Ab initio multireference study of the reactions of CH₃N·NH₂ + OH

The H-abstraction reactions of MMH by the OH radical generate three radical intermediates: CH₃N·NH₂, CH₃NHN·H, and C·H₂NHNH₂, which undergo further decomposition to smaller radicals and stable products. It was found that the β-scission of H atom from the CH₃N·NH₂ radical is not feasible as those of the β-scission of NH₂ and CH₃ from the C·H₂NHNH₂ and CH₃NHN·H radicals due to its higher energy barrier. Consequently, it may react with the OH radical for further oxidation and decomposition. The reactions of CH₃N·NH₂ with the OH radical was studied in details using ab initio and DFT methods. The geometries of the stationary points were optimized using multireference CASPT2(4e,3o)/aug-cc-pVDZ and density functional B3LYP/6-311++G(d,p) theories, with energies refined at the QCISD(T) level with the basis set extrapolated to the CBS limit via extrapolation of the cc-pVTZ and cc-pVQZ results. The transition state of the CH₃N·NH₂ + OH reaction for the formation CH₃N(OH)NH₂ adduct was evaluated with the variable reaction coordinate transition state theory employing the multireference CASPT2(4e,3o)/aug-cc-pV∞Z interaction energies. It was found that the CH₃N(OH)NH₂ adduct undergoes direct decomposition via a four-center transition states to CH₂=NNH₂ + H₂O products, it also can isomerize to CH₃NNH₂OH and then further decompose to CH₃N=NH+H₂O via a small energy barrier. The results reveal that the H-abstractions of the primary amine hydrogen by the OH radical via a hydrogen bonded complex are important to methyldiazene product formation.