The mechanisms for the abstraction reactions of alkylidenecarbenes and its substituted species with oxirane have been characterized in detail by density functional theory. All the stationary points were determined at the B3LYP/6-311G(d,p) level of the theory. The transition states both to the reactant and the product directions in the reaction paths were examined by using the intrinsic reaction coordinate. A configuration mixing model based on the work of Pross and Shaik was used to rationalize the computational results. The results show that the electro negativity of the substituents played an important role to predict its activity for the abstraction reactions. The major conclusion was that the stronger the π-donation or the more electronegative the substituents, the smaller the ΔE_ST of XYC＝C: and the lower the activation en-ergy for the abstraction reactions. In other words, it is the electronic factors, rather than the steric ones, that play a decisive role in the chemistry of the alkylidenecarbene species.

Key words: alkylidenecarbene, ab initio, density functional theory (DFT), transition state