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.

Photochemical Strategies Enable the Synthesis of Tunable Azetidine-Based Energetic Materials.

Despite their favorable properties, azetidines are often overlooked as lead compounds across multiple industries. This is often attributed to the challenging synthesis of densely functionalized azetidines in an efficient manner. In this work, we report the scalable synthesis and characterization of seven azetidines with varying regio- and stereochemistry and their application as novel azetidine-based energetic materials, enabled by the visible-light-mediated aza Paternò-Büchi reaction. The performance and stark differences in the physical properties of these new compounds make them excellent potential candidates as novel solid melt-castable explosive materials, as well as potential liquid propellant plasticizers. This work highlights the scalability and utility of the visible-light aza Paternò-Büchi reaction and demonstrates the impact of stereochemical considerations on the physical properties of azetidine-based energetics. Considering the versatility and efficiency of the presented synthetic strategies, we expect that this work will guide the development of new azetidine-based materials in the energetics space as well as other industries.

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.

Synthesis and Characterization of the Potential Melt-Castable Explosive 3-(1,2,4-Oxadiazolyl)-5-Nitratomethyl Isoxazole.

The synthesis of 3-(1,2,4-oxadiazolyl)-5-nitratomethyl isoxazole (C6 H4 N4 O5 ), its physical properties, and its theoretical performances are described. This energetic material was found to have a melting point range of 76.6-79.2 °C, and a thermal onset decomposition temperature of 184.5 °C. These thermal features put this material into the standalone melt-castable explosive class. The material was found to have TNT performance, and was found to be insensitive to impact, friction, and electrostatic discharge, despite having a nitric ester functionality. A critical reaction in making this molecule was the desymmetrization of diaminoglyoxime. The optimization of this transformation is described. Previous reports of this desymmetrization were found to be inaccurate, as the desymmetrization reaction produces a co-crystal of mono- and bi-1,2,4-oxadiazole products.

Bis(Nitroxymethylisoxazolyl) Furoxan: A Promising Standalone Melt-Castable Explosive.

The synthesis and crystal structure of the heterocyclic explosive bis(nitroxymethylisoxazolyl) furoxan, C10 H6 N6 O10 , are described. In addition, we report its physical properties and theoretical performance. This material was found to exhibit standalone melt-castable explosive properties, with a melting point of 89.8 °C and an onset decomposition temperature of 193.8 °C. Bis(nitroxymethylisoxazolyl) furoxan features an insensitive behavior to impact, friction, and electrostatic discharge, with a calculated detonation pressure about 25 % higher than the state-of-the-art melt-castable explosive TNT.

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.

Bis(1,2,4-oxadiazolyl) Furoxan: A Promising Melt-Castable Eutectic Material of Low Sensitivity.

A scalable synthesis of bis(1,2,4-oxadiazoyl) furoxan, C6 H2 N6 O4 , its physical properties, and its theoretical performance values are described. Previous attempts to synthesize this compound required expensive reagents, and/or time-consuming synthesis processes and low overall yields. In addition to disclosing a streamlined synthesis of bis(1,2,4-oxadiazolyl) furoxan, we report its molecular configuration and crystal structure, as well as its correct melting point. Bis(1,2,4-oxadiazolyl) furoxan exhibits a very insensitive behavior to impact, friction, and electrostatic discharge, with a calculated detonation pressure 20 % higher than that of TNT. Given its physical properties and theoretical performance values, this material can be classified as a promising ingredient in the development of melt-castable eutectic technology.

Synthesis and crystal structures of 5,5'-(propane-2,2-di-yl)bis-(2-hy-droxy-benzaldehyde) and 5,5'-(propane-2,2-di-yl)bis-(2-hy-droxy-isophthalaldehyde).

