Relationships between geometries and energies of identity SN2 transition states: the dominant role of the distortion energy and its origin

At the 4-31G computational level, the intrinsic barriers of eight identity S N 2 reactions X − + CH 3 X → XCH 3 + X − are less than the energies required to distort CH 3 X from its ground state geometry to its transition state geometry by a constant 25 kcal/mol. The distortion energy and its C—X stretching and H—C—X bending components can be calculated directly, or, alternatively, estimated from the force constants and bond dissociation energies of CH 3 X. Regardless of the mode of computation, the distortion energies are found to be dominated by the C—X stretching deformations, and these are linearly correlated with the intrinsic barriers. The total deformation energies are also linearly correlated with the intrinsic barriers. The transition vectors of the eight identity reactions have been calculated; each is dominated by the C—X stretch. The percentage of C—X stretching at the transition state, here termed the Distortion Index (DI), reflects the "tightness" or "looseness" of this structure. The intrinsic barrier increases as the DI increases, i.e., as the transition state becomes more "exploded". This result does not agree with the predictions of a More O'Ferrall–Jencks potential energy surface diagram. However, as shown in some detail, all of the computational results of the present work are in harmony with the surface crossing diagram model of the barrier. The essential feature of this model is the notion that the activation process in an identity S N 2 reaction is a result of the distortion that is required to transfer one electron from X − to CH 3 X.