PRIMARY C-11/C-14-KINETIC AND SECONDARY H-1/H-2-KINETIC ISOTOPE EFFECTS IN THE SN2 REACTION OF HYDROXIDE ION WITH METHYL-IODIDE - THE RELATIONSHIP BETWEEN DIFFERENT CARBON ISOTOPE EFFECTS

The short-lived radionuclide 11C has been used in kinetic isotope effect(KIE) studies. The primary 11C/14C KIE for the reaction of hydroxide ion with labeled methyl iodide in 50% dioxane/water at 25 °C was determined to be 1.192 ± 0.001. A trend of progressively increasing KIE with mass difference is found when this value is compared to previously reported 12C/13C and 12C/14C KIEs. Simple theory predicts an almost linear relationship. The validity of the values of r = ln (k12/k14)/ln (k12/k13) and In (k11/k14)/ln (k12/k14) obtained by a simple theoretical treatment is confirmed by bebovib calculations of the KIEs. Transition-state (TS) models investigated were varied from reactant-to product-like and employed three different types of reaction coordinate movement in the TS; (A) the methyl group moving as one rigid mass unit in the decomposition mode; (B) the methyl hydrogens show Walden inversion the amount of which is independent of TS geometry; and (C) the Walden component is varied with TS geometry and most pronounced for the symmetric TS. The discrepancy between theory and experiment regarding the relation between different carbon KIEs is discussed, but no rationalization is given. The α-secondary 1H/2H KIE was determined to be 0.881 ± 0.012 and 0.896 ± 0.011 by using 11C and,4C as tracers in two double label experiments where kH/kD is calculated from the primary carbon KIE and the observed value of k11H/k14D or k11H/k14D, respectively. Comparison of experimental with theoretically calculated primary carbon and secondary deuterium KIEs shows the best agreement for reaction coordinate model C at a bond order between 0.1 and 0.3 for the forming C-O bond. The small discrepancy between the values of the secondary deuterium KIE is discussed in terms of tunneling. © 1990, American Chemical Society. All rights reserved.