS (318 °C) while the detonation performance and sensitivity (D = 8889 m s^-1; IS = 18 J) are better than those of HNS (D = 7612 m s^-1; IS = 5 J) and traditional high-energy explosive RDX (D = 8795 m s^-1; IS = 7.4 J), which is rarely reported in heat-resistant explosives. Moreover, compounds 4 and 6 show excellent performances (IS > 15 J, D > 9090 m s^-1, P > 37.0 GPa), illustrating that compounds 4 and 6 may be used as secondary explosives. All these results enrich prospects for the development of energetic materials.

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.

Interheteromolecular Hyperconjugation Boosts (De)hydrogenation for Reversible H2 Storage

Interheteromolecular hyperconjugation is ubiquitous in organic systems, affecting bond length, dipole moments, conformations and so on, while its effect on (de)hydrogenation reactivity in a heterogeneous thermo-catalytic system has rarely been explored. Herein, the N-heterocycles containing a benzene ring and aliphatic chain [N-ethylcarbazole (NEC) and N-propylcarbazole (NPC)] were utilized to study the correlation between interheteromolecular hyperconjugation and catalytic (de)hydrogenation. Density functional theory calculations, variable-temperature ¹ H nuclear magnetic resonance spectroscopy, and catalytic experiments showed that the presented hyperconjugation between NEC and NPC weakened the electron cloud density of aromatic rings and thus facilitated the reactivity with hydrogen featuring unpaired electrons. Therefore, an extremely low temperature of 80 °C was enough for the hydrogenation. Moreover, this interheteromolecular hyperconjugation was general in other N-heterocycles (e. g., N-methyindole and NPC) and was also effective to (de)deuterate as revealed by isotope experiments. This work expands the application of interheteromolecular hyperconjugation to heterogeneous thermocatalysis for reversible H₂ storage.

Size-matched hydrogen bonded hydroxylammonium frameworks for regulation of energetic materials

Size matching molecular design utilizing host-guest chemistry is a general, promising strategy for seeking new functional materials. With the growing trend of multidisciplinary investigations, taming the metastable high-energy guest moiety in well-matched frameworks is a new pathway leading to innovative energetic materials. Presented is a selective encapsulation in hydrogen-bonded hydroxylammonium frameworks (HHF) by screening different sized nitrogen-rich azoles. The size-match between a sensitive high-energy guest and an HHF not only gives rise to higher energetic performance by dense packing, but also reinforces the layer-by-layer structure which can stabilize the resulting materials towards external mechanic stimuli. Preliminary assessment based on calculated detonation properties and mechanical sensitivity indicates that HHF competed well with the energetic performance and molecular stability (detonation velocity = 9286 m s-1, impact sensitivity = 50 J). This work highlights the size-matched phenomenon of HHF and may serve as an alternative strategy for exploring next generation advanced energetic materials.

Density functional theory studies on N4 and N8 species: Focusing on various structures and excellent energetic properties

All-nitrogen materials, as a unique branch of energetic materials, have gained huge attentions, of which cyclo-N₅⁻ derivatives are the representative synthetically reported materials. However, the energetic performance of cyclo-N₅⁻ compounds has certain limitations and cannot go beyond that of CL-20. In order to reach the higher energy, in this work, we presented two kinds of polynitrogen species, N₄ and N₈. Two isomers of N₄ and four isomers of N₈ were fully calculated by using density functional theory (DFT). Theoretical results show that all these polynitrogen materials exhibit excellent heats of formation (7.92–16.60 kJ g⁻¹), desirable detonation performance (D: 9766–11620 m s⁻¹; p: 36.8–61.1 GPa), as well as the remarkable specific impulses (330.1–436.2 s), which are much superior to CL-20. Among them, N₄-2 (tetraazahedrane) (D: 10037 m s⁻¹; p: 40.1 GPa; I_sp: 409.7 s) and cube N₈-4 (D: 11620 m s⁻¹; p: 61.1 GPa; I_sp: 436.2 s) have the highest energetic properties, which are expected to become promising high-energy-density-materials. Moreover, electrostatic surface potentials, Frontier molecular orbitals, infrared spectra, natural bond orbital charges, and weak interactions were also investigated to further understand their relationship between structure and performance.

2-(1,2,4-triazole-5-yl)-1,3,4-oxadiazole as a novel building block for energetic materials

The exploration of novel nitrogen-rich heterocyclic building blocks is of importance in the field of energetic materials. A series of 2-(1,2,4-triazole-5-yl)-1,3,4-oxadiazole derivatives based on a new energetic skeleton have been first synthesized by a simple synthetic strategy. All three compounds are well-characterized by IR spectroscopy, NMR spectroscopy and thermal analysis. The compounds 5 and 8 are further characterized by single-crystal X-ray diffraction analysis. 8 and its salts (8a-8c) possess relative high decomposition temperature and low sensitivity, while 5 exhibits low decomposition temperature and high sensitivity. According to EXPLO5 calculation results of detonation performance, both 5 and 8 display acceptable detonation velocities (D) of 8450 m/s and 8130 m/s and detonation pressures (P) of 31.6 GPa and 29.2 GPa, respectively. Furthermore, 5 containing a rare diazonium ylide structure shows high impact sensitivity (4.5 J), making it has a potential as a primary explosive.

