“Green” Pyrotechnics: A Chemists' Challenge

Fused Triazole-Tetrazine Assembled with Different Functional Moieties: Construction of Multipurpose Energetic Materials

Azido, amino, and azo functionalities were introduced into tetrazine backbones to access multifunctional energetic materials. AzNTT demonstrates effective initiation capability (MPC = 50 mg), whereas NTTA balances well between the energy and stability. Azo-functionalized BNTTD has a high density of 1.908 g cm-3, with performance comparable to that of the benchmark material HMX. This work underscores the scope of energetic functionalization and the outstanding comprehensive performance of polycyclic tetrazines.

Convergent Paired Electrolysis for [3+2] Cycloaddition of Azidotrimethylsilane with N-Heterocycles

A widely used method to obtain tetrazoles is through the azide and nitrile [3+2] cycloaddition. However, this process often involves using non-recyclable transition metals or Lewis acid catalysts and stoichiometric amounts of oxidants and additives, which reduces atom efficiency. We have discovered a convergent paired electrochemical reaction to perform this cycloaddition reaction, without the need for metal catalysts or oxidants. This tetrazolation strategy uses azidotrimethylsilane (TMSN3) and N-heterocycles in an undivided cell at a constant current. We use a mixture of CH3CN and equivalent amounts of H2O as co-solvent at room temperature. It is crucial to produce a stoichiometric amount of active hydroxyl ions through the cathodic reduction of water. Cyclic voltammetry (CV) studies and control experiments confirm that the cycloaddition reaction is specific to the electrode electron transfer process, eliminating the need for a mediator to shuttle electrons. This metal- and oxidant-free strategy is highly compatible with different functional groups and produces products with moderate to good yields. We have successfully tetrazolated bioactive compounds at a late stage, scaled up batches efficiently, and synthesized free amino-containing N-heterocycles via denitrogenation of tetrazoles.

Synthesis and Characterization of High-Energy Anti-Perovskite Compounds Cs3X[B12H12] Based on Cesium Dodecahydro-Closo-Borate with Molecular Oxoanions (X- = [NO3]-, [ClO3]- and [ClO4]-)

Three novel anti-perovskite compounds, formulated as Cs3X[B12H12] (X- = [NO3]-, [ClO3]-, and [ClO4]-), were successfully synthesized through the direct mixing of aqueous solutions containing Cs2[B12H12] and CsX (X-: [NO3]-, [ClO3]-, [ClO4]-), followed by isothermal evaporation. All three compounds crystallize in the orthorhombic space group Pnma, exhibiting relatively similar unit-cell parameters (e.g., Cs3[ClO3][B12H12]: a = 841.25(5) pm, b = 1070.31(6) pm, c = 1776.84(9) pm). The crystal structures were determined using single-crystal X-ray diffraction, revealing a distorted hexagonal anti-perovskite order for each. Thermal analysis indicated that the placing oxidizing anions X- into the 3 Cs+ + [B12H12]2- blend leads to a reduction in the thermal stability of the resulting anti-perovskites Cs3X[B12H12] as compared to pure Cs2[B12H12], so thermal decomposition commences at lower temperatures, ranging from 320 to 440 °C. Remarkably, the examination of the energy release through DSC studies revealed that these compounds are capable of setting free a substantial amount of energy, up to 2000 J/g, upon their structural collapse under an inert-gas atmosphere (N2). These three compounds represent pioneering members of the first ever anti-perovskite high-energy compounds based on hydro-closo-borates.

Metal Salts of 4-Chloro-3,5-dinitropyrazole for Promising Eco-Friendly Primary Colors Pyrotechnics

The construction and design of pyrotechnics with superior performance is not only a task of great significance but also a tremendous challenge. In this regard, we present the syntheses of novel green primary colors pyrotechnics (red, green, and blue light-generating pyrotechnics) by employing 4-chloro-3,5-dinitropyrazole (CDNP) as a multifunctional raw material. CDNP contains a flame enhancer, oxygen-rich functional group, and nitrogen heterocyclic combustibles, which contribute to the high performance of the pyrotechnics. The characteristic elements (strontium, barium, and copper) that impart color to the flame are combined with the CDNP to synthesize the primary colors pyrotechnics by an "all-in-one" strategy. The structures of three energetic metal salts (EMS-1, EMS-2, and EMS-3) are completely characterized, and their thermal stability, sensitivity, ignition performance, and color purity are systematically evaluated. All EMSs show excellent thermal stability and low mechanical sensitivities (>330 °C, >40 J, >360 N). Moreover, the EMSs demonstrate successful ignition and combustion under laser conditions and roasting test conditions, producing bright characteristic flames. Chromaticity analysis reveals that the three EMSs possess good color purities of 91, 80, and 70%, respectively. Consequently, the three integrated pyrotechnics exhibit exceptional combustion properties, highlighting their potential for use in various pyrotechnic applications.

