Stabilization of the Dual-Aromatic cyclo-N5- Anion by Acidic Entrapment

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.

Revisiting Pentazole: An Investigation into the Intriguing Molecule Exhibiting Dual Organic and Inorganic Characteristics

Pentazole (cyclo-HN5) is a unique heterocycle categorized as both an organic and inorganic compound. However, attempts to synthesize and characterize cyclo-HN5 have been unsuccessful thus far. In this study, we synthesized a cyclo-HN5 solution and investigated the spectra, structure, aromaticity, acidity, and stability of cyclo-HN5. The lone pair of electrons on the protonated N atom of cyclo-HN5 participates in π-electron delocalization, forming two N═N bonds. Further investigations suggest that cyclo-HN5 exhibits significantly decreased π aromaticity and slightly lower σ aromaticity than cyclo-N5-. Experimental results suggest that pure cyclo-HN5 is unstable at ambient temperatures and pressures, but it can be isolated at high pressures or stabilized in solution by abundant hydrogen bonds. The pKa of cyclo-HN5 was determined as 1.63 (H2O, 25 °C) via potentiometric titration, indicating that cyclo-HN5 is a medium-strong acid. This study reveals the fundamental structure and properties of cyclo-HN5, thereby providing important data for advancing cyclo-HN5 chemistry.

Insight into the Stability of Pentazolyl Derivatives based on Covalent Bond

Pentazole is regarded as a unique inorganic molecule that possess organic heterocyclic structure. Therefore, the research on pentazolyl derivatives represents a cutting-edge direction in both contemporary inorganic chemistry and heterocyclic chemistry. Moreover, their synthesis is regarded as the most significant research topic in the field of energetic materials due to the great potential of pentazolyl derivatives to breakthrough the energy bottleneck of CHNO-based energetic materials. However, synthesizing pentazolyl derivatives is challenging. To provide a theoretical support for the synthesis, we conducted theoretical studies on six single-ring pentazolyl derivatives with different functional groups. The results suggest that derivatization reduces the bond strength and weakens the aromaticity of the pentazolate ring. Further analysis showed that derivatization mainly affects the π aromaticity of the pentazolate ring, and ultimately causing poor stability of the pentazolyl derivatives. Among the six derivatives investigated in this study, fluoro pentazole (cyclo-N5-F) and hydroxyl pentazole (cyclo-N5-OH) possess good aromaticity, which is similar to the reported cyclo-N5-NCHN(CH3)2. Further calculations show that the kinetic stability of cyclo-N5-OH is higher than that of cyclo-N5-F. These results collectively indicate that cyclo-N5-OH is a promising candidate for synthesizing single-ring pentazolyl derivatives.

Pentazolate Anion: A Rare and Preferred Five-Membered Ligand for Constructing Pentasil-Zeolite Topology Architectures

As the fourth full-nitrogen structure, the pentazolate anion (cyclo-N5 - ) was highly coveted for decades. In 2017, the first air-stable non-metal pentazolate salt, (N5 )6 (H3 O)3 (NH4 )4 Cl, was obtained, representing a milestone in this field. As the latest member of the azole family, cyclo-N5 - is comprised of five nitrogen atoms. Although significant attention has been paid to the potential of cyclo-N5 - as an energetic material, its poor thermostability hinders any practical application. However, the unique ring structure and multiple coordination capability of cyclo-N5 - provide a platform for the fabrication of various structures, among which pentasil-zeolite topologies are the most intriguing. In addition, the introduction of structure-directing auxiliaries enables the self-assembly of diverse topological architectures, potentially imparting cyclo-N5 - with the potential to impact wide-ranging areas of coordination chemistry and topology. In this minireview, different pentasil-zeolite topologies based on metal-pentazolate frameworks are evaluated. To date, three zeolitic and zeolite-like topologies have been reported, namely the melanophlogite (MEP), chibaite (MTN), and unj topologies. The MEP topology consists of two nanocages, Na20 N60 and Na24 N60 , whereas the MTN topology contains Na20 N60 and Na28 N80 nanocages. Furthermore, the unj topology features multiple homochiral channels consisting of two helical chains. Various possible strategies for obtaining additional pentasil-zeolite topologies are also discussed.

