G2(+) investigation on the alpha-effect in the S(N)2 reactions at saturated carbon

As a continuing theoretical study on the alpha-effect in the S(N)2 reactions at saturated carbon centers, 28 gas-phase reactions have been examined computationally by using the high-level G2(+) method. The reactions include: Nu(-)+CH3X -> CH(3)Nu+ X- (X=F and Cl; Nu(-)=HO-, HS-, CH3O-, Cl-, Br-, HOO-, HSO-, FO-, ClO-, BrO-, NH2O-, and HC(=O)OO-). It was found that all alpha-nucleophiles examined exhibit downward deviations from the correlation line between the overall barriers and proton affinities for normal nucleophiles, indicating the existence of the a-effect in the gas phase. The transition states (TS) for the alpha-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 alpha-effect is related to the electron density on the a-atom, and increases when the position of a-atom is changed from left to right and from bottom to top in the periodic table. The reaction with CH3F exhibits a larger alpha-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 center dot center dot center dot CH3 center dot center dot center dot F](-double dagger). Thus, a higher electron density on the alpha-atom and a more positive methyl moiety at the TS result in a larger alpha-effect.