Evaluation and testing of emission on Sargassum wightii-based flash powder composition

Fireworks that are used in our day-to-day life are also becoming a part of greenhouse emission in the environment. Hence, it becomes the need of the hour to act immediately to reduce environmental pollution so that the future becomes safer. The objective of the current research work focuses on the reduction of pollution caused due to the emission from burning fireworks and this work is intended to reduce the sulphur emissions from crackers. One of the important ingredients used in pyrotechnics is flash powder. The traditional flash powder composition contains aluminium powder as fuel, potassium nitrate as an oxidizer and sulphur as an igniter at defined levels. To reduce the impact of sulphur emissions in flash powder, an organic compound like Sargassum wightii brown seaweed powder is used as a replacement at prescribed levels and the experimentation is carried out. It is found that the sulphur weightage in the flash powder composition can be replaced up to 50% with the Sargassum wightii brown seaweed powder without affecting the traditional performance of the flash powder. To study the emissions taking place in the flash powder composition, a specially designed flash powder emission testing chamber has been developed. Three different compositions of flash powder compositions were made and they are named SP (no seaweed powder), SP5 (5% seaweed powder) and SP10 (10% seaweed powder) respectively as per the use of Sargassum wightii seaweed powder in the traditional flash powder composition. During testing, it has been found that the emission of sulphur has been reduced up to a maximum of 17% in SP% and 24% in SP10 flash powder composition. It is also evident that the flash powder composition with Sargassum wightii can result in a reduction in toxic sulphur emission by up to 21% on the modified flash powder. Furthermore, it has been found that the auto-ignition temperature of the existing and modified flash powder compositions was in the range of 353-359 °C, 357-363 °C and 361-365 °C for SP, SP5 and SP10 compositions respectively. FTIR analysis proved the presence of carbon, sulphur and nitro compounds in the Sargassum wightii seaweed powder.

From the laboratory to the field: Chemical analysis of colored smoke pyrotechnic formulations via mass spectrometry techniques

Smoke dyes are complex molecular systems that have the potential to form many molecular derivatives and fragments when deployed. The chemical analysis of smoke samples is challenging due to the adiabatic temperature of the pyrotechnic combustion and the molecular complexity of the physically dispersed reaction products. Presented here is the characterization of the reaction byproducts of a simulant Mk124 smoke signal on a multigram scale, which contain the dye disperse red 9 (1-(methylamino)anthraquinone), by ambient ionization mass spectrometry. Our previous work has examined the thermal decomposition of a simplified smoke system consisting of disperse red 9, potassium chlorate, and sucrose by anaerobic pyrolysis gas chromatography mass spectrometry performed at the laboratory milligram scale. The results from the lab scale test were compared with a fully functioned Mk124 in the field. To achieve this, Mk124 smokes were functioned in the presence of sampling swabs that collected byproduct residues from the smoke plume in the ambient environment. These swabs were then analyzed using ambient ionization mass spectrometry to identify the expended pyrotechnic residues, with particular interest in halogenated species. Previous work determined the toxicity of unforeseen byproducts identified on the laboratory scale, which were also detected in the field demonstrating the correlation of the laboratory testing to the fielded systems. By understanding the chemical composition of smokes and their reaction products, potential toxicity effects can be easily assessed, leading to safer formulations with improved performance. These results can help assess how smoke byproducts may impact Warfighter performance, personnel health, and the environment.

'Green firecrackers' with reduced barium emissions in particulate matter

Ambient air quality is affected due to the emission of pollutants on a large scale after the bursting of firecrackers. Traditionally in all firecrackers, barium (Ba) compounds are used as oxidizers and also to impart green colour flame. Combustion products of barium compounds are water soluble and readily absorbed by the body affecting human health. Thus, the inherent risk of Ba pollution due to the bursting of firecrackers has consequent health effects. To reduce the ambient air pollution caused due to burning of conventional firecrackers, CSIR NEERI has developed reduced emission firecrackers (green crackers). This is achieved by reducing the amount of chemicals, barium nitrate, shell size and addition of additives such as zeolite and iron oxide. This study aims to specifically investigate the influence of additives on the level of barium in reduced emission firecrackers. Four types of conventional and reduced emission firecrackers were selected and tested inside a firecracker emission testing facility to check the levels of barium in PM₁₀ and PM_2.5. The measured mean concentrations of all types of reduced emission crackers (green crackers) provided by fireworks manufacturers show significantly reduced barium concentration by 30-60% compared to conventional crackers depending on the type of firecrackers, shell size and amount of chemicals used. The possible reason for reduced Ba level is attributed to i) reduced usage of Ba(NO₃)₂ and ii) formation of heavy density compounds, leading to soil fallout.

