Cis-Trans Isomerization and Thermal Decomposition Mechanisms of a Series of Nx (x = 4, 8, 10, 11) Chain-Catenated Energetic Crystals.

Nitrogen-rich compounds based on heteroaromatic rings with different lengths of nitrogen chains are at the forefront of the energetic materials field. We studied the decomposition processes and reaction kinetics of a series of Nx (x = 4, 8, 10, 11) chain-catenated energetic crystals at various temperatures (2400-3000 K) based on a combinational strategy based on density functional tight binding molecular dynamics (DFTB-MD) simulations and density functional theory (DFT). The results show that the thermal decomposition and reaction kinetics are dependent on both the temperature and nitrogen chain's length. There are two sequential stages in the initial decomposition process for the crystals N8 and N10: (i) competition between cis-trans isomerization and initial unimolecular decomposition and (ii) subsequent complicated global decomposition reactions. Increasing either the temperature or nitrogen chain's length will accelerate the competition and make initial decomposition dominate. However, cis-trans isomerization does not occur in the crystals N4 and N11. The dominant initiation paths for N4, N8, and N10 occur in the heterocycle and in the bond between the heterocycle and azo group, while that for N11 is ring elimination. The decomposition reactions exhibit a clear first-order kinetics character. The energy paths based on DFT calculations are determined as an addition to the DFTB-MD results. Our findings provide insights into the comprehensive understanding of thermal decomposition behaviors of nitrogen chain-catenated and even all-nitrogen energetic materials.