Theoretical Kinetics of Radical-Radical Reaction NH₂ṄH + ṄH₂ and Its Implications for Monomethylhydrazine Pyrolysis Mechanism

Significant discrepancies were observed between the experiments and the simulations for ṄH₂ time-histories in monomethylhydrazine pyrolysis with the robust mechanism proposed by Pascal and Catoire. The rate of formation analyses for ṄH₂ indicated the significance of the reaction NH₂ṄH + ṄH₂ = H₂NN + NH₃, which has not been well-defined. In this study, ab initio calculations were performed for the theoretical description of the NH₂ṄH + ṄH₂ chemistry. Most stationary points on the potential energy surface were quantified at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level, and the multireference methods were employed for barrier-less reaction and some transition states. The temperature- and pressure-dependent rate coefficients were determined using classical and microcanonical variational transition state theories. Four primary reaction channels were identified as competitive: 1) The H atom abstraction reaction yielding N₂H₂(T) + NH₃, dominating at 1350-3000 K across the 0.001-100 atm pressure range. 2) The H atom abstraction reaction forming N₂H₂(S) + NH₃, dominating at 800-1350 K and competing with the processes of chemical activation and collisional stabilization below 800 K. 3) The chemical-activated reaction resulting in H₂NN(S) + NH₃, dominating below 800 K at 0.001 atm. 4) The collisional-stabilized recombination reaction leading to N₃H₅, becoming significant as pressure increases and dominating below 600 and 650 K at 1 and 100 atm, respectively. The implications of newly calculated NH₂ṄH + ṄH₂ kinetics for the monomethylhydrazine pyrolysis mechanism were evaluated, and updates were implemented. Sensitivity analyses indicated the necessity of additional research efforts to comprehend the dynamics of CH₃NH₂ unimolecular and N₂H₂ + ṄH₂ reaction systems. The rate coefficients presented in this study can be employed to develop the chemical kinetic model of nitryl-containing systems.