Lithium-Promoted Formation of (M = N2H5+ or NH3OH+ and = Anion of 1-Hydroxytetrazole-5-hydrazide)-Type "Quaternary" Complexes with Nitrogen-Rich Characteristics: Construction of Novel Insensitive Energetic Materials

Lithium-based nitrogen-rich complexes are important research objects in the field of high-energy materials. However, the weak coordination abilities of lithium ions relative to those of other metal ions with greater atomic numbers have hindered their applications in the field of nitrogen-rich complexes. Herein, we successfully prepared novel lithium-based nitrogen-rich complexes (N₂H₅-2AZTO-Li and NH₃OH-2AZTO-Li) by exploiting the structural properties of 1-hydroxytetrazolium-5-hydrazine (HAZTO). Both N₂H₅-2AZTO-Li and NH₃OH-2AZTO-Li were found to exhibit physicochemical parameters (including the density, stability, and energetic properties) that were intermediate between those of the simple ionic compounds (3 and 4) and the complexes (5) that formed them, enabling a favorable balance between high energy, high stability, and environmental friendliness (for N₂H₅-2AZTO-Li: detonation velocity (D) = 9005 m s^-1, detonation pressure (P) = 35.5 GPa, decomposition temperature (T_dec) = 238.1 °C, impact sensitivity (IS) = 24 J, friction sensitivity (FS) = 210 N, and detonation product (DP) (CO) < 2%; for NH₃OH-2AZTO-Li: D = 9028 m s^-1, P = 35.7 GPa, T_dec = 211.2 °C, IS = 20 J, FS = 180 N, and DP (CO) < 2%). This study transcends the conventional structural forms of nitrogen-rich complexes, opening new horizons for the design of novel insensitive energetic materials.

Chemical characterization of PM10 and PM2.5 combusted firecracker particles during Diwali of Lucknow City, India: air-quality deterioration and health implications

Urban air pollution is a growing menace leading to human discomfort, increased hospitalizations, morbidity, and mortality. This study deals with deteriorated air quality due to firecracker bursting during Diwali in Lucknow. Inhalable particulates and gaseous pollutants were monitored during Diwali 2020 using air samplers. Elements, ions, and surface morphology of particles were analyzed using ICP-MS, ion chromatograph, and SEM-EDX, respectively. PM₁₀, PM_2.5, SO₂, and NO₂ were 558, 352, 44, and 86 μg/m³ during Diwali night and 233, 101, 17, and 40 μg/m³ on pre-Diwali night while 241, 122, 24, and 43 μg/m³ on Diwali day. Concentrations surged for PM₁₀: 139% and 132%, PM_2.5: 249% and 189%, SO₂: 159% and 83%, and NO₂: 115% and 100% on Diwali night compared to pre-Diwali night and corresponding Diwali day, respectively. Al, K, Ba, and B showed dominance in PM₁₀ whereas Zn, Al, Ba, and K in PM_2.5 on Diwali night. The order of metal abundance in PM_2.5 was Cd < Co < Ag < As < Cr < Ni < Cu < Bi < Pb < Mn < Sr < Fe < B < Zn < Al < Ba < K. Cations NH₄⁺, K⁺, Mg²⁺, Ca²⁺, and anions F^-, Cl^-, NO₃^-, Br^-, NO₂^-, SO₄^-2, PO₄^3- showed a 2-8 fold increase on Diwali night relative to pre-Diwali night. Average metal concentrations varied by 2.2, 1.6, and 0.09 times on Diwali than pre-Diwali in residential, commercial, and industrial areas, respectively. PM₁₀ concentration increased by 458% and 1140% while PM_2.5, 487%, and 2247% than respective NAAQS and WHO standards. Tiny firecracker particles vary in toxicity as compared to vehicular emissions and have enhanced bioavailability leading to severe threat in terms of LRI, COPD, and atherosclerosis for city dwellers. It is imperative to recognize the present status of ambient air quality and implement regulatory strategies for emission reduction.

