The existence and the nature of the α-effect in the S_N2 reactions have been examined computationally by using the high level G2(+) method. The reactions include: Nu^- + RX → RNu + X^- [R = Me, Et, and i-Pr; X = F and Cl; Nu^- = HO^-, CH₃O^-, HS^-, Cl^-, Br^-, NH₂O^-, HOO^-, FO^-, HSO^-, ClO^-, BrO^-, and HC(=O)OO-]. It was found that all α-nucleophiles examined exhibit downward deviations from the correlation line between the overall barriers and basicities for normal nucleophiles, indicating the existence of the α-effect in the gas phase. The transition states (TSs) for the α-nucleophiles are characterized by less advanced C-X bond cleavages than the normal nucleophiles, leading to smaller deformation energies and overall barriers. The size of the α-effect is related to the electron density on the α-atom, and increases when the position of α-atom is changed from left to right and from bottom to top in the periodic table. The reaction with CH₃F exhibits a larger α-effect than that with CH₃Cl, which can be explained by a later TS and a more positively charged methyl group at the TS for CH₃F, [Nu---CH₃---F]^-≠. Thus, a higher electron density on the α-atom and a more positive methyl moiety at the TS result in a larger size of the α-effect.

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1. Hydrolysis of α-Alkyl-α-(methylthio) Methylene Meldrum's Acides. A Kinetic and Computational Investigation of Steric Effects.