Functional group effects on thermal stability of high-nitrogen fused-ring zwitterionic energetic materials: a DFT...

High-nitrogen fused-ring zwitterionic energetic materials have attracted considerable attention owing to their high enthalpies of formation, excellent thermal stability, and low sensitivity. However, the molecular mechanism by which functional groups regulate thermal stability remains insufficiently understood. In this work, three TYX-based fused zwitterionic energetic compounds (TYX-1-TYX-3) containing identical fused-ring skeletons but different substituents (amino, nitro, and nitramino groups) were systematically investigated using density functional theory (DFT). LBO, BDE, ESP, IRI, Hirshfeld surface, QTAIM, and transition-state analyses were employed to elucidate the effects of substituents on electronic structure, intermolecular interactions, and decomposition behavior. The results show that the thermal stability follows the order TYX-1 > TYX-3 > TYX-2, consistent with the experimental decomposition temperatures. Amino substitution enhances electron delocalization and reduces molecular surface polarization, whereas nitro and especially nitramino groups strengthen local charge separation and oxygen-involved polar interactions. Transition-state calculations reveal that the preferred initial decomposition pathways are dominated by intramolecular hydrogen transfer rather than direct cleavage of the weakest bonds, indicating that the initial decomposition behavior is governed primarily by kinetic factors rather than solely by bond strength. QTAIM analyses further confirm the continuous redistribution of electron density during decomposition. The minimum Gibbs free energy barriers of TYX-1, TYX-3, and TYX-2 are 52.675, 51.540, and 48.949 kcal mol⁻1, respectively. Overall, the thermal stability of TYX compounds is governed by the competition between electron delocalization and local polarization induced by functional groups. This work provides theoretical insights for the design of thermally stable energetic materials.

Using the Gaussian 16 package, molecular structure optimization and frequency calculation were carried out at the M06-2X/def2-TZVPP level. Stable equilibrium structures and transition-state reaction pathways were confirmed via imaginary frequency verification and IRC calculations. The Shermo program was employed to calculate Gibbs free energy with the initial reactant taken as the energy reference. Combined with Multiwfn, VMD and CrystalExplorer 17 software, wavefunction analyses, including IRI and QTAIM analyses, Hirshfeld surface characterization and intermolecular interaction feature analysis were further carried out.