Mechanism, Kinetics and Thermodynamics of Decomposition for High Energy Derivatives of [1,2,4]Triazolo[4,3-b][1,2,4,5]tetrazine

This paper presents the data of research studies on the mechanisms, kinetics and thermodynamics of decomposition of three high-energy compounds: [1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine-3,6-diamine (TTDA), 3-amino-6-hydrazino[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine (TTGA) and 3,6-dinitroamino[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazine (DNTT). The points of change of the reaction mechanisms under thermal effects with different intensities from 0.1 to 2000 s⁻¹ have been established. The values of activation and induction energies for the limiting stages of decomposition have been obtained. The formation of nanostructured carbon nitride (α-C₃N₄) in condensed decomposition products, cyanogen (C₂N₂) and hydrogen cyanide (HCN) in gaseous products have been shown. Concentration-energy diagrams for the reaction products have been compiled. The parameters of heat resistance and thermal safety proved to be: 349.5 °C and 358.2 °C for TTDA; 190.3 °C and 198.0 °C for TTGA; 113.4 °C and 114.1 °C for DNTT. The energy and thermodynamic properties have also been estimated. This work found the activation energy of the decomposition process to be 129.0 kJ/mol for TTDA, 212.2 kJ/mol for TTGA and 292.2 kJ/mol for DNTT. The average induction energy of the catalytic process (Ecat) for TTGA was established to be 21 kJ/mol, and for DNTT-1500–1700 kJ/mol. The induction energy of the inhibition process (Eing) of TTDA was estimated to be 800–1400 kJ/mol.

Macrocycle as a "Container" for Dinitramide Salts

Dinitramic acid salts are promising components as oxidizers and burning-rate modifiers of high-energy compositions. However, most of these salts are not free of drawbacks such as hygroscopicity. Therefore, their application under special conditions of use and storage is limited. The synthesis and storage of stable dinitramic acid salts is a topical issue. Here, we synthesized an adduct starting from the nickel salt of dinitramic acid with carbohydrazide and glyoxal to settle the problem of stability and storage of that salt. The chemical composition of the adduct was confirmed by infrared spectroscopy and elemental analysis. The Ni content was determined by atomic emission spectroscopy. Thermogravimetric DSC and TGA analyses showed the adduct to have three decomposition stages. The adduct exhibits a good thermal stability and a low sensitivity to mechanical stimuli. Here, the adduct is demonstrated to be a promising burning-rate inhibitor of pyrotechnic compositions.

Controllable Structural Modulation: Assembling Variable Dimension Energetic Metal-Organic Frameworks via Free Protons

Herein, we provide an efficient strategy for constructing three-dimensional (3D) energetic coordination polymers (ECPs), namely, metal-organic frameworks (EMOFs), avoiding solvent coordination without changing the organic ligands or metal nodes. Three ECPs with the same ligand and metal center, namely, two-dimensional (2D) layer ECP [Pb(HOBTT)(H₂O)₂]_n (1), 3D solvent-free EMOFs [Pb(HOBTT)]_n (2), and dense [Pb₃(OBTT)₂]_n (3) (H₃OBTT = 4,5-bis(1-hydroxytetrazol-5-yl)-2H-1,2,3-triazole), were rationally designed and synthesized via free protons. As expected, the theoretical density of 3 (4.080 g·cm^-3) is greater than those of 2 (3.299 g·cm^-3) and 1 (3.055 g·cm^-3). Thermal stabilities indicate that their decomposition temperature exceeds 300 °C. Theoretical calculations show that the detonation performance of 3 is better than that of 1 and 2. The detonation performance of 1-3 was further proven by laser irradiation.

Access to Green Pyrotechnic Compositions via Constructing Coordination Polymers: A New Approach to the Application of 3,4-Dinitropyrazole

Alkali and alkaline-earth metal salts with 3,4-dinitropyrazole (DNP) were synthesized by the reaction of DNP with stoichiometric amounts of the corresponding metal hydroxide-, oxide-, or carbonate-based highly pure salts, and products were fully characterized. Determination of single-crystal structures of all new complexes except for the lithium and strontium salts was performed by X-ray diffraction techniques. The cesium salt crystallized no water among them. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) manifested that these salts have satisfactory thermal stabilities with decomposition temperatures above 210 °C. They also showed that there exists strong bonding of crystallized water among lattices, which disappeared at temperatures equal to or above 115 °C except for salts MES-3, MES-4, and MES-9. In addition, the percentage of water contents was confirmed by using DSC and TGA methods. The constant-volume combustion heats of these metal salts containing DNP anions were measured using an oxygen bomb calorimeter due to their expectant interest in energetic materials, and their standard molar formation enthalpies were obtained. The investigated salts were found to be insensitive toward friction and impact. Findings of burning tests performed with experimental formulations using MES-1, MES-7, MES-8, and MES-9 certify that these four salts might be more promising candidates for application in green pyrotechnics.

