Sequential, Electrochemical-Photochemical Synthesis of 1,2,4-Triazolo-[4,3-a]pyrazines.

1,2,4-triazolo-[4,3-a]pyrazine was prepared via a two-step electrochemical, photochemical process. First, a 5-substituted tetrazole is electrochemically coupled to 2,6-dimethoxypyrazine to yield 1,5- and 2,5- disubstituted tetrazoles. Subsequent photochemical excitation of the 2,5-disubstituted tetrazole species using an ultraviolet lamp releases nitrogen gas and produces a short-lived nitrilimine intermediate. Subsequent cyclization of the nitrilimine intermediate yields a 1,2,4-triazolo-[4,3-a]pyrazine backbone. The scope of this reaction was explored using various tetrazoles and pyrazines. Materials produced were identified using chemical analytical techniques and computationally studied for potential application as an insensitive energetic material.

Exploring Cuneanes as Potential Benzene Isosteres and Energetic Materials: Scope and Mechanistic Investigations into Regioselective Rearrangements from Cubanes.

Cuneane is a strained hydrocarbon that can be accessed via metal-catalyzed isomerization of cubane. The carbon atoms of cuneane define a polyhedron of the C2v point group with six faces─two triangular, two quadrilateral, and two pentagonal. The rigidity, strain, and unique exit vectors of the cuneane skeleton make it a potential scaffold of interest for the synthesis of functional small molecules and materials. However, the limited previous synthetic efforts toward cuneanes have focused on monosubstituted or redundantly substituted systems such as permethylated, perfluorinated, and bis(hydroxymethylated) cuneanes. Such compounds, particularly rotationally symmetric redundantly substituted cuneanes, have limited potential as building blocks for the synthesis of complex molecules. Reliable, predictable, and selective syntheses of polysubstituted cuneanes bearing more complex substitution patterns would facilitate the study of this ring system in myriad applications. Herein, we report the regioselective, AgI-catalyzed isomerization of asymmetrically 1,4-disubstituted cubanes to cuneanes. In-depth DFT calculations provide a charge-controlled regioselectivity model, and direct dynamics simulations indicate that the nonclassical carbocation invoked is short-lived and dynamic effects augment the charge model.

Investigation into a Conformationally Locked (Z)-Azidoxime.

High nitrogen compounds find wide use in the development of new propellants and explosives as well as pharmaceutical chemistry as bioisosteres, bacterial stains, and antifungal agents. A class of underexplored high-nitrogen materials includes azidoximes and their 1-hydroxytetrazole isomers. Azidoximes possess an energetic azide group and are quite sensitive to impact, spark, and friction. Therefore, these materials are generated in situ and cyclized under mild acidic conditions to their 1-hydroxytetrazole isomers. Recently, we synthesized a novel 1,2,4-triazine-derived azidoxime; however, upon subjecting this material to established cyclization conditions, no reaction was observed, even after prolonged reaction times with heating. Additional 1,2,4-triazine-derived azidoximes also displayed a similar lack of reactivities. This observation led us to probe the reactivity of these materials with both a DFT investigation and crystallographically based electrostatic potential mapping. In all, the lack of reactivity toward cyclization was found to be due to an inability of 1,2,4-triazine-based azidoximes to isomerize into the reactive (E)-conformation, requiring an activation energy of 26.4 kcal mol-1.

Energetic Salts of a Tri-Annulated Ring System.

Energetic salts of a triazolyl-tetrazinyl-aminotriazine ring system are characterized as energetic materials. Previously known sodium, ammonium, hydrazinium, barium, and triaminoguanidinium salts as well as the parent free acid were synthesized according to literature procedures and fully characterized for the first time as energetic materials. The silver salt was also synthesized and characterized for the first time as an energetic material. Generally, these materials form hydrates that are insensitive to mechanical stimuli; however, in cases in which anhydrous materials can be obtained, high sensitivities are possible.

Approach to High-Nitrogen Materials with Dual-Use Properties via Tetraaza-Nazarov Cyclization.

