On Tautomerism and Amphoterism: An In-Depth Structural and Physicochemical Characterization of Ammeline and Some of Its Salts.

Ammeline is a simple, readily available, molecular compound, which has been known for nearly 200 years. Despite that, no proper structural characterization of ammeline has been conducted so far. For this reason, the prevalent tautomeric form of ammeline in the solid remained unknown to this date. In the course of this study, its crystal structure was finally established by single-crystal X-ray diffraction. In this structure, ammeline is exclusively found as its 4,6­diamino-1,3,5-triazin-2(1H)-one tautomer and adopts layered structure with an exceptionally high hydrogen bond density. Ammeline shows an interesting amphoteric behavior. Therefore, the synthesis and structural characterization of some of its salts were carried out to investigate the influence of the protonation degree on its molecular structure. In particular, the crystal structure of silver ammelinate monohydrate was solved as the first reported structure containing deprotonated ammeline. Moreover, the crystal structures of three different modifications of ammelinium perchlorate were elucidated and the transformation conditions between them were studied. Lastly, the crystal structure of ammelinediium diperchlorate monohydrate, containing unprecedented doubly protonated ammeline, was determined. The products' thermal behavior was studied by differential thermal analysis and thermogravimetric analysis. The perchlorate salts were additionally examined for their potential as insensitive high-energy-density materials.

1- and 2-Tetrazolylacetonitrile as Versatile Ligands for Laser Ignitable Energetic Coordination Compounds.

1- and 2-Tetrazolylacetonitrile (1- and 2-TAN) have been synthesized by the reaction of chloroacetonitrile with 1H-tetrazole under basic conditions. They further were reacted with sodium azide in the presence of zinc(II) chloride to form 5-((1H-tetrazol-1-yl)methyl)-1H-tetrazole (1-HTMT) and 5-((2H-tetrazol-2-yl)methyl)-1H-tetrazole (2-HTMT). The nitrogen-rich compounds have been applied as ligands for Energetic Coordination Compounds (ECCs) and show interesting coordinative behavior due to different bridging modes. The structural variability of the compounds has been proved by low-temperature X-ray analysis. The ECCs were analyzed for their sensitivities to provide information about the safety of handling and their capability to serve as primary explosives in detonator setups to replace the commonly used lead styphnate and azide. All colored ECCs were evaluated for their ignitability by laser initiation in translucent polycarbonate primer caps. In addition, the spin-crossover characteristics of [Fe(1-TAN)6](ClO4)2 were highlighted by the measurement of the temperature-dependent susceptibility curve.

Synthesis of Bridged Tetrazoles with Promising Properties and Potential Applications by a One-Step Finkelstein Reaction.

Numerous nitramine bridged compounds which show promising combinations of properties have already been identified in the area of energetic materials. In this work, four new nitrazapropane bridged tetrazoles, as well as four new trinitrazaheptane tetrazoles and three oxapropane bridged tetrazoles were synthesized and fully characterized. These new compounds can all be synthesized by a simple, one-step synthesis using Finkelstein conditions. All of these new energetic materials were characterized using NMR spectroscopy, single crystal X-ray diffraction, vibrational analysis and elemental analysis. The thermal behaviour of these compounds was studied by differential thermal analysis (DTA) and partly by thermogravimetric analysis (TGA). The BAM standard method was used to determine the sensitivities towards impact (IS) and friction (FS). The enthalpies of formation were calculated at the CBS-4M level, and the energetic performances were calculated using the EXPLO5 (V6.06.01) computer code. The properties of the new compounds were compared to each other as well as to the known energetic material RDX. Moreover, the iron(II) and copper(II) perchlorate complexes with 1,3-bis-1,1-tetrazolylnitrazapropane as ligand were prepared and investigated.

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

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

Synthesis and Characterization of Azido- and Nitratoalkyl Nitropyrazoles as Potential Melt-Cast Explosives.

Desirable advancements in the field of explosive materials include the development of novel melt-castable compounds with melting points ranging from 80 to 110 °C. This is particularly important due to the limited performance and high toxicity associated with TNT (trinitrotoluene). In this study, a series of innovative melt-castable explosives featuring nitratoalkyl and azidoalkyl functionalities attached to the 3-nitro-, 4-nitro-, 3,4-dinitropyrazole, or 3-azido-4-nitropyrazole scaffold are introduced. These compounds were synthesized using straightforward methods and thoroughly characterized using various analytical techniques, including single-crystal X-ray diffraction, IR spectroscopy, multinuclear nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, elemental analysis, and DTA. Furthermore, the energetic properties such as (theoretical) performance data, sensitivities, and compatibilities of the compounds were evaluated and compared among the different structures.

Sensitive 1,4-Disubstituted Nitro-Containing Cubanes: Structures and Properties.