Approaches to 1,4-Disubstituted Cubane Derivatives as Energetic Materials: Design, Theoretical Studies and Synthesis

Novel 1,4-disubstituted cubane derivatives have been designed and selected ones have been successfully synthesized and characterized by various analytical and spectroscopic techniques, including single-crystal X-ray analysis. A detailed computational study at B3LYP/6-311++G(d,p) level of theory revealed that all newly designed 1,4-disubstituted cubane derivatives possess higher densities, higher density-specific impulse and superior ballistic properties when compared to conventional fuels, for example, RP-1. These compounds also exhibit acceptable kinetic and thermodynamic stabilities which were evaluated in terms of their HOMO-LUMO energy gap and bond dissociation energies, respectively, and are superior to TEX and many other compounds containing explosophoric groups. These results provide novel insights into the possible application of cubane-based energetic materials.

Detection and toxicity modeling of anthraquinone dyes and chlorinated side products from a colored smoke pyrotechnic reaction

"Green" pyrotechnics seek to remove known environmental pollutants and health hazards from their formulations. This chemical engineering approach often focuses on maintaining performance effects upon replacement of objectionable ingredients, yet neglects the chemical products formed by the exothermic reaction. In this work, milligram quantities of a lab-scale pyrotechnic red smoke composition were functioned within a thermal probe for product identification by pyrolysis-gas chromatography-mass spectrometry. Thermally decomposed ingredients and new side product derivatives were identified at lower relative abundances to the intact organic dye (as the engineered sublimation product). Side products included chlorination of the organic dye donated by the chlorate oxidizer. Machine learning quantitative structure-activity relationship models computed impacts to health and environmental hazards. High to very high toxicities were predicted for inhalation, mutagenicity, developmental, and endocrine disruption for common military pyrotechnic dyes and their analogous chlorinated side products. These results underscore the need to revise objectives of "green" pyrotechnic engineering.

Crystal structure of silver pentazolates AgN5 and AgN6

Silver pentazolate, a high energy density compound containing the cyclo-N₅^- anion, has recently been synthesized under ambient conditions. However, due to high sensitivity to irradiation, its crystal structure has not been determined. In this work, silver-nitrogen crystalline compounds under ambient conditions and at high pressures, up to 100 GPa, are predicted and characterized by performing first-principles evolutionary crystal structure searching with variable stoichiometry. It is found that newly discovered AgN₅ and AgN₆ are the only thermodynamically stable silver-nitrogen compounds at pressures between 42 and 80 GPa. In contrast to AgN₅, the pentazolate AgN₆ compound contains N₂ diatomic molecules in addition to cyclo-N₅^-. These AgN₅ and AgN₆ crystals are metastable under ambient conditions with positive formation enthalpies of 54.95 kJ mol^-1 and 46.24 kJ mol^-1, respectively. The underlying cause of the stability of cyclo-N₅^- silver pentazolates is the enhanced aromaticity enabled by the charge transfer from silver atoms to nitrogen rings. To aid in the experimental identification of these materials, calculated Raman spectra are reported at ambient pressure: the frequencies of N₅^- vibrational modes of AgN₅ are in good agreement with those measured in the experiment.

Isostructural rubidium and caesium 4-(3,5-di-nitro-pyrazol-4-yl)-3,5-di-nitro-pyrazolates: crystal engineering with polynitro energetic species

In the structures of the title salts, poly[[μ4-4-(3,5-di-nitro-pyrazol-4-yl)-3,5-di-nitro-pyrazol-1-ido]rubidium], [Rb(C6HN8O8)] n , (1), and its isostructural caesium analogue [Cs(C6HN8O8) n , (2), two independent cations M1 and M2 (M = Rb, Cs) are situated on a crystallographic twofold axis and on a center of inversion, respectively. Mutual inter-molecular hydrogen bonding between the conjugate 3,5-dinito-pyrazole NH-donor and 3,5-di-nitro-pyrazole N-acceptor sites of the anions [N⋯N = 2.785 (2) Å for (1) and 2.832 (3) Å for (2)] governs the self-assembly of the translation-related anions in a predictable fashion. Such one-component modular construction of the organic subtopology supports the utility of the crystal-engineering approach towards designing the structures of polynitro energetic materials. The anionic chains are further linked by multiple ion-dipole inter-actions involving the 12-coordinate cations bonded to two pyrazole N-atoms [Rb-N = 3.1285 (16), 3.2261 (16) Å; Cs-N = 3.369 (2), 3.401 (2) Å] and all of the eight nitro O-atoms [Rb-O = 2.8543 (15)-3.6985 (16) Å; Cs-O = 3.071 (2)-3.811 (2) Å]. The resulting ionic networks follow the CsCl topological archetype, with either metal or organic ions residing in an environment of eight counter-ions. Weak lone pair-π-hole inter-actions [pyrazole-N atoms to NO2 groups; N⋯N = 2.990 (3)-3.198 (3) Å] are also relevant to the packing. The Hirshfeld surfaces and percentage two-dimensional fingerprint plots for (1) and (2) are described.