In this report, we describe the application of an electrocyclization toward the synthesis of a high-nitrogen heterocycle. It entails the synthesis of a novel, high-nitrogen, 2-3-disubstituted tetrazolium salt via the tetraaza-Nazarov cyclization (4π electrocyclization) of 3-bromo-1,5-bis(3-nitro-1,2,4-triazole-1H-5-yl)-formazan (BDNF). The cyclization takes place under mild conditions using the oxidant phenyliodine(III) diacetate (PIDA). The proposed electrocyclic mechanism is supported by density functional theory (DFT) calculations and data from previous studies of formazan cyclizations. This is noteworthy because while 4π electrocyclizations with one or two nitrogen atoms have been documented previously, this case represents the first example of generation and cyclization of a conjugated intermediate with four nitrogen atoms. The experimental behavior of electrocyclization is consistent with the predictions of DFT.

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

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

4,4'-Dinitrimino-5,5'-diamino-3,3'-azo-bis-1,2,4-triazole: A High-Performing Zwitterionic Energetic Material.

Mixed acid nitration of electrochemically generated 4,4',5,5'-tetraamino-3,3'-azo-bis-1,2,4-triazole (TAABT) generated the novel energetic material 4,4'-dinitrimino-5,5'-diamino-3,3'-azo-bis-1,2,4-triazole (DNDAABT). Various energetic salts of DNDAABT were also prepared and characterized to confirm their structures and determine their explosive sensitivities and performances. The free acid of DNDAABT exists as a zwitterionic molecule that leads to a high-density material with predicted detonation parameters comparable to those of TKX-50 (bis(hydroxylammonium) 5,5'-bis(tetrazolate-1 N-oxide). Due to the insensitive nature of TAABT, it was predicted that DNDAABT would demonstrate remarkably low sensitivities for a primary N-nitramine. However, it was found that DNDAABT and all salts produced have primary explosive sensitivities, albeit with relatively high thermal stabilities for primary N-nitramines.

1,3,4,5-Tetraamino-1,2,4-triazolium Cation: An Energetic Moiety.

The amination of 3,4,5-triamino-1,2,4-triazole with O-tosylhydroxylamine yielded the nitrogen-rich 1,3,4,5-tetraamino-1,2,4-triazolium cation as its tosylate salt. Subsequent metathesis reactions produced energetic salts with various energetic anions, including perchlorate, nitrate, nitrotetrazolate, and bistetrazolate diolate. All energetic salts possess relatively high heats of formation, thermal sensitivities, and detonation velocities and pressures. The prepared energetic salts were characterized chemically using single-crystal X-ray crystallography, elemental analysis, and 1H NMR, 13C NMR, and IR spectroscopy and energetically by measuring their thermal, impact, and friction sensitivities. 15N NMR was carried out on the tosylate salt. Energetic performances were determined by a combined experimental-computational method using calculated heats of formation and experimental crystal densities.

Heterocyclic Nitrilimines and Their Use in the Synthesis of Complex High-Nitrogen Materials.

We show the ability of a nitrilimine prepared from 3-amino-5-nitro-1,2,4-triazole to undergo various cyclization and rearrangement reactions, giving a beautiful diversity of nitrogen-rich heterocyclic products. This chemistry includes the first cyclization of a nitrilimine with a diazonium species, giving a tetrazole, a previously unknown transformation, as well as leading to the creation of several new energetic materials with backbones not available by traditional techniques. New materials prepared were characterized both chemically (multinuclear NMR, IR, mass spectrometry, and elemental analysis) and energetically, with sensitivities and performances reported.

Tetrazole Azasydnone (C2 N7 O2 H) And Its Salts: High-Performing Zwitterionic Energetic Materials Containing A Unique Explosophore.

This work reports the first compound containing both a tetrazole and an azasydnone ring, a unique energetic material. Several energetic salts of the tetrazole azasydnone were synthesized and characterized, leading to the creation of new secondary and primary explosives. Molecular structures are confirmed by 1 H and 13 C NMR, IR spectroscopy, and X-ray crystallographic analysis. The high heats of formation, fast detonation velocities, and straight-forward synthesis of energetic azasydnones should capture the attention of future energetics research.

Synthesis of Erythritol Tetranitrate Derivatives: Functional Group Tuning of Explosive Sensitivity.

