Relationships between geometries and energies of identity S_N2 transition states: the dominant role of the distortion energy and its origin

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

Concertedness and E2 elimination reactions: prediction of transition state position and reaction rates using two-dimensional reaction surfaces based on quadratic and quartic approximations

“…There is a basis for expecting G to be less for the less substituted alkyl bromides. Mitchell et al (60) have reported that G is proportional to the deformation energy for CH3-X. They were considering SN2 reactions but it appears likely that similar effects would apply to SN1 reactions.…”

Intrinsic selectivity and its geometric significance in SN2 reactions

“…Such a correlation exists the exchange reactions of P~S -, HS-, HO-, and NC-, ln because the energy requried to dissociate the TS into three fragments accord with eq. [5], the halide exchange series is predicted to in Table 1. The experimental data set is seen to obey the above Table This i index of the reaction series, the looser the average transition state of the series.…”

“…1. Large Ix: -ARX gaps and delocalization indices (WR:) will require a greater C-X deformation and will thereby lead to looser transition state geometries (5). Since barriers and transition state geometries behave alike, it follows then that the intrinsic selectivity that derives from plots of barriers AE* vs. (Ix, -Am) may serve as an indicator of transition state geometry.…”

“…See ref. 5 for a detailed treatment of this correlation. Such a correlation exists the exchange reactions of P~S -, HS-, HO-, and NC-, ln because the energy requried to dissociate the TS into three fragments accord with eq.…”

“…[5] and [8], both the barriers and the looseness of the transition state vary in proportion to WR: and, therefore, a somewhat stricter definition of IS can be formulated. Thus, intrinsic selectivity parameters, which derive from plots of AE' vs. (Ix, -ARX), measure not only the average I looseness of the transition state, but also the sensitivity of the reaction series to geometric loosening as X is varied.…”