Low-perchlorate blue-flame pyrotechnic compositions

An ammonium perchlorate (AP) and copper(II) benzoate pyrotechnic blue-flame composition was gradually “diluted” by adding an experimental perchlorate-free blue-flame composition based on aminoguanidinium nitrate (AGN), malachite, PVC powder and shellac resin. Flame’s luminous intensity and specific luminous intensity were recorded and analyzed. A copper-aminoguanidinium (CuAG) complex was also synthesized and tested as an energetic additive in perchlorate-free blue-flame composition. Green-flame color was observed when testing chlorine-free energetic compositions with CuAG.

Novel metal-organic frameworks assembled from the combination of polynitro-pyrazole and 5-nitroamine-1,2,4-oxadiazole: synthesis, structure and thermal properties

Energetic metal organic frameworks (EMOFs) is a hot topic in the field of energetic materials research. This paper reports two kinds of EMOFs based on methylene-linked polynitropyrazole and nitroamine 1,2,4-oxadiazole. Their structures were fully characterized by crystallography and their detonation performance and stability performance were explored. The results showed that the crystals of compounds 4 and 5 exhibited a 3D stacking phenomenon due to the action of a large number of hydrogen bonds and coordination bonds inside the crystal. In terms of stability, both 4 and 5 showed good thermal stability (T_SADT (4) = 204.4 °C and T_SADT (5) = 216.2 °C), but due to the difference in the number of energetic groups (-NO₂), the sensitivity of 4 (IS = 6.0 J and FS = 100 N) to mechanical stimuli is significantly lower than that of compound 5 (IS = 1.2 J and FS = 40 N). In terms of energy performance, it is this great advantage in the number of energetic groups that makes compound 5's (D_v = 8.059 km s^-1 and P = 30.9 GPa) detonation performance superior to that of 4 (D_v = 7.704 km s^-1 and P = 26.9 GPa). This research broadens the horizon for the development of EMOFs based on polynitropyrazole derivatives.

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.

Synthesis and characterization of promising insensitive energetic salts based on 3-amino-5-hydrazinopyrazole

The development of green energetic materials is based on environmental friendliness, safety and performance improvement. It is of great significance to design and synthesize new nitrogen rich salts for a new generation of green energetic materials. In the present work, a series of 3-amino-5-hydrazinopyrazole energetic salts comprising energetic anions were synthesized and were characterized using elemental analysis, IR spectroscopy and differential scanning calorimetry (DSC). Compounds 1-5 were further confirmed by single crystal X-ray diffraction and the sensitivities were measured by the standard BAM methods. Additionally, the structure-property relationship was elucidated from the experimental results and theoretical calculations. Energetic salts of 2 and 5 exhibited high heat of formation (5, 1160.06 kJ mol-1), high decomposition temperature (2, 172 °C; 5, 186 °C), excellent detonation performance (2, Dv, 9076 m s-1, P 34.1 GPa; 5, Dv, 8974 m s-1, P 31.9 GPa), moderate sensitivity towards outer stimuli and high nitrogen contents (2, 41.03%; 5, 63.84%). This work increases future prospects for the design of insensitive and novel high-energy green energetic material.

Crystallographic study of the energetic salt 1,2,4-triazolium perchlorate

The molecular and crystal structure of 1H-1,2,4-triazolium perchlorate, C₂H₄N₃⁺·ClO₄^-, was determined as detailed crystallographic data had not been available previously. The structure has monoclinic (P2₁/m) symmetry. It is of interest in the field of energetic compounds because nitrogen-rich azoles are the backbone of high-density energetic compounds, and salt-based energetic materials can exhibit preferential energy-release behaviour. The bond angles of the 1,2,4-triazolium cation in this study were similar to those of a cationic triazole ring reported previously and were different from those of the neutral triazole ring. This study contributes to the available data that can be used to analyse the relationship between the structures and properties of energetic materials.

