A quantum-based molecular dynamics study of the ICM-102/HNO3 host-guest reaction at high temperatures

Computational insights into the structure and decomposition behaviors of 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide under high pressure up to 10 GPa

CONTEXT

Inspired by the recent successful synthesis of the energetic compound 2,4,6-triamino-5-nitropyrimidine-1,3-dioxide (ICM-102), which displayed a good balance between high energy and sensitivity, the response of the structure and decomposition behaviors of ICM-102 to high pressure was systematically investigated using first principle calculations. ICM-102 exhibited a graphite-like layer structure, with the c-axis and the a-axis mainly contributing to the distance between the molecular planes. As the pressure increased from 1 atm to 10 GPa, this distance decreased from 3.166 to 2.689 Ǻ. The hydrogen bonds had the most contribution to the non-covalent interactions within the same molecular planes, resulting in the b-axis discontinuity. However, van der Waals interactions gradually appeared between molecular planes as the pressure increased to 2.5 GPa. Based on the analysis of crystal orbitals, the distribution of π bonds and the Laplacian bond order (LBO), it was determined that the generation mechanism of H2O molecules involved the cleavage of N-Oc (coordinated oxygen atoms), followed by intermolecular hydrogen transfer reactions, and ultimately the formation of H2O molecules through competition with H atoms in the amino groups within the same molecules. More importantly, the pressure dependence of LBO values for N-Oc revealed that high pressure could inhibit the ICM-102 decomposition process due to reinforcing hydrogen bonds and van der Waals interactions. This work will deepen our understanding of the stability of ICM-102 under high pressure and provide a helpful reference for its potential detonation applications.

METHODS

All simulations, including geometry optimization and vibration analysis under quasi-hydrostatic pressure, were conducted using the CP2K code. The PBE function and the Goldk-Teter-Hutter (GTH) pseudopotential with the double-ζ-with-polarization (DZVP) basis set were employed. Additionally, the semiempirical dispersion correction D3 (BJ) was used to account for the intermolecular dispersion force. The simulations were performed under periodic boundary conditions, with a finest grid level cutoff set to 400 Ry for the Γ point. The Broyden-Flecher-Goldfarb-Shanno (BFGS) optimization method was used, with tighter convergence criteria applied for the subsequent calculations of infrared spectra. Finally, the wave-function analysis, such as non-covalent interaction and LBO, was conducted using the Multiwfn and VMD packages.

Effects of Different Guests on Pyrolysis Mechanism of α-CL-20/Guest at High Temperatures by Reactive Molecular Dynamics Simulations at High Temperatures

The host-guest inclusion strategy has the potential to surpass the limitations of energy density and suboptimal performances of single explosives. The guest molecules can not only enhance the detonation performance of host explosives but also can enhance their stability. Therefore, a deep analysis of the role of guest influence on the pyrolysis decomposition of the host-guest explosive is necessary. The whole decomposition reaction stage of CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH was calculated by ReaxFF-MD. The incorporation of CO₂, N₂O and NH₂OH significantly increase the energy levels of CL-20. However, different guests have little influence on the initial decomposition paths of CL-20. The E_a1 and E_a2 values of CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are higher than the CL-20/H₂O system. Clearly, incorporation of CO₂, N₂O, NH₂OH can inhibit the initial decomposition and intermediate decomposition stage of CL-20/H₂O. Guest molecules become heavily involved in the reaction and influence on the reaction rates. k₁ of CL-20/N₂O and CL-20/NH₂OH systems are significantly larger than that of CL-20/H₂O at high temperatures. k₁ of CL-20/CO₂ system is very complex, which can be affected deeply by temperatures. k₂ of the CL-20/CO₂, CL-20/N₂O systems is significantly smaller than that of CL-20/H₂O at high temperatures. k₂ of CL-20/NH₂OH system shows little difference at high temperatures. For the CL-20/CO₂ system, the k₃ value of CO₂ is slightly higher than that for CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems, while the k₃ values of N₂ and H₂O are slightly smaller than that for the CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems. For the CL-20/N₂O system, the k₃ value of CO₂ is slightly smaller than that for CL-20/H₂O, CL-20/CO₂, CL-20/NH₂OH systems. For the CL-20/NH₂OH system, the k₃ value of H₂O is slightly larger than that for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O systems. These mechanisms revealed that CO₂, N₂O and NH₂OH molecules inhibit the early stages of the initial decomposition of CL-20 and play an important role for the decomposition subsequently.

