Theoretical study on reaction mechanism of the cyanogen radical with nitrogen dioxide

The complex singlet potential energy surface for the reaction of CN with NO₂, including 9 minimum isomers and 10 transition states, is explored computationally using a coupled cluster method and a density functional method. The most favorable association of CN with NO₂ was found to be a barrierless carbon-to-nitrogen approach process forming an energy-rich adduct a (NCNO₂) followed by C-N bond rupture along with C-O bond formation to give b₁ (trans-NCONO), which can easily convert to b₂ (cis-NCONO). Our results show that the product P₁ (NCO + NO) is the major product, while the product P₂ (CNO + NO) is a minor product. The other products may be of significance only at high temperatures. Product P₁ (NCO + NO) can be obtained through path 1 P₁: R → a → b₁ (b₂) → P ₁ (NCO + NO), whereas the product P₂ (CNO + NO) can be formed through path P₂: R → a → b₁ → b ₂ → C₁ (c₂) → P₂ (CNO + NO). Because the intermediates and transition states involved in the above two channels are all lower than the reactants in energy, the CN + NO₂ reaction is expected to be rapid, as is confirmed by experiment. Therefore, it may be suggested as an efficient NO₂-reduction strategy. These calculations indicate that the title reaction proceeds mostly through singlet pathways and less go through triplet pathways. The present results can lead us to understand deeply the mechanism of the title reaction and can be helpful for understanding NO₂-combustion chemistry.