Theoretical mechanistic study on the reaction of the methoxymethyl radical with nitrogen dioxide

Mechanism of the reaction of CH₃OCH₂ with NO₂ is explored theoretically at the M062X/MG3S and G4 levels. The calculated results indicate two stable association intermediates, CH₃OCH₂NO₂ (IM1) and CH₃OCH₂ONO (IM2), which can be produced by the attack of the nitrogen or oxygen atom of NO₂ to terminal carbon atom of CH₃OCH₂ without barrier involved. IM2 is found to take trans (IM2a)-cis (IM2b) conversion and isomerization to IM1, with the following stability order IM2a > IM2b > IM1. Starting from IM2a, the most feasible pathway is the direct O-NO bond cleavage leading to P1 (CH₃OCH₂O + NO) or the H-shift and O-NO bond rupture to produce P2 (CH₃OCHO + HNO), both of which have comparable contribution to the title reaction. There also involves an H-transfer from the methyl group of IM2a to the N atom with the simultaneous dissociations of the C-O and O-N bonds to produce P4 (2CH₂O + HNO). In addition, another dissociation pathway is open to IM2b which decompose to P5 (CH₂O + CH₃ONO) by the O-N and C-O bond scissions and the recombination of CH₃O and NO. Because all the intermediates and transition states involved in the above pathways lie below reactants, the CH₃OCH₂ + NO₂ reaction is expected to be rapid. Subsequent dissociation of IM1 and direct H-abstraction between CH₃OCH₂ and NO₂ are kinetically almost inhibited due to significantly high barriers. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for understanding NO₂ combustion chemistry.