The isomerization reaction of CH₂XH→CH₃X (X＝O, S, Se) has been studied by the ab initio and quantum topological analysis method. The breakage and formation of the chemical bonds in the reactions have been discussed by the topological analysis method of electronic density, and the calculated results show that there is a transitional structure of three-membered ring on the isomerization reaction paths. The “energy transition state” and the “structure transition state” in all of the studied reactions have been found. And in all these reactions, the structure transition state appeared after the energy transition state. The more the reaction energy is, the wider scope the three-membered ring appears in.

Key words: isomerization reaction, topological analysis of electronic density, reaction mechanism, structure transition state