Regioisomerism effects on the thermal decomposition mechanism of fused triazole-based high-nitrogen compounds: a DFT study.

Context: High-nitrogen fused-ring compounds are promising candidates for energetic materials due to their high energy density and good thermal stability. As a molecular design strategy, regioisomerism has been widely applied in pharmaceuticals and functional materials. However, its effect on the performance of high-nitrogen fused energetic materials has not yet been systematically investigated. In this study, density functional theory (DFT) was employed to explore the structural and energetic implications of regioisomerism by examining four representative compounds: 3-nitro-1H-[1,2,4]triazolo[4,3-b][1,2,4]triazole (A1), 6-nitro-1H-[1,2,4]triazolo[4,3-b][1,2,4]triazole (A2), [1,2,4]triazolo[4,3-a][1,3,5]triazine-3,5,7-triamine (B1), and [1,2,4]triazolo[1,5-a][1,3,5]triazine-2,5,7-triamine (B2). A comparative analysis was conducted from multiple perspectives, including molecular conformation, bond strength, non-covalent interactions, and surface electrostatic potential distributions. Computational results revealed notable differences among the regioisomers in terms of internal steric repulsion, key bond strength, and charge distribution, which in turn influenced their initial decomposition pathways and corresponding energy barriers. Specifically, A2 and B2 exhibited more compact molecular geometries, reduced intramolecular repulsion, and generally higher activation barriers along key bond cleavage pathways, indicating superior thermal stability compared to their respective isomers. This work highlights the important role of regioisomerism in tuning the thermal stability of high-nitrogen fused-ring energetic compounds and provides theoretical guidance for the rational design of high-performance energetic materials.

Methods: All computations in this study were performed using Gaussian 16 at the M06-2X/def2-TZVPP level of theory. The intrinsic reaction coordinate (IRC) calculations were conducted to verify the validity of all transition states. The Shermo program was employed to compute the Gibbs free energy of the molecules, while wavefunction analysis was carried out using Multiwfn and VMD.