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Inference of transition state geometries from kinetic isotope effects: An ab initio study of an E2 model system

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  • Sanne Schroeder Glad, University of Odense
  • ,
  • F. Jensen
Transition states for the E2 reaction of ethyl chloride with 11 different nucleophiles have been calculated at the MP2/6-31+G* level. The geometries comply with the rules of Thornton and the postulate of Hammond, and More O'Ferrall-Jencks diagrams show that the reactions are central and do not indicate E1-like mechanisms. Three different kinetic isotope effects have been calculated for each nucleophile, and the variation with respect to different nucleophiles is shown to be due to hydrogen bending vibrations. Contrary to commonly accepted theories of kinetic isotope effects, the primary isotope effect does not pass through a maximum for the most symmetric transition structures. Furthermore, there is no simple overall correlation between secondary kinetic isotope effects and the geometry of the transition states. It is shown that the charge distribution at the transition state is not directly correlated with the geometry and that the common assumption that bond distances and force constants are proportional may not be valid. This casts some doubt on the validity of drawing conclusions about the transition state geometry from kinetic isotope effects.
Original languageEnglish
JournalJournal of the American Chemical Society
Pages (from-to)9302-9310
Number of pages9
Publication statusPublished - 1994
Externally publishedYes

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