Comparative DFT study of vibrational, electronic, and optical properties of energetic alkali metal salts based on nitrogen-rich 5-aminotetrazole

This article presents a thorough density functional theory based comparative study on nitrogen-rich 5-aminotetrazole alkali metal salts M 5-At (M = Li, K, Rb, Cs). The calculated structural parameters using plane-wave pseudopotential method are consistent with the experimental results. The computed vibrational frequencies at ambient pressure show that vibrational modes in high energy region are due the NH bond of NH₂ group. Pressure variation IR spectra of these materials show clear frequency shifts where Li 5-At shows an overall red shift below 900 cm^-1 contrary to the blue shift seen in other materials in this range. The born effective charge values reveal the presence of strong covalency between N, H, and C atoms whereas an increased ionic nature is seen as the atomic number of metal atoms increases. Furthermore, we used full potential linear augmented plane wave (FP-LAPW) method for calculating electronic structure and optical properties with TB-mBJ potential which provides an enhanced band gap for all materials compared to standard GGA functional. Electronic structure calculation reveals that all the compounds are indirect band gap insulators with the exception of Li 5-At. The computed partial density of states show mixed ionic-covalent nature in metal-N/C bonds and covalent nature in NC bonds. In addition, we are also presenting the optical properties such as real and imaginary dielectric constant, absorption, refraction, reflection, loss spectrum as functions of photon energy. From the optical properties we can conclude that all the studied compounds are optically anisotropic in nature and are good absorbers in the ultraviolet (UV) region.

Laser ignition of energetic complexes: impact of metal ion on laser initiation ability

Alkali metal-containing energetic complexes were easy to initiate, followed by the free ligand, whereas the alkaline-earth metal complexes exhibited longer initiation delay times.

Inorganic oxidizer detection from propellants, pyrotechnics, and homemade explosive powders using gradient elution moving boundary electrophoresis

Advancement in rapid targeted chemical analysis of homemade and improvised explosive devices is critical for the identification of explosives-based hazards and threats. Gradient elution moving boundary electrophoresis (GEMBE), a robust electrokinetic separation technique, was employed for the separation and detection of common inorganic oxidizers from frequently encountered fuel-oxidizer mixtures. The GEMBE system incorporated sample and run buffer reservoirs, a short capillary (5 cm), an applied electric field, and a pressure-driven counterflow. GEMBE provided a separation format that allowed for continuous injection of sample, selectivity of analytes, and no sample cleanup or filtration prior to analysis. Nitrate, chlorate, and perchlorate oxidizers were successfully detected from low explosive propellants (e.g., black powders and black powder substitutes), pyrotechnics (e.g., flash powder), and tertiary explosive mixtures (e.g., ammonium nitrate- and potassium chlorate-based fuel-oxidizer mixtures). Separation of these mixtures exhibited detection without interference from a plethora of additional organic and inorganic fuels, enabled single particle analysis, and demonstrated semiquantitative capabilities. The bulk counterflow successfully excluded difficult components from fouling the capillary, yielding estimated limits of detection down to approximately 10 μmol/L. Finally, nitrate was separated and detected from postblast debris collected and directly analyzed from two nitrate-based charges.

Designing high-performance hypergolic propellants based on materials genome

A new generation of rocket propellants for deep space exploration, ionic liquid propellants, with long endurance and high stability, is attracting more and more attention. However, a major defect of ionic liquid propellants that restricts their application is the inadequate hypergolic reactivity between the fuel and the oxidant, and this defect results in local burnout and accidental explosions during the launch process. We propose a visualization model to show the features of structure, density, thermal stability, and hypergolic activity for estimating propellant performances and their application abilities. This propellant materials genome and visualization model greatly improves the efficiency and quality of developing high-performance propellants, which benefits the discovery of new advanced functional molecules in the field of energetic materials.

New examples of Ru(II)-tetrazolato complexes as thiocyanate-free sensitizers for dye-sensitized solar cells

A set of three new Ru(ii) polypyridyl complexes decorated with 5-aryl tetrazolato ligands (R-CN4)-, (D series, namely D1, D3 and D4), is presented herein. Whereas complex D1 represents the pyrazinyl tetrazolato analogue of a previously reported Ru(ii) complex (D2) with the general formula cis-[(dcbpy)2Ru(N^N)]+, in which dcbpy is 2,2'-bipyridine-4,4'-dicarboxylic acid and N^N is the chelating 2-pyridyl tetrazolato anion, the design of the unprecedented Ru(ii) species D3 and D4 relied upon a completely different architecture. More specifically, the molecular structure of thiocyanate-based species cis-[(dcbpy)2Ru(NCS)2], that is typically found in benchmark Ru(ii) dyes for dye sensitized solar cell (DSSCs), was modified with the replacement of two of the -NCS ligands in favour of the introduction of 5-aryl tetrazolato anions, such as the deprotonated form of 5-(4-bromophenyl)-1H-tetrazole, for complex D3 and 5-(4-cyanophenyl)-1H-tetrazole in the case of complex D4. To streamline the behavior of the D series of Ru(ii) complexes as photosensitizers for DSSCs, an in-depth analysis of the excited state properties of D1, D3 and D4 was performed through TDDFT calculations and TDAS (nanosecond transient difference absorption spectroscopy). The obtained results highlight a trend that was confirmed once D1, D3 and D4 were tested as photosensitizers for DSSC under different conditions. Along the series of the Ru(ii) complexes, the neutrally charged species D3 and D4 displayed the best photovoltaic performances.

