Abstract: The mechanism of cycloaddition between singlet difluoro-vinylidene and formaldehyde was investigated with MP2/6-31G^* method, including geometry optimization, vibrational analysis, and energetics for the involved stationary points on the potential energy surface. The results show that the title reaction has two major channels. The first one brings about a three-membered ring product(P1) via a, b and c paths, of which c is the leading path that consists of two steps: (Ⅰ) an intermediate (INT1c) is formed between difluoro vinylidene and formaldehyde through a free barrier exothermic reaction of 219.18 kJ/mol; (Ⅱ) the intermediate then isomerizes to a product, difluoro-methylene ethane oxide, with an energy barrier of 134.71 kJ/mol. The reaction path of channel 2 consists of three steps: (Ⅰ) an energy-rich intermediate (INT1b) is firstly formed between difluoro-vinylidene and formaldehyde through a free barrier exothermic reaction of 142.77 kJ/mol; (Ⅱ) the intermediate(INT1b) then isomerizes to the other one intermediate(INT2), with an energy barrier of 22.31 kJ/mol; (Ⅲ) the intermediate(INT2) isomerizes to one 4-membered ring products (P2), with an energy barrier of 11.98 kJ/mol.

Key words: Difluoro vinylidene carbene, Cycloaddition reaction, Second-order Moller-Plesset perturbation theory, Potential energy surface