Polymerization of Nitrogen in Nitrogen-Fluorine Compounds under Pressure

Free-standing cubic gauche nitrogen stable at 760 K under ambient pressure

Cubic gauche nitrogen (cg-N) has received wide attention for its exceptionally high energy density and environmental friendliness. However, traditional synthesis methods for cg-N predominantly rely on high-pressure techniques or the utilization of nanoconfined effects using highly toxic and sensitive sodium azide as precursor, which substantially restrict its practical application. On the basis of the first-principles simulations, we found that adsorption of potassium on the cg-N surface exhibits superior stabilization compared to sodium. Then, we chose safer potassium azide as precursor for synthesizing cg-N. Through plasma-enhanced chemical vapor deposition treatment, the free-standing cg-N was successfully synthesized without the need for high-pressure and nanoconfined effects. It demonstrated excellent thermal stability up to 760 K, and then rapid and intense thermal decomposition occurred, exhibiting typical thermal decomposition behaviors of high-energy-density materials. The explosion parameters were also measured using laser-induced plasma spectroscopy. Our work has substantially promoted the practical application of cg-N as HEDMs.

Free and Complexed Fluoropentazoles: Anomalous Ring Charge and Appreciable Kinetic Stability

Fluoronitrogens are strongly related to high-energy density materials. Fluoropentazole (cyclo-FN₅) with hitherto the highest N/F ratio at ambient pressure belongs to the family of well-known and 119-year-old pentazoles. Due to the presence of the overwhelmingly electronegative F element, cyclo-FN₅ is the only pentazole to date that contains a cationic N₅ ring. However, also due to F's large electronegativity, the reported ring-destruction barrier of cyclo-FN₅ is nearly the lowest (6.7 kcal/mol), which has made its synthesis and characterization quite difficult. Here, cyclo-FN₅ was re-predicted to bear an almost doubled ring-destruction barrier (∼12-14 kcal/mol) at the CBS-QB3, G4, W1BD, CCSD(T)/CBS//CCSD/cc-pVTZ, and CCSD(T)/CBS//CCSD(T)/aug-cc-pVTZ levels. Promisingly, upon further complexation with Lewis acid(s), the ring-destruction barrier of cyclo-FN₅ might reach 23 kcal/mol, which is comparable to that of the well-known and already synthesized arylpentazoles (∼20 kcal/mol), and the positive charge on the N₅ ring can be increased to 0.66 |e|.

High-Pressure Studies of Trimethylsilane Azide by Raman Scattering and Synchrotron X-ray Diffraction

We have reported the high-pressure behavior of energetic material trimethylsilane azide ((CH₃)₃SiN₃, TMSiN₃) by in situ Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) measurements in diamond anvil cells with a pressure up to ∼40 GPa at ambient temperature. The analyses of Raman spectra showed two liquid-liquid phase transitions at 0.13 and 7.9 GPa. The XRD results verified that TMSiN₃ remained in a liquid state throughout the phase transitions. In the pressure range of 0.13-2.6 GPa, the distortion of the Si-C₃ bond and the shortening of the bond between azide group and silicon atom lead to the first phase transition. The second transition is ascribed to the rotation of methyl group at 7.9 GPa. With further compression, the azide groups become increasingly asymmetric, and completely amorphous at 33.8 GPa. This paper is useful to understand the behavior of azide group and the molecular structural evolution of organic azide under high pressure. The unique high-pressure behavior of the azide group in TMSiN₃ may be useful for improving in the formation of the polynitrogen compound with azides.