Ab Initio Kinetics of Methylamine Radical Thermal Decomposition and H-Abstraction from Monomethylhydrazine by H-Atom

Methylamine radicals (CH₃NH) and amino radicals (NH₂) are major products in the early pyrolysis/ignition of monomethylhydrazine (CH₃NHNH₂). Ab initio kinetics of thermal decomposition of CH₃NH radicals was analyzed by RRKM master equation simulations. It was found that β-scission of the methyl H-atom from CH₃NH radicals is predominant and fast enough to induce subsequent H-abstraction reactions in CH₃NHNH₂ to trigger ignition. Consequently, the kinetics of H-abstraction reactions from CH₃NHNH₂ by H-atoms was further investigated. It was found that the energy barriers for abstraction of the central amine H-atom, two terminal amine H-atoms, and methyl H-atoms are 4.16, 2.95, 5.98, and 8.50 kcal mol^-1, respectively. In units of cm³ molecule^-1 s^-1, the corresponding rate coefficients were found to be k₈ = 9.63 × 10^-20T^2.596 exp(-154.2/T), k₉ = 2.04 × 10^-18T^2.154 exp(104.1/T), k₁₀ = 1.13 × 10^-20T^2.866 exp(-416.3/T), and k₁₁ = 2.41 × 10^-23T^3.650 exp(-870.5/T), respectively, in the 290-2500 K temperature range. The results reveal that abstraction of the terminal amine H-atom to form trans-CH₃NHNH radicals is the dominant channel among the different abstraction channels. At 298 K, the total theoretical H-abstraction rate coefficient, calculated with no adjustable parameters, is 8.16 × 10^-13 cm³ molecule^-1 s^-1, which is in excellent agreement with Vaghjiani's experimental observation of (7.60 ± 1.14) × 10^-13 cm³ molecule^-1 s^-1.