The cubane cage system is characteristic and well known for its high strain energy, qualifying it as a promising precursor for energetic materials. 1,4-Disubstituted cubanes are the easiest accessible derivatives. A further developed laboratory-scale procedure for cubane-1,4-dicarboxylic acid dimethyl ester is presented. From this central precursor, the bis-trinitroethyl and bis-nitromethyl esters as well as the bis-methylcarbamate and bis-methylnitrocarbamate were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallography. In addition, their physical and energetic properties were determined and studied.

4,5-Bisnitratomethyl and Similar Representatives of the Rare Class of 2-Nitro-1,2,3-triazoles.

A [3+2]-cycloaddition toward bishydroxymethyl-1,2,3-triazole makes the title compound available through selective nitration. The obtained sensitive 2-nitrotriazole was shown to have a high density of 1.764 g cm-3 and a detonation velocity of 8590 m s-1. It can also be classified as an oxidizer with an oxygen balance toward CO of 12%. Further representatives of this rare class of 2-nitro-1,2,3-triazoles were synthesized. One- and two-dimensional 15N NMR spectroscopy, crystal structure, and elemental analysis were performed.

"Green" PBX Formulations Based on High Explosives (RDX and HMX) and Water-Soluble pH-Sensitive Polymeric Binders.

The increasingly harsher and more complex international and European environmental legislation drives the current development of "greener" energetics materials and munitions. The aerospace and defense industries rely on extensive research in the formulation and scale-up production of polymer-bonded explosives (PBX). In this context, this paper aims to present a versatile method for obtaining "green" PBX formulations based on two high explosives (hexogen (RDX) and octogen (HMX)) and acrylic acid-ethyl acrylate copolymeric binders. This study developed an innovative "eco-friendly" technology for coating the RDX and HMX crystals, allowing straightforward and safer manufacture of PBX, avoiding the use of traditional organic solvents. At the same time, these polymeric binders are soluble in water at a slightly alkaline pH and insoluble at acidic or neutral pH, thus ensuring a safer manipulation of the energetic materials during their entire life cycle and a facile recovery of the explosive in its original shape and morphology in demilitarization. The PBX formulations were characterized via specific analytical tools to evaluate the influence of their composition on the safety and performance characteristics: scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), alkaline pH solubility tests, differential thermal analysis (DTA), impact sensitivity test (BAM Fall Hammer Test), friction sensitivity test (BAM Friction Test), electrostatic sensitivity test (ESD), vacuum stability test, small scale shock reactivity test (SSRT), detonation velocity test. The "green" PBX formulations obtained through a simple and innovative coating method, based on the polymeric binders' adjustable water solubility, demonstrated remarkable energetic performances and a facile recovery of the explosive crystals by the dissolution of the polymeric binder at pH 11 and 30 °C.

Development and Characterization of New Energetic Composites Based on HNTO/AN Co-Crystal and Nitro-Cellulosic Materials.

To develop advanced cellulose-based energetic composites, new types of high-energy-density formulations containing hydrazine 3-nitro-1,2,4-triazol-5-one (HNTO)/ammonium nitrate (AN) cocrystals combined with nitrocellulose or nanostructured cellulose nitrate (NC and NMCC) were experimentally characterized. The prepared energetic formulations were analyzed in terms of their physicochemical properties, mechanical sensitivities, structural features, and thermal behavior. Their heats of combustion and theoretical energetic performance were assessed as well. Experimental results exhibited the inherent characteristics of the designed NC@HNTO/AN and NMCC@HNTO/AN, including improved density, specific impulse, and impact sensitivity compared to their raw compounds. Besides that, thermo-kinetic findings revealed that the as-prepared insensitive and high-energy-density composites undergo two exothermic decomposition processes, and that NC@HNTO/AN has higher thermal activity. The present study demonstrated the outstanding characteristics of the new composites and could serve as a reference for developing more advanced cellulose-based energetic formulations.

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

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

Melt Castable Derivatives of Pentaerythritol Tetranitrate.

In the search for high-performance and environmentally friendly energetic materials, the derivatization of known materials is an often-applied concept to fulfill modern-day demands. Surprisingly, the long know pentaerythritol tetranitrate (PETN) has only been derivatized to a limited extent. PETN shows a brought application in energetic materials or pharmaceutics. In this work, the PETN backbone is modified by introducing nitramine, ionic nitramine, amine, ionic amine and tetrazole functionalities. The obtained and structurally similar compounds allow good comparability and insights into functional group effects on sensitivity, thermal behavior and performance. The functionalizations result in melting points in the range of 64 to 126 °C. Some compounds are therefore potential candidates to replace toxic TNT.

