Theoretical study on triplet potential energy surface of the CH(²II) + NO₂ reaction

A detailed theoretical survey on the triplet potential energy surface (PES) for the CH + NO₂ reaction is carried out at the B3LYP and CCSD(T) (single-point) levels in order to gain a deeper mechanistic knowledge of this important radical reaction. Thirty-eight minimum isomers and 107 transition states are located. It is shown that the CH and NO₂ radicals can be brought together barrierlessly via the triplet PES to form the initial N-attack adduct HCNO₂ (1) that lies 50.6 kcal/mol below the reactant R. Subsequently, the most feasible channel is the direct O-extrusion of 1 to produce P6 (HCNO + 3O) with the barrier 31.2 kcal/mol. The less competitive channel is a 1,3-H shift conversion of 1 to the branched isomer HON(O)C (10) (34.2 kcal/mol below R) with a barrier 37.1 kcal/mol, which can directly dissociate to product Py (CNO + OH) with a small : barrier 9.0 kcal/mol. However, the oxygen-shift conversion of. 1 leading to the very low lying isomers OC(H)NO (2) (2') that can directly dissociate to PI (HCO + NO) and then Pn (H + CO + NO) needs a much larger barrier, 43.3 kcal/mol. This is in sharp contrast to the mechanism of the title reaction via the singlet PES (Tao; et al. J. Phys. Chem. A 2001, 705, 3388) that the exclusive feasible channel proceeds via the almost barrierless oxygen-shift conversion of HCNO₂ to OC(H)NO followed by formation of HCO + NO (major), HNO + CO, and HON + CO (minor), each of which can take secondary dissociation to the final product H + CO + NO. Since all appropriate transition states and intermediates lie below R, the overall rate constant of the title reaction via the triplet PES is expected to be fast, as confirmed by the simple RRKM calculations (&298K = 5.14 x 10~12 cm3molecule~1 s~')- Since the predicted products of the title reaction via the triplet PES is completely different from that via the singlet PES, we feel that future experimental investigations on this radical reaction are desirable.