Nonbonding Electron Delocalization Stabilizes the Flexible N8 Molecular Assembly

Electron delocalization has an important impact on the physical properties of condensed materials. However, the L-electron delocalization in inorganic, especially nitrogen, compounds needs exploitation to improve the energy efficiency, safety, and environmental sustainability of high-energy-density materials (HEDMs). This Letter presents an intriguing N8 molecule, ingeniously utilizing nitrogen's L-electron delocalization. The molecule, exhibiting a unique lollipop-shaped conformation, can fold at various angles with very low energy barriers, self-assembling into environmentally stable, all-nitrogen crystals. These crystals demonstrate unparalleled stability, high energy density, low mechanical sensitivity, and optimal electronic thermal conductivity, outperforming existing HEDMs. The remarkable properties of these designed materials are attributed to two distinct delocalized systems within nitrogen's L-shell: π- and lone pair σ-electrons, which not only stabilize the molecular structure but also facilitate interconnected 3D networks of intermolecular nonbonding interactions. This work might pave the way to the experimental synthesis of environmentally stable all-nitrogen solids.

Insight into the high-temperature oxidation kinetics of acetylene: A first-principles molecular dynamics study

The study on high-temperature oxidation kinetics and kinetic modeling of acetylene (C2H2) has significant importance for its engineering applications. In this paper, the first-principles molecular dynamics method is used to simulate the C2H2 oxidation under high temperatures for the first time. Our results show that there are 38 intermediates and 225 elementary reactions in the process of C2H2 oxidation. The formation mechanisms of "prompt" CO2, as well as gas pollutants CHOCHO and HCOOH are revealed in depth. Four intermediates, CHCHO, CHOCO, CHOCHO and HCOOH, which have significant controversy in current kinetic models, are verified. And a new intermediate, CHOCO2, is discovered. Meanwhile, our simulation also shows that radicals, such as HO2, OH, O, etc. play a key role in promoting the oxidation of intermediates in the early stage of C2H2 oxidation. Combined with quantum chemical calculations, a detailed chemical kinetic model of C2H2/air (FP-C2H2) is built and verified by simulating ignition delay time, species concentration in the flow reactor and premixed laminar flame speed. Our studies provide novel insight for understanding the complex chemical reaction kinetics, and environmental and human health threats from air pollutant formation during C2H2 combustion.

Discriminative Mechanical and Thermal Response of the H-N Bonds for the Energetic LLM-105 Molecular Assembly

Molecular interactions in energetic materials form the key not only to the "structure stability, energy storage, ignition, and detonation" dynamics but also to the sensitivity to the loading of perturbation and the power intensity of radiation for the energetic substance, with the nature of the interactions remaining elusive. With the aid of perturbative Raman spectroscopy and the pressure-resolved density functional theory, we uncovered that the H-N bond of the intermolecular O:H-N bonds for LLM-105 shares the same negative compressibility and thermal expansivity of the H-O bond for the coupling O:H-O bond of water [Phys. Rep. 2023, 998, 1-68]. In contrast, the dangling H-N bond vibrating at a 3440 cm-1 high frequency does otherwise due to the absence of coupling interaction and the undercoordination-driven bond contraction. These findings should deepen our insight into interactions involving electron lone pairs and offer an efficient means for discriminating the performance of individual bonds.

Hydrogen Bond and π-π Stacking Interaction: Stabilization Mechanism of Two Metal Cyclo-N5 --Containing Energetic Materials

In recent years, cyclo-N₅ ^- has attracted extensive attention because all-nitrogen high-energy-density materials (HEDMs) have been expected to reach a TNT equivalent of over 3.0. However, for cyclo-N₅ ^--containing HEDMs, the stabilization mechanism has remained enigmatic. In this study, two typical cyclo-N₅ ^--containing metal hydrates, [Na(H₂O)(N₅)]·2H₂O (Na-cyclo-N₅ ^-) and [Mg(H₂O)₆(N₅)₂]·4H₂O (Mg-cyclo-N₅ ^-), are selected to gain insights into the factors affecting their stability by the first-principles method. Both binding/lattice energy calculations and density of states analysis show that Mg-cyclo-N₅ ^- is more stable than Na-cyclo-N₅ ^-. Hydrogen bonding is the main stabilization mechanism for stabilizing crystals and cyclo-N₅ ^-. Two types of hydrogen bonds, O-H···O and O-H···N, are clarified, which construct a 3D hydrogen bond network in Mg-cyclo-N₅ ^- and an intralayer 2D hydrogen bond network in Na-cyclo-N₅ ^-. Moreover, nonuniform stress causes distortion of cyclo-N₅ ^-. Comparing the two samples, the distortion degree of cyclo-N₅ ^- is higher in Na-cyclo-N₅ ^-, which indicates that cyclo-N₅ ^- decomposition is easier. These findings will enhance the future prospects for the design and synthesis of cyclo-N₅ ^--containing HEDMs.

