Theoretical study on the reaction mechanism of the methyl radical with nitrogen oxides

The radical-molecule reaction mechanism of CH₃ with NO _x (x = 1, 2) has been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels of theory. For the singlet potential energy surface (PES) of the CH₃ + NO₂ reaction, it is found that the carbon to middle nitrogen attack between CH₃ and NO₂ can form energy-rich adduct a (H₃CNO₂) with no barrier followed by isomerization to b₁ (CH₃ONO-trans), which can easily convert to b₂ (CH₃ONO-cis). Subsequently, starting from b (b₁, b₂), the most feasible pathway is the direct N-O bond cleavage of b (b₁, b₂) leading to P₁ (CH₃O + NO) or the 1,3-H-shift and N-O bond rupture of b₁ to form P₂ (CH₂O + HNO), both of which may have comparable contribution to the reaction CH₃ + NO₂. Much less competitively, b₂ can take a concerted H-shift and N-O bond cleavage to form product P₃ (CH₂O + HON). Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the CH₃ + NO₂ reaction is expected to be rapid, as is consistent with the experimental measurement in quality. For the singlet PES of the CH₃ + NO reaction, the major product is found to be P₁ (HCN + H₂O), whereas the minor products are P₂ (HNCO + H₂) and P₃ (HNC +H ₂O). The CH₃ + NO reaction is predicted to be only of significance at high temperatures because the transition states involved in the most feasible pathways lie almost above the reactants. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. The present study may be helpful for further experimental investigation of the title reactions.