Intramolecular H-bonds in LLM-105 and its derivatives: a DFT study

Discriminative Mechanical and Thermal Response of the H-N Bonds for the Energetic LLM-105 Molecular Assembly

Molecular interactions in energetic materials form the key not only to the "structure stability, energy storage, ignition, and detonation" dynamics but also to the sensitivity to the loading of perturbation and the power intensity of radiation for the energetic substance, with the nature of the interactions remaining elusive. With the aid of perturbative Raman spectroscopy and the pressure-resolved density functional theory, we uncovered that the H-N bond of the intermolecular O:H-N bonds for LLM-105 shares the same negative compressibility and thermal expansivity of the H-O bond for the coupling O:H-O bond of water [Phys. Rep. 2023, 998, 1-68]. In contrast, the dangling H-N bond vibrating at a 3440 cm-1 high frequency does otherwise due to the absence of coupling interaction and the undercoordination-driven bond contraction. These findings should deepen our insight into interactions involving electron lone pairs and offer an efficient means for discriminating the performance of individual bonds.

Application of a multi-channel in-situ infrared spectroscopy: The case of LLM-105

The thermal decomposition process of 2,6-diamino-3,5-dinitropyrazine-1-oxide(LLM-105)under several constant temperatures (100 °C, 115 °C, 130 °C, and 145 °C) have been studied by a multi-channel in-situ reaction system. Almost 1000 spectra were obtained within 24 days by Fourier-transform infrared spectroscopy (FT-IR). The thermal decomposition activation energies (E_α) of C-NH₂ and C-NO₂ in LLM-105 were calculated by the Arrhenius equation to be 89.65 and 145.09 kJ mol^-1, respectively. The thermal decomposition process of LLM-105 under long-term constant temperature is divided into two paths: intramolecular H-transfer and C-NO₂ partition. It is feasible to study the aging process of materials using a combination of a multi-channel in-situ reaction system and FT-IR, which can effectively monitor the evolution of structure.

First-Principles-Based Force Field for 2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a highly promising energetic material (EM) with high safety. Understanding its microscopic response mechanisms within the external stimulus is meaningful for the design of EMs. In order to comprehend the complicated phenomena, it is necessary to employ molecular simulation methods to investigate the response mechanisms with the force field (FF) at an atomic level. In this work, we developed a tailored FF for LLM-105 based on first-principles calculations. The validity of the FF was evaluated by molecular dynamics simulations. The structural parameters of LLM-105 predicted by FF are in good agreement with the experimental values, such as lattice constant, bond length, bond angle, dihedral angle and center of mass, and so forth. Moreover, the FF possesses good performance to describe the structural response on pressure accurately. In general, our work not only builds a balanced FF in gas and condensed phases, but also provides a useful tool to study the properties about LLM-105 at a large scale, which is helpful to improve the understanding about the balance between energy and safety in EMs.

Computational analysis of mesoscale thermomechanical ignition behavior of impacted LLM-105 based explosives

LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide) is an insensitive high explosive crystal which has performance between that of HMX and TATB. An elastoviscoplastic dislocation model is developed for LLM-105 crystal, which accounts for the dislocation evolutions at the crystal interior and crystal wall and strain-rate dependent work hardening. Three different crystal morphology (cubic, icosahedral, rodlike) of LLM-105 based explosive computational models were constructed and subjected to an impact velocity of 200 m s⁻¹ and 500 m s⁻¹. Effects of crystal morphology and initial dislocation density on thermomechanical ignition behavior of LLM-105 based explosives were analyzed. Dislocation density of both crystal interiors and crystal walls in the rodlike LLM-105 based explosive increases slower than that in the cubic and icosahedral explosives. Both the volume averaged and localized stress and dislocation density are the lowest for the rodlike explosive. At the impact velocity of 500 m s⁻¹, a temperature rise due to volumetric work, plasticity work and chemical reaction is sufficiently high to lead to the ignition of the cubic explosive, which shows that the rodlike explosive is the least sensitive among the three explosives. Moreover, with the increase of initial dislocation density, the corresponding volume averaged and localized stress and temperature increase as well. Results presented bridge the macroscale thermomechanical ignition response with the mesoscale deformation mechanisms, which is essential for better understanding the ignition mechanisms and guiding the design of LLM-105 based formulations.

Mesoscale thermomechanical ignition behavior of LLM-105 based explosives was quantified through a developed elastoviscoplastic dislocation model for LLM-105 crystal.

Initial Mechanisms for the Unimolecular Thermal Decomposition of 2,6-Diamino-3,5-dinitropyrazine-1-oxide

The initial channels of thermal decomposition mechanism of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) molecule were investigated. The results of quantum chemical calculations revealed four candidates involved in the reaction pathway, including the C⁻NO₂ bond homolysis, nitro⁻nitrite rearrangement followed by NO elimination, and H transfer from amino to acyl O and to nitro O with the subsequent OH or HONO elimination, respectively. In view of the further kinetic analysis and ab initio molecular dynamics simulations, the C⁻NO₂ bond homolysis was suggested to be the dominant step that triggered the decomposition of LLM-105 at temperatures above 580 K. Below this temperature, two types of H transfer were considered as the primary reactions, which have advantages including lower barrier and high rate compared to the C⁻NO₂ bond dissociation. It could be affirmed that these two types of H transfer are reversible processes, which could buffer against external thermal stimulation. Therefore, the excellent thermal stability of LLM-105, that is nearly identical to that of 1,3,5-triamino-2,4,6-trinitrobenzene, can be attributed to the reversibility of H transfers at relatively low temperatures. However, subsequent OH or HONO elimination reactions occur with difficulty because of their slow rates and extra energy barriers. Although nitro⁻nitrite rearrangement is theoretically feasible, its rate constant is too small to be observed. This study facilitates the understanding of the essence of thermal stability and detailed decomposition mechanism of LLM-105.

Structural, mechanical properties, and vibrational spectra of LLM-105 under high pressures from a first-principles study

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 ₃₃ (38.5 GPa) > C ₁₁ (24.0 GPa) > C ₂₂ (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.

First-principles study of the structural transformation, electronic structure, and optical properties of crystalline 2,6-diamino-3,5-dinitropyrazine-1-oxide under high pressure

Periodic first-principles calculations have been performed to study the effect of high pressure on the geometric, electronic, and absorption properties of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) under hydrostatic pressures of 0-50 GPa. Obvious irregular changes in lattice constants, unit-cell angles, bond lengths, bond angles, and band gaps showed that crystalline LLM-105 undergoes four structural transformations at 8, 17, 25, and 42 GPa, respectively. The intramolecular H-bonds were strong at pressures of 0-41 GPa but weakened in the range 42-50 GPa. The lengths of the intermolecular H-bonds (<1.47 Å) indicated that these H-bonds have covalent character and tend to induce the formation of a new twelve-membered ring. Analysis of the DOS showed that the interactions between electrons, especially the valence electrons, strengthen under the influence of pressure. The p states play a very important role in chemical reactions of LLM-105. The absorption spectrum of LLM-105 displayed more bands--as well as stronger bands--in the fundamental absorption region when the pressure was high rather than low. A new absorption peak due to O-H stretching appeared at 18.3 eV above 40 GPa, indicating that covalent O-H bonds and a new twelve-membered ring are present in LLM-105.