Raman spectra and vibrational properties of FOX-7 under pressure and temperature: First-principles calculations

FOX-7 (1,1-diamino-2,2-dinitroethene) as one of the widely studied insensitive high explosives exists five polymorphs (α, β, γ, α', ε) whose crystal structures have been determined by XRD (X-rays Diffraction) and which are investigated by a density functional theory (DFT) approach in this work. The calculation results show that the GGA PBE-D2 method can reproduce the experimental crystal structure of FOX-7 polymorphs better. The calculated Raman spectra of FOX-7 polymorphs were compared in detail and fully with the experimental Raman spectra data and it was found that the calculated Raman spectra frequencies have an overall red-shift in middle band (800-1700 cm^-1), and that the maximum deviation does not exceed 4 % (The maximum point is the mode of CC in plane bending). The high-temperature phase transform path (α → β → γ) and the high-pressure phase transform path (α → α'→ε) can be well represented in the computational Raman spectra. In addition, crystal structure of ε-FOX-7 was performed up to 70 GPa to probe Raman spectra and vibrational properties. The results showed that the NH₂ Raman shift is jittering with pressure (not smooth compared to other vibrational modes) and NH₂ anti-symmetry-stretching appears red-shifted. The vibration of hydrogen mixes in all of other vibrational modes. This work shows that the dispersion-corrected GGA PBE method can reproduce the experimental structure, vibrational properties and Raman spectra very well.

Triggering the mechanism of the initial reaction of energetic materials under pressure based on Raman intensity analysis

The Raman intensity and other stoichiometric calculations of nitromethane (NM) and 2-nitrimino-5-nitro-hexahydro-1,3,5-triazine (NNHT) have been made by using first-principles density functional theory. We propose a method to judge the initial reaction mechanism of NM and NNHT under pressure based on the Raman intensity. Both the resulting NM and NNHT undergo hydrogen transfer and conventional trigger bond cleavage. And the results obtained from the Raman peak intensities infer a reaction path that is not inferior to the traditional C-NO₂ and N-NO₂ bond cleavage, thus verifying our results.

Highly Selective Nitroamino Isomerization Guided by Proton Transport Dynamics: Full-Nitroamino Imidazole[4,5-d]pyridazine Fused-Ring System

Due to the advantage of the hydrogen bond system formed by nitroamino isomerization, by the calculations of hydrogen transfer in reported nitroamino explosives, the proton transport dynamics was first proposed to predict the nitroamino isomerization of energetic materials. With the calculated results of zero-point energy, the full-nitroamino fused energetic materials, 2,4-nitroamino-7-nitroimino-1,5-dihydro-4H-imidazolo[4,5-d]pyridazine (FNPI-1) and 2,2',7,7'-tetranitromino-4,4'-azo-imidazolo[4,5-d]pyridazine (FNPI-2) were designed and successfully synthesized. The highly selective nitroamino isomerization of neutral compound FNPI-1 is shown by X-ray diffraction. After the hydrogen transfer occurs, the intermolecular hydrogen bonds will greatly promote tight stacking, which enhances the density and thus a series of comprehensive properties of energetic materials. The theoretical calculations of zero-point energy explain perfectly the selectivity of hydrogen transfer between the nitroamino groups and the fused-ring skeleton for FNPI-1. The hydrogen atom transfer and selective isomerization of nitroamino energetic materials can be accurately predicted following proton transport dynamics, which provides computational bases and new ideas for the efficient design of fully nitroamino-based explosives.

Reactive molecular dynamics study on the thermal decomposition reaction of a triple-base solid propellant

The study of the combustion property of newly designed propellant by means of computational simulation is an efficient pathway for assessment and could avoid exposure to hazardous chemicals. An RDX-modified triple-base solid propellant formula was proposed in this study. Reactive molecular dynamics simulations employing ReaxFF-lg force field were performed to explore the thermal decomposition property of the propellant for a variety of temperatures. The reaction kinetics of the system and major ingredients were analyzed, and the apparent decomposition activation energies were calculated. The population of decomposition intermediates and products is thoroughly investigated. H₂O is consumed at high temperatures indicating a water-gas reaction that could reduce carbon clusters during the combustion of solid propellant. The water-gas reaction, as well as the population of H₂ at high temperature, points out the way of adjusting the formula of the propellant, which is adding fuel and oxidizer to improve combustion temperature and oxygen balance.

Hydrogen bond system generated by nitroamino rearrangement: new character for designing next generation energetic materials

Hydrogen bond systems stabilize molecules and shorten intermolecular distances to give higher density and lower sensitivity.

Review of the molecular and crystal correlations on sensitivities of energetic materials

Highly efficient design on the levels of molecule and crystal, as well as formulation, is highly desired for accelerating the development of energetic materials (EMs). Sensitivity is one of the most important characteristics of EMs and should be compulsorily considered in the design. However, owing to multiple factors responsible for the sensitivity, it usually undergoes a low predictability. Thus, it becomes urgent to clarify which factors govern the sensitivity and what is the importance of these factors. The present article focuses upon the progress of the molecular and crystal correlations on the sensitivity, and the molecule-based numerical models for sensitivity prediction in the past decades. On the molecular level, composition, geometric structure, electronic structure, energy and reactivity can be correlated with the sensitivity; while the sensitivity can be also related with molecular packing pattern, intermolecular interaction, crystal morphology, crystal size and distribution, crystal surface/interface and crystal defect on the crystal level. And most of these factors, in particle on the crystal level, have been employed as variables in numerical models for predicting sensitivity of categorized EMs. Besides, we stress that more attention should be paid to the sensitivity correlations on the inherent structures of EMs, molecule and crystal packing, because they can be readily dealt by molecular simulations nowadays, facilitating to reveal the physical nature of sensitivity.

Toward the Mechanistic Understanding of the Additives' Role on Ammonium Nitrate Decomposition: Calcium Carbonate and Calcium Sulfate as Case Studies

The reaction mechanism involved in the decomposition of ammonium nitrate (AN) in the presence of CaCO3 and CaSO4, commonly used for stabilization and the reduction of explosivity properties of AN, was theoretically investigated using a computational approach based on density functional theory. The presented computational results suggest that both carbonate and sulfate anions can intercept an acid proton from nitric acid issued from the first step of decomposition of AN, thus inhibiting its runaway decomposition and the generation of reactive species (radicals). The reaction then leads to the production of stable products, as experimentally observed. Our modeling outcomes allow for tracing a relationship between the capability of proton acceptance of both carbonate and sulfate anions and the macroscopic behavior of these two additives as inhibitor or inert in the AN mixture.