The title compounds 5,5'-(propane-2,2-di-yl)bis-(2-hy-droxy-benzaldehyde), C17H16O4, (1), and 5,5'-(propane-2,2-di-yl)bis-(2-hy-droxy-isophthalaldehyde), C19H16O6, (2), crystallize with one mol-ecule in the asymmetric unit. In mol-ecule (1), a >C(CH3)2 group bridges two nearly planar salicyl-aldehyde groups [r.m.s deviations = 0.010 (1) and 0.025 (2) Å], each comprising a planar phenyl ring bonded with a hydroxyl and an aldehyde group. Similarly, compound (2) has the same bridging group, but it connects two nearly planar appendants [r.m.s deviations = 0.034 (1) and 0.035 (1) Å], each comprising a phenyl ring bonded with a hydroxyl and two aldehyde groups. Mol-ecule (1) exhibits a bridge angle of 109.5 (2)° with the salicyl-aldehyde planes subtending a dihedral angle of 88.4 (1)°. In contrast, mol-ecule (2) presents a bridge angle of 108.9 (2)° with its appendants subtending a dihedral angle of 79.6 (3)°. Both mol-ecules exhibit two intra-molecular O-H⋯O hydrogen bonds involving the phenolic H atoms and carboxyl O-atom acceptors. In the crystal of (2), O-H⋯O hydrogen bonds between one of the hydroxyl H atoms and a carboxyl O atom from a symmetry-related mol-ecule form a chain along [10]. In addition, (2) exhibits a strong visible luminescence when excited with ultraviolet radiation.

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).

Crystal structures of 5,5'-bis(-hydroxy-methyl)-3,3'-biisoxazole and 4,4',5,5'-tetrakis-(hydroxy-methyl)-3,3'-biisoxazole.

The mol-ecular structure of 5,5'-bis(-hydroxy-methyl)-3,3'-biisoxazole, C8H8N2O4 (1), is composed of two trans planar isoxazole rings [r.m.s deviation = 0.006 (1) Å], each connected with a methyl hydroxyl group. Similarly, the structure of 4,4',5,5'-tetrakis-(hydroxy-methyl)-3,3'-biisoxazole, C10H12N2O6 (2), is composed of two planar isoxazole rings [r.m.s. deviation = 0.002 (1) Å], but with four hydroxymethyl groups as substituents. Both mol-ecules sit on a center of inversion, thus Z' = 0.5. The crystal structures are stabilized by networks of O-H⋯N [for (1)] and O-H⋯O hydrogen-bonding inter-actions [for (2)], giving rise to corrugated supra-molecular planes. The isoxazole rings are packed in a slip-stacked fashion, with centroid-to-centroid distances of 4.0652 (1) Šfor (1) (along the b-axis direction) and of 4.5379 (Å) for (2) (along the a-axis direction).

Synthesis and crystal structures of (2E)-1,4-bis-(4-chloro-phen-yl)but-2-ene-1,4-dione and (2E)-1,4-bis-(4-bromo-phen-yl)but-2-ene-1,4-dione.

The mol-ecular structure of (2E)-1,4-bis-(4-chloro-phen-yl)but-2-ene-1,4-dione [C16H10Cl2O2, (1)] is composed of two p-chlorophenyl rings, each bonded on opposite ends to a near planar 1,4-trans enedione moiety [-C(=O)-CH=CH-(C=O)-] [r.m.s. deviation = 0.003 (1) Å]. (2E)-1,4-Bis(4-bromo-phen-yl)but-2-ene-1,4-dione [C16H10Br2O2, (2)] has a similar structure to (1), but with two p-bromophenyl rings and a less planar enedione group [r.m.s. deviation = 0.011 (1) Å]. Both mol-ecules sit on a center of inversion, thus Z' = 0.5. The dihedral angles between the ring and the enedione group are 16.61 (8) and 15.58 (11)° for (1) and (2), respectively. In the crystal, mol-ecules of (1) exhibit C-Cl⋯Cl type I inter-actions, whereas mol-ecules of (2) present C-Br⋯Br type II inter-actions. van der Waals-type inter-actions contribute to the packing of both mol-ecules, and the packing reveals face-to-face ring stacking with similar inter-planar distances of approximately 3.53 Å.

Crystal structure of 3,3'-biisoxazole-5,5'-bis(methyl-ene) dinitrate (BIDN).