Understanding the factors that affect explosive sensitivity is paramount to the safe handling and development of new explosives molecules. Erythritol tetranitrate (ETN) is an explosive that recently has attracted significant attention in the explosives community because of its ease of synthesis and physical properties. Herein, we report the synthesis of ETN derivatives using azide, nitramine, and nitrate ester functional groups. Impact, spark, and friction sensitivity measurements, computationally calculated explosive properties, and the crystal structure analysis of the ETN derivatives are reported. Mixing explosive functional groups led to changes in the explosive sensitivity, explosive performance as well as physical properties including melting point and physical state at room temperature. Overall, we have demonstrated that combining functional groups can enable the tuning of explosive and physical properties of a molecule. This tunability can potentially aid in the development of new explosives in which characteristics are varied to meet certain specifications.

Heuristics for chemical species identification in dense systems.

A new approach to identify chemical species from molecular dynamics (MD) simulations of reacting materials under extreme temperatures and pressures is presented. The approach is based on bond-distance and vibrational criteria, derived from the examination of atomic behavior during a density functional theory MD simulation of an overdriven shock of the explosive pentaerythritol tetranitrate. For comparison, the trajectory was analyzed using popular bonding criteria commonly used in analysis of reactive MD simulations, including distance, distance-time, and bond-order criteria. Cluster analyses using the new time-dependent bond definition approach presented here and a bond-order approach revealed that species and their corresponding lifetimes were strongly dependent on the chosen approach, indicating significant implications for the development of chemical mechanisms and chemical kinetics models using the results of reactive MD simulations.

Impact of Stereo- and Regiochemistry on Energetic Materials.

The synthesis, physical properties, and calculated performances of six stereo- and regioisomeric cyclobutane nitric ester materials are described. While the calculated performances of these isomers, as expected, were similar, their physical properties were found to be extremely different. By alteration of the stereo- and regiochemistry, complete tunability in the form of low- or high-melting solids, stand-alone melt-castable explosives, melt-castable explosive eutectic compounds, and liquid propellant materials was obtained. This demonstrates that theoretical calculations should not be the main factor in driving the design of new materials and that stereo- and regiochemistry matter in the design of compounds of potential relevance to energetic formulators.

Ray tracing calculations in simulated propellant flames with detailed chemistry.

Optical measurements in propellant flames are necessary to understand the combustion physics, yet these conditions provide challenges in probing the flame and may introduce uncertainty into the measurement. This work reports the use of simulations of an ammonium perchlorate propellant flame with finite rate chemistry to understand the role of ammonium perchlorate particle size and pressure on the uncertainty of imaging-based measurements on propellant flames. A two-dimensional ray tracing code was developed to incorporate the effects of the species concentration and temperature gradients on ray refraction within propellant flames. It was determined that the effects of the flame structure based upon pressure and oxidizer particle size increases the amount of ray deflection particularly at high pressures explaining a cause for challenges of aluminum agglomerate measurements at elevated pressure. This framework shows promise for understanding limitations and uncertainties of optical measurements for reacting and turbulent flows.

Density Functional Theory and Experimental Studies of the Molecular, Vibrational, and Crystal Structure of Bis-Oxadiazole-Bis-Methylene Dinitrate (BODN).

Density function theory (DFT) and experimental characterization of energetic materials play important roles in understanding molecular structure-property relations and validating models for their predictive capabilities. Here, we report our modeling and experimental results on the molecular, vibrational, and crystal structure of energetic bis-oxadiazole-bis-methylene dinitrate (BODN) obtained by molecular DFT (M-DFT) at the B3LYP- 6-31G** level, crystal DFT (C-DFT) using the Perdew-Burke-Ernzerhof functional optimized with norm-conserving pseudopotentials, X-ray diffractometry, infrared and Raman spectroscopy, and thermogravimetric analysis. Both models predict well the experimental bond lengths, bond angles, and torsion angles of BODN. The C-DFT lattice constant values are in excellent agreement with those determined experimentally, with unit cell length and angle values differing by less than 1.2 and 0.7%, respectively. BODN presents van der Waals O···H and O···C bifurcated intramolecular contacts and short N···H and O···O intermolecular contacts. Overall, the predicted vibrational energies of both models are in line with experiment. M-DFT thermodynamic calculations predict well the experimentally derived lattice energy (-131 kJ/mol) and the M-DFT electrostatic potential calculations reveal a low sensitivity to impact. In addition, C-DFT band gap calculations predict a value of 3.80 eV for BODN, resulting predominantly from the ring O and N atoms, suggesting it is insensitive to impact. These results are compared and contrasted with those obtained in this study or reported previously for 3,3-bis-isoxazole-5,5'-bis-methylene dinitrate (BIDN).

