The 1,2-H shift reactions of simple carbenes (CH2Y-C-X) have been studied using density functional theory (DFT). The influence of the substituent X and Y groups on the activation energy (Ea) of 1,2-H shifts were examined. The 'bystander' Y substituents lower Ea in the order of Me, F > Cl, Br > H. Our analysis shows that the X effect is more significant than the bystander Y effect. X substitutions increase Ea of carbenes in the order of F > Cl > Br > Me > H. The influence of X on Ea is governed by the singlet-triplet energy separation (ΔES-T) of the carbene, i.e., Ea of a carbene is larger as its AES-T increases due to an X substitution. The X effect was also found to be related to the magnitude of the exothermicity: Ea of reaction is smaller when the reaction is more exothermic. Origin of the Y effect is attributed to the interplay between two factors: 'lateness' of transition state on the potential energy surface, and the exothermicity of the reaction.
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