The reaction coordinate shown in Figure 1 for the gas-phase displacement X-+ CH3Y XCH3 + Y" is characterized by two energy minima, corresponding to reactant (1) and product (3) ion-molecule clusters, separated by a central barrier (2), corresponding to the Sn2 transition state. As discussed by Brauman and his co-workers,*

No abstract

At the 4-31G computational level, the intrinsic barriers of eight identity SN2 reactions X 3-CH3X XCH3 + X-are less than the energies required to distort CH,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 CH3X. 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 (Dl), reflects the "tightness" or "looseness" of this structure. The intrinsic barrier increases as the D1 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 SN2 reaction is a result of the distortion that is required to transfer one electron from X-to CH,X. DAVID JOHN MITCHELL, H. BERNHARD SCHLEGEL, SASON S. SHAIK et SAUL WOLFE. Can. J. Chem. 63, 1642 (1985).Des calculs au niveau 4-3 IG indiquent que les Cnergies des barrieres intrinseques de 8 reactions SN2 d'identite: X-+ CH,X 4 XCH3 + X-sont systematiquement 25 kcal/mol de moins que celles requises pour dCformer le CH3X de sa gtomktrie de