The mechanisms of three cycloaddition reactions of singlet alkylidenecarbene and formaldehyde have been studied by using second- order Moller-Plesset perturbation theory. The geometrical parameters, harmonic vibrational frequencies and energies of stationary points on the potential energy surface are calculated by MP2/6-31G~* method. According to the data of energy, we predict that path a of the cycloaddition reaction (1) would be the major reactive channel of the cycloaddition reactions of singlet alkylidenecarbene with formaldehyde, which proceeds in two steps: (I) Alkylidenecarbene and formaldehyde form an energy-rich intermediate, through an exothermal reaction with no energy barrier, and (II) the intermediate isomerizes to the product alkylideneoxirane. The energy barrier for the latter step is 24.1 kJ· mol~(-1) at MP2/6-31G~* level.

Key words: CARBENE, CYCLOADDITION REACTION, POTENTIAL ENERGY SURFACES, perturbation theory, FORMALDEHYDE, REACTION MECHANISM