Effects of oxidizing molecules on the thermal decomposition of TTDO by ab initio molecular dynamics simulations

Oxidizing molecules play a very important role in improving the comprehensive properties of energetic materials. Recently, a series of energetic cocrystals containing 2,4,6-triamino-1,3,5-triazine-1,3-dioxide (TTDO) and oxidizing molecule have been successfully prepared. Therefore, ab initio molecular dynamics were used to simulate the thermal decomposition process of TTDO, TTDO:H₂O₂, TTDO:HNO₃, and TTDO:HClO₄ crystals at 3000 K to study the role of oxidizing molecules during the thermal decomposition of TTDO. The initial decomposition paths of the TTDO crystal include N-H bond breaking, C-N bond breaking, and intramolecular and intermolecular H transfers. The formation mechanisms of H₂O, N₂, and CO₂ in the four crystals are completely different. The key formation mechanism of H₂O is the combination of O with OH, that of N₂ is the formation of the -N-N- structure, and that of CO₂ is to form the intermediate CO-R with carbonyl structure that form the fragment with the -O-C-O- structure. All the oxidizers H₂O₂, HNO₃, and HClO₄ involve in the formation of H₂O, N₂, and CO₂. The formation mechanisms of urea during the decomposition process of the four crystals are completely different, but the key step is to produce the structure of -N-CO-N-. An analysis of N_x shows that H₂O₂, HNO₃ and HClO₄ affect not only the types of N_x, but also its formation mechanisms. Among them, HNO₃ has the greatest influence on N_x.

Theoretical Studies on the Role of Guest in α-CL-20/Guest Crystals

The contradiction between energy and safety of explosives is better balanced by the host-guest inclusion strategy. To deeply analyze the role of small guest molecules in the host-guest system, we first investigated the intermolecular contacts of host and guest molecules through Hirshfeld surfaces, 2-D fingerprint plots and electrostatic interaction energy. We then examined the strength and nature of the intermolecular interactions between CL-20 and various small molecules in detail, using state-of-the-art quantum chemistry calculations and elaborate wavefunction analyses. Finally, we studied the effect of the small molecules on the properties of CL-20, using density functional theory (DFT). The results showed that the spatial arrangement of host and guest molecules and the interaction between host and guest molecules, such as repulsion or attraction, may depend on the properties of the guest molecules, such as polarity, oxidation, hydrogen content, etc. The insertion of H2O2, H2O, N2O, and CO2 had significant influence on the electrostatic potential (ESP), van der Waals (vdW) potential and chemical bonding of CL-20. The intermolecular interactions, electric density and crystal orbital Hamilton population (COHP) clarified and quantified the stabilization effect of different small molecules on CL-20. The insertion of the guest molecules improved the stability of CL-20 to different extents, of which H2O2 worked best.

Mechanism of the improvement of the energy of host-guest explosives by incorporation of small guest molecules: HNO3 and H2O2 promoted C-N bond cleavage of the ring of ICM-102

Host–guest materials exhibit great potential applications as an insensitive high-energy–density explosive and low characteristic signal solid propellant. To investigate the mechanism of the improvement of the energy of host–guest explosives by guest molecules, ReaxFF-lg reactive molecular dynamics simulations were performed to calculate the thermal decomposition reactions of the host–guest explosives systems ICM-102/HNO₃, ICM-102/H₂O₂, and pure ICM-102 under different constant high temperatures and different heating rates. Incorporation of guest molecules significantly increased the energy level of the host–guest system. However, the initial reaction path of the ICM-102 molecule was not changed by the guest molecules. The guest molecules did not initially participate in the host molecule reaction. After a period of time, the H₂O₂ and HNO₃ guest molecules promoted cleavage of the C–N bond of the ICM-102 ring. Stronger oxidation and higher oxygen content resulted in the guest molecules more obviously accelerating destruction of the ICM-102 ring structure. The guest molecules accelerated the initial endothermic reaction of ICM-102, but they played a more important role in the intermediate exothermic reaction stage: incorporation of guest molecules (HNO₃ and H₂O₂) greatly improved the heat release and exothermic reaction rate. Although the energies of the host–guest systems were clearly improved by incorporation of guest molecules, the guest molecules had little effect on the thermal stabilities of the systems.