The reaction mechanisms of singlet and triplet CH₂ with CH₂CO have been studied by using the density functional theory (DFT), natural bond orbital (NBO), coupled cluster single and double excitation [CCSD(T)] and atoms in molecules (AIM) methods. The geometries of reactants, transition states and products were completely optimized by B3LYP/6-311＋G(d,p). All the energy values of the species were obtained at the CCSD(T)/6-311G＋(d,p) level. The calculated results indicated that the singlet CH₂ could not only insert the C—H (reaction I) but also could react with C＝C, C＝O (reactions II and III). There are three main pathways existing when the products CO and C₂H₄ are generated. Reaction II happens more easily according to the energy changes and the barrier in its rate-controlling step. In addition, the important geometries in the main pathways have been studied by AIM theory and NBO analysis. There are three reactive pathways as triplet CH₂ reacts with CH₂CO, the first one is the triplet CH₂ insertion into a C—H; the second one is addition reaction by the triplet CH₂ to the C＝C of CH₂CO, which is easier for its lower potential barrier; however, the last pathway involving a diradical is the most difficult to occur for its biggest activation energy.

Key words: ketene, methylene radical, DFT, NBO, AIM