The mechanism of cycloaddition reaction between singlet alkylidene carbene and propylene has been investigated with second-order Moller- Plesset perturbation theory. Using MP2/6-31G* method, geometry optimization, vibrational analysis, and energetic calculation have been done for the involved stationary points on the potential energy surface. According to the data of energy on the surface energy profile calculated, we predict that path a of the cycloaddition (1) and path b of cycloaddition (2) would be two major competition channels of the title cycloaddition between singlet alkylidene carbene and propylene, both proceeding in two steps: (I) the two reactants form two energy- rich intermediates INTla and BMT2b, which are exothermic reactions without energy barrier releasing 60.28 and 26.33 kj/mol, respectively; (II) two intermediates INTla and INT2b isomerize to a three-membered ling product (Pi) and a 4-membered ring product (P2) via transition states TSla and TS2b with energy barriers of 16.43 and 12.73 kj/mol, respectively.

Key words: CARBENE, CYCLOADDITION REACTION, POTENTIAL ENERGY SURFACES, TRANSITION STATE, PROPENE, PERTURBATION THEORY, REACTION MECHANISM