Ab initio kinetics for thermal decomposition of CH ₃N• NH ₂, cis-CH ₂NHN•H, trans-CH ₃NHN•H, and C•H ₂NNH ₂ radicals

The thermal decomposition of the CH ₃N•NH ₂, cis-CH ₃NHN•H, trans-CH ₃NHN•H, and C•H ₂NNH ₂ radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH ₂ to form methyleneimine is the predominant channel for the decomposition of the C•H ₂NNH ₂ radical due to its small energy barrier of 13.8 kcal mol ^-1. The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10 ¹⁹T ^-1.672 exp(-9665.13/T) s ^-1. In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH ₃NHN•H and trans-CH ₃NHN•H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol ^-1 for their isomerization. The β-scission of CH3 from the cis-CH ₃NHN•H radical to form transdiazene has an energy barrier of 35.2 kcal mol ^-1, and the β-scission of CH ₃ from the trans-CH ₃NHN•H radical to form cisdiazene has an energy barrier of 39.8 kcal mol ^-1. The CH ₃N•NH ₃ radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH ₂=NNH ₂, trans-CH ₃N=NH, and cis-CH ₃N=NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol ^-1, respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01- 100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.