Simple and Efficient Synthesis of Explosive Cocrystals containing 3,5-Dimethylpyrazol-1-yl-substituted-1,2,4,5-tetrazines.

The reaction of 3,4-dinitropyrazole, 5-nitrotetrazole, or 4-nitro-1,2,3-triazole with 1,2,4,5-tetrazines substituted with 3,5-dimethylpyrazolyl (dmp) groups results in energetic cocrystals after 1 minute of reflux and cooling to room temperature in yields of 89-92 %. Hydrogen-bonding between the dmp group to the N-H of the energetic heterocycles are the predominant interaction that stabilizes the new cocrystals. Each cocrystal packs in a different lattice structure and the cocrystals with sheet-like and herring-bone crystal packing orientations are less sensitive than the cocrystal with the interlocked structure. Electrostatic potential mapping helps rationalize why dmp-substituted tetrazines readily form cocrystals, whereas more electron-deficient pyrazolyl tetrazines do not. The calculated energetic performance of the new cocrystals approaches that of 2,4,6-trinitrotoluene (TNT) and importantly, these materials will aid in the rational design of new cocrystalline energetic materials.

Evaluation of electrostatic descriptors for predicting crystalline density.

This study evaluates the importance of electrostatic corrections to earlier quantum-mechanically based methods to predict crystal densities of neutral and ionic molecular energetic materials. Our previous methods (B. M. Rice et al., J. Phys. Chem. A 2007, 111, 10874) use the molecular volumes of the isolated molecule or formula unit to estimate the crystal density; this volume is defined to be that inside the quantum-mechanically determined 0.001 a.u. isosurface of electron density surrounding the isolated molecule. The electrostatic corrections to these volumetric estimates are based on features of the electrostatic potential mapped onto this isosurface of electron density, and have been parameterized using information from 180 neutral and 23 ionic CHNO molecular systems. The quality of the electrostatically corrected methods was assessed through application to 38 neutral and 48 ionic compounds not used in the parameterization. The root mean square (rms) percent deviation and average absolute error of predictions for the 38 neutral species relative to experiment are 2.7% and 0.035 g/cm(3), respectively, decreases of 0.9% and 0.015 g/cm(3) from the earlier predictions (3.6% and 0.050 g/cm(3), respectively). The rms percent deviation and average absolute error of predictions for the 48 ionic compounds relative to experiment are 3.7% and 0.045 g/cm(3), respectively, decreases of 2.6% and 0.043 g/cm(3) from the earlier predictions that used the formula unit volumes only. The results clearly show a significant improvement to the earlier method upon inclusion of electrostatic corrections.

Shock Hugoniot calculations of polymers using quantum mechanics and molecular dynamics.

Using quantum mechanics (QM) and classical force-field based molecular dynamics (FF), we have calculated the principle shock Hugoniot curves for numerous amorphous polymers including poly[methyl methacrylate] (PMMA), poly[styrene], polycarbonate, as well as both the amorphous and crystalline forms of poly[ethylene]. In the FF calculations, we considered a non-reactive force field (i.e., polymer consistent FF). The QM calculations were performed with density functional theory (DFT) using dispersion corrected atom centered pseudopotentials. Overall, results obtained by DFT show much better agreement with available experimental data than classical force fields. In particular, DFT calculated Hugoniot curves for PMMA up to 74 GPa are in very good agreement with experimental data, where a preliminary study of chain fracture and association was also performed. Structure analysis calculations of the radius of gyration and carbon-carbon radial distribution function were also carried out to elucidate contraction of the polymer chains with increasing pressure.

3-Methyl-1,2,3-triazolium-1N-dinitromethylylide and the strategy of zwitterionic dinitromethyl groups in energetic materials design.

3-Methyl-1,2,3-triazolium-1N-dinitromethylylide, an exemplary zwitterionic energetic molecule, is the first fully-studied energetic material making use of the zwitterionic dinitromethyl functional group. This compound has impact and friction sensitivities of 8 J and 144-160 N respectively with a detonation velocity of 8162 m s-1.