Symmetrical cyclo-N5- hydrogen bonds: stabilization mechanism of four non-metallic cyclo-pentazolate energetic salts

Pairing different cations (R⁺) to stabilize cyclo-N₅^- is the main synthesis path for non-metallic cyclo-pentazolate (cyclo-N₅^-) salts. As novel energetic materials (EMs), crystalline packing-force of cyclo-N₅^- salts has been a puzzle, and whether cyclo-N₅^- is protonated also is a controversial issue. In this paper, four non-metallic cyclo-N₅^- salts, PHAC, N₂H₅N₅, NH₃OHN₅, and NH₄N₅, are quantitatively studied by coupling first-principle method and bond-strength analyzing technology. Different from the traditional CHON-EMs (molecular crystal) and azide-EMs (ionic crystal), the four salts are stabilized by 3D hydrogen bond (HB) networks. One new type of hydrogen bond, protonated HB (p-H, R-H⋯N₅^-), is discovered to be a key stabilizing factor for cyclo-N₅^-. Proton competition mechanism between R and cyclo-N₅^- in p-H HB showed that cyclo-N₅^- cannot be protonated into HN₅. In general, p-H HB can be adopted to estimate the stability of novel non-metallic cyclo-N₅^- EMs. Such findings have great significance for future design and performance prediction of novel cyclo-N₅^- EMs in both theoretical and experimental aspects.

Planar Inorganic Five-Membered Heterocycles with σ+π Dual Aromaticity in Both S0 and T1 States

Cyclic species being aromatic in both the lowest singlet and triplet electronic states (so-called adaptive aromaticity) are scarce. To date, the reported systems are mostly organometallic heterocycles with the aromaticities...

Alleviating the stability–performance contradiction of cage-like high-energy-density materials by a backbone-collapse and branch-heterolysis competition mechanism

Key role of cage-like conformations in alleviating the stability–performance contradiction of HEDMs.

Thermal Decomposition Mechanism and Energy Release Law of Novel Cyclo-N5--Based Nitrogen-Rich Energetic Salt

Detonation energy of novel cyclo-N₅^--based nitrogen-rich energetic salts is expected to exceed 3 times the equivalent of TNT. PHAC([(N₅)₆(H₃O)₃(NH₄)₄Cl]) was selected as the prototype to investigate the thermal decomposition reaction of PHAC in the solid phase for the first time by the first-principles molecular dynamics method. At about 38 ps, the final state of the reaction was reached. It was found that there were mainly five final products, among which the proportion of N₂ molecules was the maximum and accounted for 60% (mole fraction) of all final products. The reaction pathways of PHAC were analyzed, and more than 30 elementary reactions were found. The initial reaction of the PHAC thermal decomposition was the ring-opening of cyclo-N₅^- ion and proton transfer. The energy release of PHAC thermal decomposition is divided into two stages. The first stage is a slow release of energy before the formation of the HN₃ molecule. The second stage is the rapid release of energy after the formation of HN₃ molecules. The HN₃ molecule is an essential junction, and the unimolecular dissociation of HN₃ is the rate-determining step. Such an understanding of reaction mechanism and energy release law greatly promotes the application and synthesis of novel cyclo-N₅^--based nitrogen-rich energetic salts.

Acidic Stabilization of the Dual-Aromatic Heterocyclic Anions

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second σ-aromaticity in addition to the prototypical π-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional π-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid−base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N5¯, tetrazole anion N4C1H1¯, and 1,2,4-triazole anion N3C2H2¯) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid−base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

Applying machine learning to balance performance and stability of high energy density materials

The long-standing performance-stability contradiction issue of high energy density materials (HEDMs) is of extremely complex and multi-parameter nature. Herein, machine learning was employed to handle 28 feature descriptors and 5 properties of detonation and stability of 153 HEDMs, wherein all 21,648 data used were obtained through high-throughput crystal-level quantum mechanics calculations on supercomputers. Among five models, namely, extreme gradient boosting regression tree (XGBoost), adaptive boosting, random forest, multi-layer perceptron, and kernel ridge regression, were respectively trained and evaluated by stratified sampling and 5-fold cross-validation method. Among them, XGBoost model produced the best scoring metrics in predicting the detonation velocity, detonation pressure, heat of explosion, decomposition temperature, and lattice energy of HEDMs, and XGBoost predictions agreed best with the 1,383 experimental data collected from massive literatures. Feature importance analysis was conducted to obtain data-driven insight into the causality of the performance-stability contradiction and delivered the optimal range of key features for more efficient rational design of advanced HEDMs.