The mol-ecular structure of the title energetic compound, C8H6N4O8, is composed of two planar isoxazole rings and two near planar alkyl-nitrate groups (r.m.s deviation = 0.006 Å). In the crystal, the mol-ecule sits on an inversion center, thus Z' = 0.5. The dihedral angle between the isoxazole ring and the nitrate group is 69.58 (8)°. van der Waals contacts dominate the inter-molecular inter-actions. Inversion-related rings are in close slip-stacked proximity, with an inter-planar separation of 3.101 (3) Š[centroid-centroid distance = 3.701 (3) Å]. The measured and calculated densities are in good agreement (1.585 versus 1.610 Mg m-3).

Time-resolved imaging and optical spectroscopy of plasma plumes during pulsed laser material deposition.

We employ fast imaging photography and emission spectroscopy to study plasma plumes resulting from the 248-nm ablation of barium strontium titanate, and we utilize x-ray diffraction analysis and scanning electron microscopy to characterize the deposited thin films. Hydrodynamic plume analyses yield initial velocities of approximately 20 km/s, whereas spectral simulations of the Ba I lines between 739 and 770 nm yield temperatures of approximately 17000 K at early times in vacuum. Analyses of the Stark broadened Ba II lines at 614 and 649 nm reveal an electron number density of approximately 10+18 cm-3 near the surface. Several Pa of oxygen reduces these values while improving the film quality.

Nanomechanics of RDX Single Crystals by Force-Displacement Measurements and Molecular Dynamics Simulations.

Nanoenergetic material modifications for enhanced performance and stability require an understanding of the mechanical properties and molecular structure-property relationships of materials. We investigate the mechanical and tribological properties of single-crystal hexahydro-1,3,5-trinitro-s-triazine (RDX) by force-displacement microscopy and molecular dynamics (MD). Our MD simulations reveal the RDX reduced modulus (Er) depends on the particular crystallographic surface. The predicted Er values for the respective (210) and (001) surfaces are 26.8 and 21.0 GPa. Further, our simulations reveal a symmetric and fairly localized deformation occurring on the (001) surface compared to an asymmetric deformation on the (210) surface. The predicted hardness (H) values are nearly equal for both surfaces. The predicted Er and H values are ∼33% and 17% greater than the respective experimental values of 0.798 ± 0.030 GPa and 22.9 ± 0.7 GPa for the (210) surface and even larger than those reported previously. Our experimental H and Er values are ∼19% and 9% greater than those reported previously for the (210) surface. The difference between the experimental values reported here and elsewhere stems in part from an inaccurate determination of the contact area. We employ the parameter √H/Er, which is independent of area, as a means to compare present and past results, and find excellent agreement, within a few percent, between our predicted and experimental results and between our results and those obtained from previous nanoindentation experiments. Also, we performed nanoscratch simulations of the (210) and (001) surfaces and nanoscratch tests on the (210) surface and present values of the dynamic coefficient of deformation friction.

Laser-based detection of TNT and RDX residues in real time under ambient conditions.

We detect thin films of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-hexanitro-1,3,5-triazine (RDX) by one- and two-laser photofragmentation-fragment detection spectroscopy in real time at ambient temperature and pressure. In the one-laser technique, a laser tuned to 226 nm excites the energetic material and both generates the characteristic NO photofragments and facilitates their detection by resonance-enhanced multiphoton ionization (REMPI) using their A-X (0,0) transitions near 226 nm. In contrast, in the two-laser technique, a 454 nm laser generates the analyte molecule in the gas phase by matrix-assisted desorption, and a second laser tuned to 226 nm both photofragments it and ionizes the resulting NO. We report the effects of laser energy, analyte concentration, and matrix concentration on the ion signal and determine the rotational temperatures of the NO photofragments from Boltzmann, rotational distribution analysis of the REMPI spectra. We achieve limits of detection (S/N = 3) of hundreds of ng/cm(2) for both techniques under ambient conditions with a positive signal identification as low as 70 pg using a single 226 nm laser pulse of approximately 50 microJ.