Crystal Structure and Noncovalent Interactions of Heterocyclic Energetic Molecules

Nitrogen-rich heterocyclic compounds are important heterocyclic substances with extensive future applications for energetic materials due to their outstanding density and excellent physicochemical properties. However, the weak intermolecular interactions of these compounds are not clear, which severely limits their widespread application. Three nitrogen-rich heterocyclic compounds were chosen to detect their molecular geometry, stacking mode and intermolecular interactions by crystal structure, Hirshfeld surface, RDG and ESP. The results show that all atoms in each molecule are coplanar and that the stacking mode of the three crystals is a planar layer style. A large amount of inter- and intramolecular interaction exists in the three crystals. All principal types of intermolecular contacts in the three crystals are N···H interactions and they account for 40.9%, 38.9% and 32.9%, respectively. Hydrogen bonding, vdW interactions and steric effects in Crystal c are stronger than in Crystals a and b. The negative ESPs all concentrate on the nitrogen atoms in the three molecules. This work is expected to benefit the crystal engineering of heterocyclic energetic materials.

Design and selection of high energy materials based on 4,8-dihydrodifurazano[3,4-b,e]pyrazine

In order to search for high energy density materials, various 4,8-dihydrodifurazano[3,4-b,e]pyrazine based energetic materials were designed. Density functional theory was employed to investigate the relationships between the structures and properties. The calculated results indicated that the properties of these designed compounds were influenced by the energetic groups and heterocyclic substituents. The -N3 energetic group was found to be the most effective substituent to improve the heats of formation of the designed compounds while the tetrazole ring/-C(NO2)3 group contributed much to the values of detonation properties. The analysis of bond orders and bond dissociation energies showed that the addition of -NHNH2, -NHNO2, -CH(NO2)3 and -C(NO2)3 groups would decrease the bond dissociation energies remarkably. Compounds A8, B8, C8, D8, E8, and F8 were finally screened as the potential candidates for high energy density materials since these compounds possess excellent detonation properties and acceptable thermal stabilities. Additionally, the electronic structures of the screened compounds were calculated.

Noncovalent Interactions and Crystal Structure Prediction of Energetic Materials

The crystal and molecular structures, intermolecular interactions, and energy of CL-20, HATO, and FOX-7 were comparatively predicted based on molecular dynamic (MD) simulations. By comparison, the 2D fingerprint plot, Hirshfeld surface, reduced density gradient isosurface, and electrostatic potential surface were studied to detect the intermolecular interactions. Meanwhile, the effects of vacuum and different solvents on the crystal habit of CL-20, HATO, and FOX-7 were studied by AE and MAE model, respectively. The energy calculation was also analysed based on the equilibrium structures of these crystal models by MD simulations. Our results would provide fundamental insights for the crystal engineering of energetic materials.

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.

Anisotropic Impact Sensitivity of Metal-Free Molecular Perovskite High-Energetic Material (C6H14N2)(NH2NH3)(ClO4)3 by First-Principles Study

Density functional theory simulations were carried out to investigate energetic molecular perovskite (C₆H₁₄N₂)(NH₂NH₃)(ClO₄)₃ which was a new type energetic material promising for future application. The electronic properties, surface energy, and hydrogen bonding of (100), (010), (011), (101), (111) surfaces were studied, and the anisotropic impact sensitivity of these surfaces were reported. By comparing the values of the band gaps for different surface structures, we found that the (100) surface has the lowest sensitivity, while the (101) surface was considered to be much more sensitive than the others. The results for the total density of states further validated the previous conclusion obtained from the band gap. Additionally, the calculated surface energy indicated that surface energy was positively correlated with impact sensitivity. Hydrogen bond content of the surface structures showed distinct variability according to the two-dimensional fingerprint plots. In particular, the hydrogen bond content of (100) surface was higher than that of other surfaces, indicating that the impact sensitivity of (100) surface is the lowest.

Polynitro-N-aryl-C-nitro-pyrazole/imidazole Derivatives: Thermally Stable-Insensitive Energetic Materials

A wide array of methoxy-substituted-polynitro-aryl-pyrazole/imidazoles with readily oxidizable -NH₂/NO₂/NHNO₂/diazo functional groups is synthesized. Single crystal X-ray diffraction (XRD) analysis confirms the molecular structure of the compounds. Energetic properties of the synthesized compounds are determined by theoretical and experimental studies. Most of the compounds are thermally stable and insensitive to impact and friction. Some of the molecules possess better detonation velocity and detonation pressure over TNT.