¹³C kinetic isotope effects and reaction coordinate motions in transition states for S_N2 displacement reactions

Abstract

Can. J. Chem. 54,1146 (1976. Reaction coordinate motions and "C kinetic isotope effects at 25 "C have been calculated for the S,2 reactions of methyl iodide with iodide, cyanide, and chloride ions and for the S,2 reaction of benzyl bromide with hydroxide ion using transition state models characterized by single interaction force constant, F,,, between the bond being formed and the bond being broken. The isotope effect calculations show that the dependence of calculated 13C isotope effects on transition state symmetry found by Willi and Sims er al. holds true for reaction barriers corresponding to small values of v,, while the symmetry dependence observed by Bron holds true for barriers corresponding to large values of v,. v, was also found to have a strong influence on the reaction coordinate motions of the transition states. In particular, for the methyl iodide reactions an increase in v, increases the distortion of the methyl group in the direction expected for a classical S,2 reaction. Finally, reaction coordinate motions were used to show that the model proposed by Bron for the borderline region between S,1 and S,2 reaction mechanisms predicts an increase in the "C kinetic isotope effect with decreasing total bond order and not the decrease suggested by Bron.WARREN EDWARD BUDDENBAUM et VERNON JACK SHINER, JR. Can. J. Chem. 54, 1146Chem. 54, (1976.On a calcule les mouvements des coordonnees de la reaction et les effets isotopiques cinetiques du 13C a 25 ' C pour les reactions S,2 de I'iodure de methyle avec les ions iodure, cyanure et chlorure et pour la reaction SN2 du bromure de benzyle avec I'ion hydroxyde en utilisant des modeles d'etats de transition caracterises par une constante de force pour une seule interaction, F,,, entre le lien qui se forme et le lien qui se brise. Les calculs d'effet isotopique montrent que la dependance des effets isotopiques de 13C sur la symetrie des etats de transition qui a ete trouvee par Willi et par Sims el al. prevaut pour des barrieres a la reaction correspondant a de petites valeurs de v,, alors que la dependance sur la symetrie observee par Bron prevaut dans les cas ou les barrieres a la reaction correspondent a des valeurs elevees de v,. On a aussi trouve que v, a une influence importante sur les mouvements des coordonnees de la reaction des etats de transition. En particulier, pour les reactions de I'iodure de methyle, une augmentation de v, augmente la distorsion du groupe mtthyle dans la direction attendue pour une reaction SN2 classique. Finalement, on a utilise les mouvements des coordonnees de reaction pour montrer quelle modele propose par Bron pour la region intermediaire entre les mecanismes de reaction S, I et SN2 predit une augrnenrarion de I'effet isotopique cinetique du 13C avec une diminution de I'ordre total de liaison et non pas une diminurion telle que sugeree par Bron.[Traduit par le journal]Theoretical considerations lead to the pre-the transfer of a carbon (C*) fragment between diction that primary hydrogen kinetic isotope two groups, X and Y ...

Prediction of Isotope Effects for Anticipated Intermediate Structures in the Course of Bacterial Denitrification

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A New Insight into Using Chlorine Leaving Group and Nucleophile Carbon Kinetic Isotope Effects To Determine Substituent Effects on the Structure of SN2 Transition States

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Experimental analyses of the chemical dynamics of ribozyme catalysis

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