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

The complex singlet potential energy surface of the CHNO₂ system is investigated at the B3LYP and CCSD-(T) (single-point) levels in order to explore the possible reaction mechanism of CH radical with NO₂. Twenty-five minimum isomers and 50 transition states are located. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are obtained. Starting from the very energy rich reactant R CH + NO₂, the side-attack adduct HCNO₂ (1) is first formed followed by oxygen-shift almost barrierlessly to give cis-OC(H)NO (2) and then to trans-OC(H)NO (2'). Subsequently, the most favorable channel is direct dissociation of 2 and 2' to product P₁ HCO + NO. The other two much less favorable channels are direct dissociation of 2' to product P₂ HNO + CO or isomerization of 2' to a complex HON···CO (21) that easily dissociates to product P₃ HON + CO. The large exothermicity released in these processes further drives the three products P₁, P₂, and P₃ to take secondary dissociation to the final product P₁₂ H + CO + NO. The pathways leading to other dissociation products such as NH + CO₂, OH + NCO and HNCO + O, however, are even much less competitive either due to thermodynamical or kinetic factors. A notable finding is that product P₃ HON + CO, which was completely ignored in previous experiments, should be considered in evaluation of the final product yields. The present calculations can excellently explain the experimental result of a very recent diode laser study of the title reaction.