Trace Detection and Chemical Analysis of Homemade Fuel-Oxidizer Mixture Explosives: Emerging Challenges and Perspectives

The chemical analysis of homemade explosives (HMEs) and improvised explosive devices (IEDs) remains challenging for fieldable analytical instrumentation and sensors. Complex explosive fuel-oxidizer mixtures, black and smokeless powders, flash powders, and pyrotechnics often include an array of potential organic and inorganic components that present unique interference and matrix effect difficulties. The widely varying physicochemical properties of these components as well as external environmental interferents and background challenge many sampling and sensing modalities. This review provides perspective on these emerging challenges, critically discusses developments in sampling, sensors, and instrumentation, and showcases advancements for the trace detection of inorganic-based explosives.

Synthesis, crystal structure, and thermal properties of Ni(NH3)4(AFT)2

Ni(NH3)4(AFT)2 [NiC6H16N18O2, AFT = 4-amino-3-(5-tetrazolate)furazan] is synthesized and characterized by elemental analysis and Fourier-transform infrared spectroscopy for the first time. X-ray diffraction measurements are used to determine the crystal structure of compound 1. The results demonstrate that compound 1 crystallized in the orthorhombic crystal system. The nickel(II) ion is six-coordinated by N atoms from two AFT-ligands and four NH3 molecules. Its thermal properties are investigated by differential scanning calorimetry and thermogravimetry-derivative thermogravimetry methods, with the results demonstrating that the differential scanning calorimetry curve exhibits two endothermic and one exothermic processes. The endothermic processes are in the range of 130–510 °C with a peak temperature of 188 °C. The temperature from 230 to 400 °C is the exothermic process in which the peak temperature is 314.58 °C. In addition, Kissinger’s and Ozawa-Doyle’s methods are used for calculating the non-isothermal kinetics parameters. Moreover, the apparent activation energy ( E), safety, and thermal stability parameters ( TSADT, TTIT, Tb) for Ni(NH3)4(AFT)2 are calculated. In addition, the calculated thermodynamic functions ( ∆S≠, ∆H≠, and ∆G≠) for the exothermic decomposition process of Ni(NH3)4(AFT)2 are 55.07 J mol−1 K−1, 196.18 kJ mol−1, and 164.90 kJ mol−1, respectively.

Guanidinium 5,5'-Azotetrazolate: A Colorful Chameleon for Halogen-Free Smoke Signals

A progressive halogen‐free multicolored smoke system to obtain white, red, violet, yellow, green, and blue smoke color is presented. The nitrogen‐rich salt guanidinium 5,5′‐azotetrazolate (GZT), which is usually applied as a gas generator or propellant ingredient, was combined with different smoke dyes (Solvent Red 1, Solvent Violet 47, Solvent Green 3, Solvent Yellow 33). These two‐component smoke mixtures offer a convenient and safe multicolor approach without the need for potassium chlorate or any other hazardous material. The common smoke characteristics with respect to burn time/burn rate, yield factor, transfer rate, as well as energetic properties were determined and compared with classic chlorate‐based formulations currently used. To the best of our knowledge, nothing comparable is known in the literature and a completely new research area in modern pyrotechnics is opened.

Thermochemistry and Initial Decomposition Pathways of Triazole Energetic Materials

A thorough investigation of the initial decomposition pathways of triazoles and their nitro-substituted derivatives has been conducted using the MP2 method for optimization and DLPNO-CCSD(T) method for energy. Different initial thermolysis mechanisms are proposed for 1,2,4-triazole and 1,2,3-triazole, the two kinds of triazoles. The higher energy barrier of the primary decomposition path of 1,2,4-triazole (H-transfer path, ∼52 kcal/mol) compared with that of 1,2,3-triazole (ring-open path, ∼45 kcal/mol) shows that 1,2,4-triazole is more stable, consistent with experimental observations. For nitro-substituted triazoles, more dissociation channels associated with the nitro group have been obtained and found to be competitive with the primary decomposition paths of the triazole skeleton in some cases. Besides, the effect of the nitro group on the decomposition pattern of the triazole skeleton has been explored, and it has been found that the electron-withdrawing nitro group has an opposite effect on the primary dissociation channels of 1,2,4-triazole derivatives and 1,2,3-triazole derivatives.