Theoretical study on the mechanism of the 3CH2+NO2 reaction

The complex doublet potential energy surface of the CH2NO2 system is investigated at the B3LYP/6-31G(d,p) and QCISD(T)/6-311G(d,p) (single-point) levels to explore the possible reaction mechanism of the triplet CH2 radical with NO2. Forty minimum isomers and 92 transition states are located. For the most relevant reaction pathways, the high-level QCISD(T)/6-311 + G(2df,2p) calculations are performed at the B3LYP/6-31G(d,p) geometries to accurately determine the energetics. It is found that the top attack of the (CH2)-C-3 radical at the N-atom of NO2 first forms the branched open-chain H2CNO2 a with no barrier followed by ring closure to give the three-membered ring isomer cC(H-2)ON-O b that will almost barrierlessly dissociate to product P-1 H2CO + NO. The lesser followed competitive channel is the 1,3-H-shift of a to isomer HCN(O)OH c, which will take subsequent cis-trans conversion and dissociation to P-2 OH + HCNO. The direct O-extrusion of a to product P-3 O-3 + H2CNO is even much less feasible. Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. Formation of the other very low-lying dissociation products such as NH2 + CO2, OH + HNCO and H2O + NCO seems unlikely due to kinetic hindrance. Moreover, the (CH2)-C-3 attack at the end-O of NO, is a barrier-consumed process, and thus may only be of significance at very high temperatures. The reaction of the singlet CH2 with NO2 is also briefly discussed. Our calculated results may assist in future laboratory identification of the products of the title reaction. (C) 2002 Wiley Periodicals, Inc.