Detonation reaction mechanisms of 3,4-dinitropyrazole: shock-induced reaction behavior and kinetic characteristics

Aromatic heterocyclic energetic compounds exhibit complex reaction networks involving both bimolecular and unimolecular reactions under shock loading, yet their chemical reaction mechanisms and energy release characteristics remain unclear. This study investigates the reaction mechanisms of representative 3,4-dinitropyrazole (DNP). First-principles molecular dynamics in combination with the multi-scale shock technique is employed to elucidate the reaction pathways and product evolution of DNP under shock loading. The results show that the early decomposition pathways can be categorized into two mechanistically different reaction modes: bimolecular H/O transfer and unimolecular nitro group dissociation. Moreover, the comprehensive reaction pathways from reactants through intermediates to final products are established, with the final product N₂ primarily originating from direct pyrazole ring cleavage. On this basis, a steric-hindrance-effect-modified collision theory model is established to calculate the reaction rates of shock-induced bimolecular reactions. Transition state theory is further employed to calculate the unimolecular reaction energy barriers and rate constants. The results indicate that, owing to lower steric hindrance, H-transfer reactions exhibit higher rate constants and are preferred over O-transfer reactions in shock-induced reactions. In later stages, the pyrazole ring undergoes highly spontaneous and exothermic ring-opening reactions, demonstrating strong thermodynamic driving forces. This study elucidates the pathway selection and kinetic competition mechanisms of DNP under shock loading. The modified collision theory provides an approach for characterizing intermolecular reaction kinetics, offering theoretical support for regulating the energy release characteristics of high-energy insensitive energetic materials.