The Nitrate and Nitrocarbamate of 1,3,5-Trinitrocyclohexane-trimethanol and Selected Salts.

With 1,3,5-trinitrocyclohexane-1,3,5-triyl trimethanol as a precursor, available from 1,3,5-trinitrobenzene, two further nitro-containing molecules were synthesized. Via modification of the hydroxyl groups, new oxygen-rich compounds were obtained, the corresponding trinitrate and trinitrocarbamate. From the latter, various salts were obtained by treatment with bases. All compounds were fully characterized by NMR and IR spectroscopy, X-ray diffraction, and elemental and differential thermal analyses. Moreover, the sensitivity toward friction and impact was determined according to BAM standard techniques and the energetic properties were calculated by using the EXPLO5 computer code.

Towards Investigating the Effect of Ammonium Nitrate on the Characteristics and Thermal Decomposition Behavior of Energetic Double Base NC/DEGDN Composite.

This research work aimed to elaborate on a new modified double-base propellant containing nitrocellulose (NC), ammonium nitrate (AN), and diethylene glycol dinitrate (DEGDN). The developed AN/NC-DEGDN formulation was successfully obtained through a casting process and fully characterized in terms of its chemical structure, morphological features, and thermal behavior. Beforehand, theoretical calculation by the CEA-NASA program was applied to select the optimal composition of the formulation. Experimental findings demonstrated the homogenous dispersion of AN oxidizer in the NC-DEGDN matrix without alteration of their molecular structures. The catalytic influence of AN on the thermal decomposition behavior of NC-DEGDN film was also elucidated by thermal analyses. When AN was incorporated into the formulation, the decomposition peak temperatures for the different decomposition processes were shifted toward lower temperatures, while the total enthalpy of decomposition increased by around 1272.24 J/g. In addition, the kinetics of the thermal decomposition of the developed modified double base propellant were investigated using DSC results coupled with model kinetic approaches. It was found that the addition of AN decreases the activation energy of nitrate esters from 134.5 kJ/mol to 118.84 kJ/mol, providing evidence for its excellent catalytic effect. Overall, this investigation could serve as a reference for developing future generation of modified double-base propellants.

1-(Nitratomethyl)-5H-tetrazole: A Highly Sensitive Ligand with Improved Oxygen Balance for Laser-Ignitable Coordination Compounds.

For the first time, the highly sensitive 1-(nitratomethyl)-5H-tetrazole (1-NAMT) was synthesized, representing the shortest possible 1-(nitratoalkyl)-5H-tetrazole with a combined nitrogen and oxygen content of 81.4%. Compared to its related ethyl derivative, 1-(nitratoethyl)-5H-tetrazole, it exhibits improved oxygen balance, resulting in higher detonation parameters. 1-NAMT was thoroughly analyzed by single-crystal diffraction experiments accompanied by elemental analysis, IR spectroscopy, and multinuclear (1H, 13C, and 14N) NMR measurements. The thermal behavior of 1-NAMT was analyzed by differential thermal analysis supported by thermogravimetric analysis. Furthermore, energetic coordination compounds (ECCs) of Cu with different inorganic (e.g., nitrate, chlorate, and perchlorate) and nitroaromatic anions (e.g., picrate and styphnate) were synthesized and thoroughly analyzed. It is shown that the formation of ECCs with nitroaromatic anions (Tdec ∼ 180 °C) is a suitable strategy to improve the rather low thermal stability of 1-NAMT (125 °C).

Chemistry of 2,5-diaminotetrazole.

1,5-Diaminotetrazole is one of the most prominent high-nitrogen tetrazole compounds described in the literature. Interestingly the isomer 2,5-diaminotetrazole is nearly undescribed due to its challenging synthetic routes. 2,5-Diaminotetrazol (1) was successfully synthesized via amination of 5-aminotetrazole followed by various purification steps to separate it from isomeric 1,5-diaminotetrazole. In addition to the extensive characterization of 2,5-DAT further derivates by protonation, methylation and amination of the tetrazole ring were synthesized and characterized. The resulting tri-functionalized, ionic tetrazolium derivatives were combined with energetic anions (nitrate, perchlorate, azide, 5,5'-bistetrazole-1,1'-diolate (BTO2-)) to adjust and tune the properties of each compound. All compounds were intensively characterized including IR and multinuclear NMR spectroscopy, thermal analysis through DTA, X-ray diffraction and sensitivity testing. The purity was verified by CHNO elemental analysis and the energetic properties were calculated using the EXPLO5 code and the calculated enthalpy of formation (CBS-4M).

1-Nitrimino-5-azidotetrazole: Extending Energetic Tetrazole Chemistry.

