Theoretical study on intra-molecule interactions in TKX-50

To fully and deeply understand the weak interactions in the gaseous structure of the TKX-50 molecule, two conformations I and II of the TKX-50 molecule confirmed in a crystal cell were optimized at the B3LYP/6-311g(d,p) level in the gas state, and the single point energy of the optimized structure was calculated at the M06-2X/ma-TZVPP level. Analyzing methods for weak interactions such as the interaction region indicator (IRI), topological basin analysis, and the extended transition state-natural orbitals for chemical valence (ETS-NOCV) theory with the help of Multiwfn code were employed to reveal the corresponding intramolecular weak interactions. The results showed that there were 5 kinds of intramolecular weak interaction in both conformations. They are two types of H bond, two types of intra-ring weak interaction, and one type of O-N bond within the molecular fragment containing the bis-tetrazole ring. The combined effect of all these weak interactions holds the bis-tetrazole ring of TKX-50 retaining an almost coplanar configuration. Meanwhile, the strength of these weak interactions is significantly different in conformation I and conformation II. The most obvious difference is that conformation II has a significant H transfer between intramolecular fragments due to the mirror rotation of almost 180° of cations (NH3OH)+ perpendicular to the N-O bond axis thereof as compared to the reference conformation I. This conformational difference not only makes the weak interaction between the two conformations very different but also forms a quasi-covalent bond in conformation II with much larger bonding energy than other H bonds, thus resulting in conformation II having lower electron energy and more stable geometry. In addition, the order of breaking various H bonds in the combustion decomposition process of TKX-50 is deduced by comparing various H bonds.

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

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

Smart Host-Guest Energetic Material Constructed by Stabilizing Energetic Fuel Hydroxylamine in Lattice Cavity of 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane Significantly Enhanced the Detonation, Safety, Propulsion, and Combustion Performances

The host-guest inclusion strategy has become a promising method for developing novel high-energy density materials (HEDMs). The selection of functional guest molecules was a strategic project, as it can not only enhance the detonation performance of host explosives but can also modify some of their suboptimal performances. Here, to improve the propulsion and combustion performances of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), a novel energetic-energetic host-guest inclusion explosive was obtained by incorporating energetic rocket fuel, hydroxylamine (HA), into the lattice cavities of HNIW. Based on their perfect space matching, the crystallographic density of HNIW-HA was determined to be 2.00 g/cm³ at 296 K, which has reached the gold standard regarding the density of HEDMs. HNIW-HA also showed higher thermal stability (T_d = 245.9 °C) and safety (H₅₀ = 16.8 cm) and superior detonation velocity (D_V = 9674 m/s) than the ε-HNIW. Additionally, because of the excellent combustion performance of HA, HNIW-HA possessed higher propulsion performances, including combustion speed (S_C = 39.5 mg/s), combustion heat (Q_C = 8661 J/g), and specific impulse (I_sp = 276.4 s), than ε-HNIW. Thus, the host-guest inclusion strategy has potential to surpass the limitations of energy density and suboptimal performances of single explosives and become a strategy for developing multipurpose intermolecular explosives.

Origin of the considerably high thermal stability of cyclo-N5− containing salts at ambient conditions

Ionization, conjugation, hydrogen bonding, coordination bonding and π–π stacking consolidate the cyclo-N₅⁻ caged in salt crystals.

Alleviating the energy & safety contradiction to construct new low sensitivity and highly energetic materials through crystal engineering

Alleviating the energy & safety contradiction of energetic materials through crystal engineering.