High-throughput design of Peierls and charge density wave phases in Q1D organometallic materials

Soft-phonon modes of an undistorted phase encode a material's preference for symmetry lowering. However, the evidence is sparse for the relationship between an unstable phonon wavevector's reciprocal and the number of formula units in the stable distorted phase. This "1/q^*-criterion" holds great potential for the first-principles design of materials, especially in low-dimension. We validate the approach on the Q1D organometallic materials space containing 1199 ring-metal units and identify candidates that are stable in undistorted (1 unit), Peierls (2 units), charge density wave (3-5 units), or long wave (>5 units) phases. We highlight materials exhibiting gap-opening as well as an uncommon gap-closing Peierls transition and discuss an example case stabilized as a charge density wave insulator. We present the data generated for this study through an interactive publicly accessible Big Data analytics platform (https://moldis.tifrh.res.in/data/rmq1d) facilitating limitless and seamless data-mining explorations.

Recent advances in synthesis and crystal structures of metal pentazolate salts

Pentazole is a good nitrogen-donor ligand to prepare intriguing structures. Herein, the synthesis and crystal structures of all reported metal pentazolate salts were summarized and the future prospects in this field were also discussed.

"Bottled" spiro-doubly aromatic trinuclear [Pd2Ru]+ complexes

Following an ongoing interest in the study of transition metal complexes with exotic bonding networks, we report herein the synthesis of a family of heterobimetallic triangular clusters involving Ru and Pd atoms. These are the first examples of trinuclear complexes combining these nuclei. Structural and bonding analyses revealed both analogies and unexpected differences for these [Pd₂Ru]⁺ complexes compared to their parent [Pd₃]⁺ peers. Noticeably, participation of the Ru atom in the π-aromaticity of the coordinated benzene ring makes the synthesized compound the second reported example of ‘bottled’ double aromaticity. This can also be referred to as spiroaromaticity due to the participation of Ru in two aromatic systems at a time. Moreover, the [Pd₂Ru]⁺ kernel exhibits unprecedented orbital overlap of Ru d_z² AO and two Pd d_xy or d_x²−y² AOs. The present findings reveal the possibility of synthesizing stable clusters with delocalized metal–metal bonding from the combination of non-adjacent elements of the periodic table which has not been reported previously.

Synthesis of a triangular [Pd₂Ru]⁺ complex with delocalized metal–metal bonding between non-adjacent elements of the periodic table, double aromaticity and overlap of d-AOs with different angular momentum.

Water electrification: Principles and applications

Deep engineering of liquid water by charge and impurity injection, charged support, current flow, hydrophobic confinement, or applying a directional field has becoming increasingly important to the mankind toward overcoming energy and environment crisis. One can mediate the processes or temperatures of molecular evaporation for clean water harvesting, HO bond dissociation for H₂ fuel generation, solidification for living-organism cryopreservation, structure stiffening for bioengineering, etc., with mechanisms being still puzzling. We show that the framework of "hydrogen bonding and electronic dynamics" has substantiated the progress in the fundamental issues and the aimed engineering. The segmental disparity of the coupled hydrogen bond (O:HO or HB with ":" being lone pair of oxygen) resolves their specific-heat curves and turns out a quasisolid phase (QS, bound at -15 and 4 °C). Electrification shows dual functionality that not only aligns, orders, polarizes water molecules but also stretches the O:HO bond. The O:HO segmental cooperative relaxation and polarization shift the QS boundary through Einstein's relation, ΔΘ_Dx ∝ Δω_x, resulting in a gel-like, viscoelastic, and stable supersolid phase with raised melting point T_m and lowered temperatures for vaporization T_V and ice nucleation T_N. The supersolidity and electro structure ordering provide additional forces to reinforce Armstrong's water bridge. QS dispersion and the secondary effect of electrification such as compression define the T_N for Dufour's electro-freezing. The T_V depression, surface stress disruption, and electrostatic attraction raise Asakawa's molecular evaporability. Composition of opposite, compatible fields eases the HO dissociation and soil wetting. Progress evidences not only the essentiality of the coupled O:HO bond theory but also the feasibility of engineering water and solutions by programmed electrification.

Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

Novel All-Nitrogen Molecular Crystals of Aromatic N10

Nitrogen has unique bonding ability to form single, double, and triple bonds, similar to that of carbon. However, a molecular crystal formed by an aromatic polynitrogen similar to a carbon system has not been found yet. Herein, a new form of stable all-nitrogen molecular crystals consisting of only bispentazole N10 molecules with exceedingly high energy density is predicted. The crystal structures and the conformation of N10 molecules are strongly correlated, both depending on the applied external pressure. These molecular crystals can be recovered upon the release of the pressure. The first-principles molecular dynamics simulations reveal that these all-nitrogen materials decompose at temperatures much higher than room temperature. The decompositions always start from breaking off N2 molecules from the nitrogen ring and can release a large amount of energy. These new polynitrogens are aromatic and are more stable than all the other polynitrogen crystals reported previously, providing a new green strategy to get all-nitrogen, nonpolluting high energy density materials without introducing any metal or other guest stabilizer.

Formation mechanism of insensitive tellurium hexanitride with armchair-like cyclo-N6 anions

The lower decomposition barriers of cyclo-N6 anions hinder their application as high-energy-density materials. Here, first-principles calculations and molecular dynamics simulations reveal that enhancing the covalent component of the interaction between cyclo-N6 anions and cations can effectively improve the stability of cyclo-N6 anions. Taking tellurium hexanitride as a representative, the exotic armchair-like N6 anions of tellurium hexanitride exhibit resistance towards electronic attack and gain extra stability through the formation of covalent bonds with the surrounding elemental tellurium under high pressures. These covalent bonds effectively improve the chemical barrier and insensitivity of tellurium hexanitride during blasting, which prevents the decomposition of solid cyclo-N6 salts into molecular nitrogen. Furthermore, the high-pressure induced covalent bonds between cyclo-N6 anions and tellurium enable the high bulk modulus, remarkable detonation performance, and high-temperature thermodynamic stability of tellurium hexanitride.

Unraveling the Mechanism of cyclo-N5- Production through Selective C-N Bond Cleavage of Arylpentazole with Ferrous Bisglycinate and m-Chloroperbenzonic Acid: A Theoretical Perspective

Very recently, the bulk synthesis of cyclo-N₅^- from arylpentazole through the treatment with m-chloroperbenzonic acid (m-CPBA) and ferrous bisglycinate ([Fe(Gly)₂]) (Zhang, C., et al. Science 2017, 355, 374) has greatly promoted the application of pentazolate anion as a novel high-performance energetic material. Yet the mechanism for this reaction is still unexplored. Herein we perform mechanistic studies on the selective C-N bond cleavage in arylpentazole by using density functional theory methods. The direct C-N bond activation by m-CPBA was computed to be kinetically inaccessible. Instead, the oxidation of [Fe(Gly)₂] by m-CPBA is much favorable, which leads to the generation of a high-valent iron(IV)-oxo product. The Fe(IV)-oxo intermediate has been examined by UV-vis absorption spectra experiments and further verified by excited-state calculations. It is found that the Fe(IV)-oxo serves as the key intermediate for the C-N bond activation of arylpentazole and the cyclo-N₅^- generation. Our calculations clarified the key mechanistic details of the cyclo-N₅^- generation, and the factors that affect the production yield are further discussed.

Molecular and Crystal Features of Thermostable Energetic Materials: Guidelines for Architecture of "Bridged" Compounds

Extensive density functional theory (DFT) calculation and data analysis on molecular and crystal level features of 60 reported energetic materials (EMs) allowed us to define key descriptors that are characteristics of these compounds' thermostability. We see these descriptors as reminiscent of "Lipinski's rule of 5", which revolutionized the design of new orally active pharmaceutical molecules. The proposed descriptors for thermostable EMs are of a type of molecular design, location and type of the weakest bond in the energetic molecule, as well as specific ranges of oxygen balance, crystal packing coefficient, Hirshfeld surface hydrogen bonding, and crystal lattice energy. On this basis, we designed three new thermostable EMs containing bridged, 3,5-dinitropyrazole moieties, HL3, HL7, and HL9, which were synthesized, characterized, and evaluated in small-scale field detonation experiments. The best overall performing compound HL7 exhibited an onset decomposition temperature of 341 °C and has a density of 1.865 g cm-3, and the calculated velocity of detonation and maximum detonation pressure were 8517 m s-1 and 30.6 GPa, respectively. Considering HL7's impressive safety parameters [impact sensitivity (IS) = 22 J; friction sensitivity (FS) = 352; and electrostatic discharge sensitivity (ESD) = 1.05 J] and the results of small-scale field detonation experiments, the proposed guidelines should further promote the rational design of novel thermostable EMs, suitable for deep well drilling, space exploration, and other high-value defense and civil applications.

