Kinetic isotope effects and transition state geometries. A theoretical investigation of E2 model systems

Ab initio calculations at the MP2/6-31+G* level have been performed on E2 model systems to investigate whether differences in kinetic isotope effects correlate with changes in transition state geometries. By combining various nucleophiles (NH2-, OH-, F-, PH2-, SH-, Cl-) and leaving groups (NH3, Br-, Cl-, F-, SH-) for reactions of the type Nu(-) + CH(3)CH(2)X, a large diversity of transition structures from reactant-like to product-like are generated. For each reaction one primary and two different alpha-secondary kinetic isotope effects are calculated. The primary kinetic isotope effects depend strongly on the nucleophilic placement in the periodic system, which mainly is due to differences in equilibrium isotope effects. When this effect is subtracted, the primary kinetic isotope effects display the expected maximum for symmetric transition structures, although the maximum is broad. The secondary kinetic isotope effects associated with the leaving group provide a qualitative correlation with the hybridization at the carbon, but the corresponding effects at the carbon where the hydrogen abstraction takes place is uncorrelated with the transition state geometry.