Pentazole anion, the best candidate of full-nitrogen energetic materials, can be isolated only from acidic solution with a puzzled reason that is critical to raising the production yield and energy density of the substance. Herein, we report for the first time the discovery of the dual aromaticity (π and ) of cyclo-N5ˉ, which makes the anion unstable in nature but confers additional stability in acidic surroundings. In addition to the usual πaromaticity, similar to that of the prototypical benzene, five lone pairs are delocalized in the equatorial plane of cyclo-N5ˉ, forming additional -aromaticity. It is the compatible coexistence of the inter-lone-pair repulsion and inter-lone-pair attraction within the aromatic system that makes the naked cyclo-N5ˉ highly reactive to electrophiles and easily broken. Only in sufficiently acid solution can the cyclo-N5ˉ become unsusceptible to the electrophilic attack and gain extra stability through the formation of hydrogen-bonded complex from surrounding electrophiles; otherwise the cyclo-N5ˉ cannot be productively isolated. The dual aromaticity discovered in cyclo-N5ˉ is expected to be a universal nature of the pnictogen five-membered ring systems.
In this work, we report the structure, mechanical properties, and vibrational spectra of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), an energetic molecular crystal, with a first-principles method based on density functional theory (DFT) using the recentely developped HASEM package. The elastic constants, acoustic velocity, and parameters of equations of state were calculated, and the predicted ordering of stiffness constants is C (38.5 GPa)> C (24.0 GPa)> C (17.7 GPa). We also investigated the structure and equation of state of LLM-105 under hydrostatic pressure up to 100 GPa. The predicted structures are in good agreement with experimental results available from ambient pressure to 20 GPa. Under compressions, the LLM-105 crystal exhibits anisotropic compressibility, with a highly incompressible response along the a-axis and c-axis. It is worth noting that there is a sudden change in the lattice parameters and change rate of volume at ~30 GPa. Based on the intermolecular interaction analysis and vibrational spectra, a phase transition at the hydrostatic pressure of ~30 GPa is predicted.
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