Prediction of stable energetic beryllium pentazolate salt under ambient conditions

The most stable BeN₁₀ salt was directly predicted at atmospheric pressure, and the energy density is up to 5.36 kJ g⁻¹.

Energetic Butterfly: Heat-Resistant Diaminodinitro trans-Bimane

Due to a significant and prolific activity in the field of design and synthesis of new energetic molecules, it becomes increasingly difficult to introduce new explosophore structures with attractive properties. In this work, we synthesized a trans-bimane-based energetic material—3,7-diamino-2,6-dinitro-1H,5H-pyrazolo-[1,2-a]pyrazole-1,5-dione (4), the structure of which was comprehensively analyzed by a variety of advanced spectroscopic methods and by X-ray crystallo-graphy (with density of 1.845 g·cm⁻³ at 173 K). Although obtained crystals of 4 contained solvent molecules in their structure, state-of-the-art density functional theory (DFT) computational techniques allowed us to predict that solvent-free crystals of this explosive would preserve a similar tightly packed planar layered molecular arrangement, with the same number of molecules of 4 per unit cell, but with a smaller unit cell volume and therefore higher energy density. Explosive 4 was found to be heat resistant, with an onset decomposition temperature of 328.8 °C, and was calculated to exhibit velocity of detonation in a range of 6.88–7.14 km·s⁻¹ and detonation pressure in the range of 19.14–22.04 GPa, using for comparison both HASEM and the EXPLO 5 software. Our results indicate that the trans-bimane explosophore could be a viable platform for the development of new thermostable energetic materials.

A Study of the Shock Sensitivity of Energetic Single Crystals by Large-Scale Ab Initio Molecular Dynamics Simulations

Understanding the reaction initiation of energetic single crystals under external stimuli is a long-term challenge in the field of high energy density materials. Herewith, we developed an ab initio molecular dynamics method based on the multiscale shock technique (MSST) and reported the reaction initiation mechanism by performing large-scale simulations for the sensitive explosive benzotrifuroxan (BTF), insensitive explosive triaminotrinitrobenzene (TATB), four polymorphs of hexanitrohexaazaisowurtzitane (CL-20) pristine crystals and five novel CL-20 cocrystals. A theoretical indicator, t_initiation, the delay of decomposition reaction under shock, was proposed to characterize the shock sensitivity of energetic single crystal, which was proved to be reliable and satisfactorily consistent with experiments. We found that it was the coupling of heat and pressure that drove the shock reaction, wherein the vibrational spectra, the specific heat capacity, as well as the strength of the trigger bonds were the determinants of the shock sensitivity. The intermolecular hydrogen bonds were found to effectively buffer the system from heating, thereby delaying the decomposition reaction and reducing the shock sensitivity of the energetic single crystal. Theoretical rules for synthesizing novel energetic materials with low shock sensitivity were given. Our work is expected to provide a useful reference for the understanding, certifying and adjusting of the shock sensitivity of novel energetic materials.

Mechanism and Functionality of Pnictogen Dual Aromaticity in Pentazolate Crystals

Our recent work (J. Phys. Chem. Lett. 2019, 10, 2378) reported the discovery of the abnormal pnictogen dual aromaticity (π and σ) in cyclo-N₅ ^- , which makes the anion unstable in nature but confers enhanced stability in sufficiently acid solution. Herein, we present systematic quantum calculations on the structures, energetics and dynamics of the pentazolate salt and metal pentazolate hydrates, focusing on the mechanism and functionality of the pnictogen dual aromaticity in these crystals, which are verified by experiments. We find that owning a net charge of -e is crucial to the formation of the dual aromaticity and the stabilization of the cyclo-N₅ ^- . The competition between the dual aromaticity and the proton affinity drives the cyclo-N₅ ^- to be unreactive to acid and remain unprotonated in these crystals. We decompose the crystal packing effect into pure mechanical compression and interspecies nonbonding interactions, and figure out that the type and number of the adjacent counterions of the cyclo-N₅ ^- anion, instead of the compression effect, accounts for the protonation state reversion in the vacuum and in the crystal. The current work supports our original conclusion (Science 2018, 359, eaas8953) and is expected to provide compelling evidence against the current debate on the cyclo-N₅ ^- stability (Science 2018, 359, eaao3672; J. Phys. Chem. Lett. 2018, 9, 7137; J. Am. Chem. Soc. 2019, 141, 2984).