| Abstract
| - The reactions of a model sulfur ylide with formaldehyde and 1,1-dicianoethylene, leading to the formation ofan epoxyde and a cyclopropane, respectively, have been studied using different computational methods, andthe results have been compared to those obtained with the CBS-QB3 method. The second step of these reactionspresents transition states similar to that of an SN2 reaction. Depending on the degree of electron delocalizationat the transition state, a different amount of exact exchange is necessary in the exchange functional to obtainaccurate energy barriers. This amount is larger for the reaction of formaldehyde, in which the transition stateis more delocalized, than for the reaction of 1,1-dicianoethylene. Similar results have been obtained forsymmetric and non-symmetric SN2 reactions. The calculation of the reaction path has shown that the errorrelative to CBS-QB3 tends to increase when approaching the transition state. Among the different computationalmethods, PBE1PBE is the one to provide the most accurate energy barriers and reaction energies, whereasBB1K leads to the best results for the reaction path before the transition state.
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