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

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