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

The radical -molecule reaction mechanism of CH3 with NOX (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 b(1) (CH3NO-trans), which can easily convert to b(2) (CH3ONO-cis). Subsequently, starting from b (b(1), b(2)), the most feasible pathway is the direct N-O bond cleavage of b (b(1), b(2)) leading to P-1 (CH3O+ NO) or the 1,3-H-shift and N-O bond rupture of b(1) to form P-2 (CH2O+ HNO), both of which may have comparable contribution to the reaction CH3 + NO2. Much less competitively, b(2) can take a concerted H-shift and N-O bond cleavage to form product P-3 (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 + NO2 reaction, the major product is found to be P-1 (HCN + H2O), whereas the minor products are P-2 (HNCO + H-2) and P-3 (HNC +H2O). 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.