Thermal decomposition of ammonium perchlorate-based molecular perovskite from TG-DSC-FTIR-MS and ab initio molecular dynamics.

(H2dabco)[NH4(ClO4)3] (DAP, dabco = 1,4-diazabicyclo[2.2.2]octane) is a recently synthesized ammonium perchlorate-based molecular perovskite energetic material. The high-symmetry perovskite configuration assembles the oxidant ClO4 - and fuel H2dabco2+ into a compact cubic crystal, realizing a high energy-releasing efficiency. In this study, the thermal decomposition of DAP has been investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) coupled with Fourier transform infrared (FTIR) spectroscopy and mass spectroscopy (MS). The TG-DSC profiles show that DAP has an intense one-stage heat release process with a weight loss of 94.7%. The evolved gas products are identified as H2O, CO2, CO, HCl, HCN, NH3, HNCO by FTIR spectrum, in which the infrared characteristic peak at 2283 and 2250 cm-1 is clarified not from N2O and assigned to HNCO. The principal products are H2O and CO2 together with significant amounts of HCl, HCN, NH3 in MS, while few nitrogen oxides and O2 are detected. The experimental results show that organic components have been the prominent media for the degradation of ClO4 -. To refine the mechanism observed in experiment, ab initio molecular dynamics simulations are carried out to reveal the atomistic reaction mechanisms. The decomposition of DAP starts with proton transfer from NH4 + and H2dabco2+ to ClO4 -. The deprotonated carbon skeleton is preferable to NH3 in capturing O atoms, realizing a faster consumption of O atoms. Amounts of H atoms enter the environment being active free radicals, realizing an efficient autocatalytic chain propagation of degradation of ClO4 -. The atomistic thermal decomposition reaction mechanism of DAP uncovers the role of organic components in promoting the degradation of ClO4 -, which will help improve the synthesis strategy of molecular perovskite energetic materials with improved performance.

Molecular dynamics simulations of a cyclotetramethylene tetra-nitramine/hydrazine 5,5'-bitetrazole-1,1'-diolate cocrystal.

An energetic ionic salt (EIS)-based cocrystal formation, cyclotetramethylene tetra-nitramine (HMX)/hydrazine 5,5'-bitetrazole-1,1'-diolate (HA·BTO), is predicted based on molecular dynamics simulations. HA·BTO is a newly-synthesized environmentally friendly energetic ionic salt with good detonation performance and low sensitivity. Calculated powder X-ray diffraction patterns and intermolecular interactions deduce the formation of the new cocrystal structure. Radial distribution function analysis suggests that hydrogen bonds and van der Waals (vdW) forces exist between the H⋯O pairs of HMX and HA·BTO, while the hydrogen bonds between the H of HA·BTO and the O of HMX play a prominent role. The cohesive energy density and mechanical properties are also analyzed. The cohesive energy density of the HMX/HA·BTO cocrystal is larger than that of the composite of HMX and HA·BTO, indicating an improvement in crystal stability by cocrystalization. Compared to both HMX and HA·BTO, HMX/HA·BTO has smaller Young modulus, bulk modulus and shear modulus values, but larger K/G values and a positive Cauchy pressure, suggesting decreased stiffness and improved ductibility. Moreover, the calculated formation energy is -405.79 kJ mol-1 at 298 K, which implies that the proposed cocrystal structure is likely to be synthesized at ambient temperature. In summary, we have predicted an EIS-based cocrystal formation in which the safety and mechanical properties of HMX have been improved via cocrystalization with HA·BTO, and this provides deep insight into the formation mechanism of the EIS-based cocrystal.