Theoretical mechanistic study on the radical-radical reaction of ketenyl with nitrogen dioxide

The radical-radical reaction between the ketenyl radical (HCCO) and nitrogen dioxide (NO₂) played a very important role in atmospheric and combustion chemistry. Motivated by recent laboratory characterization about the reaction kinetics of ketenyl radical with nitrogen dioxide, in this contribution, we applied the coupled cluster and density functional theory to explore the mechanism of the title reaction. These calculations indicate that the title reaction proceeds mostly through singlet pathways, less go through triplet pathways. It is found that the HCCO + NO₂ reaction initially favors formation of adduct OCCHNC-2 (1) with no barrier. Subsequently, starting from isomer 1, the most feasible pathway is ring closure of 1 to isomer O-cCCHN-(O)O (2) followed by CO₂ extrusion to product HCNO + CO ₂ (P₁), which is the major product with predominant yields. Much less competitively, 1 can take the successive 1,3-H- and 1,3-OH-shift interconversion to isomer OCCNOHO (3^a, 3^b, 3^c) and then to isomer OCOHCNO (4^a, 4^b), which can finally take a concerted H-shift and C-C bond fission to give HCNO + CO: (P₁)- The least competitive pathway is the ring-closure of isomer 3^a to form isomer O-cCCN(OH)0 (5^a, 5^b) followed by dissociation to HONC + CO₂ (P₂) through the direct side CO₂ elimination. Because the intermediates and transition states involved in the most favorable channel all lie below the reactants, the title reaction is expected to be rapid, as is confirmed by experiment. Therefore, it can be significant for elimination of nitrogen dioxide pollutants. The present results can lead us to a deep understanding of the mechanism of the title reaction and can be helpful for understanding NO_x-combustion chemistry.