Theoretical Kinetics of Radical-Radical Reaction NH 2 ṄH + ṄH 2 and Its Implications for Monomethylhydrazine Pyrolysis Mechanism.

Significant discrepancies were observed between the experiments and the simulations for ṄH ₂ time-histories in monomethylhydrazine pyrolysis with the robust mechanism proposed by Pascal and Catoire. The rate of formation analyses for ṄH ₂ indicated the significance of the reaction NH ₂ ṄH + ṄH ₂ = H ₂ NN + NH ₃ , which has not been well-defined. In this study , ab initio calculations were performed for the theoretical description of the NH ₂ ṄH + ṄH ₂ 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 N ₂ H ₂ (T) + NH ₃ , dominating at 1350-3000 K across the 0.001-100 atm pressure range. 2) The H atom abstraction reaction forming N ₂ H ₂ (S) + NH ₃ , 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 H ₂ NN(S) + NH ₃ , dominating below 800 K at 0.001 atm. 4) The collisional-stabilized recombination reaction leading to N ₃ H ₅ , becoming significant as pressure increases and dominating below 600 and 650 K at 1 and 100 atm, respectively. The implications of newly calculated NH ₂ ṄH + ṄH ₂ 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 CH ₃ NH ₂ unimolecular and N ₂ H ₂ + ṄH ₂ reaction systems. The rate coefficients presented in this study can be employed to develop the chemical kinetic model of nitryl-containing systems.