Azide and nitrimino functions are among the most energetic substituents that can be introduced to the skeleton to enhance the energetic properties of a compound. In this study, we report the successful synthesis of a compound that combines both, azide and nitrimino substituents directly attached to one tetrazole scaffold. 1-Nitrimino-5-azidotetrazole is prepared by nitration of 1-amino-5-azidotetrazole. Subsequent salination with ammonia and guanidinium carbonate yields two highly energetic derivatives. All energetic compounds, as well as the intermediate steps of an alternatively developed synthesis strategy, were analysed and characterized in detail. In addition to multinuclear NMR and IR spectroscopy, crystal structures of all key compounds were measured. The sensitivities (friction, impact, electrostatic discharge and thermal) were determined accordingly. In addition, the detonation parameters of all energetic substances were calculated with the EXPLO5 code, which was fed with the enthalpy of formation (atomization method based on CBS-4M) and the crystallographic densities.

Exploring the Photochemistry of Solid 1,1-Diamino-2,2-dinitroethylene (FOX-7) Spanning Simple Bond Ruptures, Nitro-to-Nitrite Isomerization, and Nonadiabatic Dynamics.

The UV photolysis of solid FOX-7 at 5 K with 355 and 532 nm photons was investigated to unravel initial isomerization and decomposition pathways. Isomer-selective single photon ionization coupled with reflectron time-of-flight mass spectrometry (ReTOF-MS) documented the nitric oxide (NO) loss channel at 355 nm along with a nitro-to-nitrite isomerization, which was observed by using infrared spectroscopy, representing the initial reaction pathway followed by O─NO bond rupture of the nitrite moiety. A residual gas analyzer detected molecular oxygen for the 355 and 532 nm photolysis at a ratio of 4.3 ± 0.3:1, which signifies FOX-7 as an energetic material that provides its own oxidant once the decomposition starts. Overall branching ratios for molecular oxygen versus nitric oxide were derived to be 700 ± 100:1 at 355 nm. It is notable that this is the first time that molecular oxygen was detected as a decomposition product of FOX-7. Computations show that atomic oxygen, which later combines to form molecular oxygen, is likely released from a nitro group involving conical intersections. The condensed phase potential energy profile computed at the CCSD(T) and CASPT2 level correlates well with the experiments and highlights the critical roles of conical intersections, nonadiabatic dynamics, and the encapsulated environment that dictate the mechanism of the reaction through intermolecular hydrogen bonds.

Microscopic Studies and Application of a Stabilized Ceric Ammonium Nitrate Oxidizer in Environmentally Friendly Pyrotechnic Strobes.

The growing reservations against the use of any kind of chlorine source in pyrotechnic items revolutionize the predominantly empirical development of strobe formulations in two ways. First, a conventionally applied ammonium perchlorate oxidizer needs to be replaced. Second, visible light emissions should no longer be generated by metastable monochloride species. Moreover, until now, toxic substances such as potassium dichromate have been added in order to achieve a more pronounced strobe effect. This work evaluates the potential of stearic acid-treated ceric ammonium nitrate to serve as an oxidizing agent in next-generation strobe compositions. For this purpose, its microscopic structure, energetic properties, and stability under ambient conditions were assessed. A two-component mixture with a magnesium-aluminum alloy was investigated, which should allow for the introduction of copper-based colorants. This potentially paves the way for the first environmentally friendly blue strobe formulations.

1-(Azidomethyl)-5H-Tetrazole: A Powerful New Ligand for Highly Energetic Coordination Compounds.

Highly energetic 1-(azidomethyl)-5H-tetrazole (AzMT, 3) has been synthesized and characterized. This completes the series of 1-(azidoalkyl)-5H-tetrazoles represented by 1-(azidoethyl)-5H-tetrazole (AET) and 1-(azidopropyl)-5H-tetrazole (APT). AzMT was thoroughly analyzed by single-crystal X-ray diffraction experiments, elemental analysis, IR spectroscopy and multinuclear (1 H, 13 C, 14 N, 15 N) NMR measurements. Several energetic coordination compounds (ECCs) of 3d metals (Mn, Fe, Cu, Zn) and silver in combination with anions such as (per)chlorate, mono- and dihydroxy-trinitrophenolate were prepared, giving insight into the coordination behavior of AzMT as a ligand. The synthesized ECCs were also analyzed by X-ray diffraction experiments, elemental analysis, and IR spectroscopy. Differential thermal analysis for all compounds was conducted, and the sensitivity towards external stimuli (impact, friction, and ESD) was measured. Due to the high enthalpy of formation of AzMT (+654.5 kJ mol-1 ), some of the resulting coordination compounds are extremely sensitive, yet are able to undergo deflagration-to-detonation transition (DDT) and initiate pentaerythritol tetranitrate (PETN). Therefore, they are to be ranked as primary explosives.