INFLUENCE OF SUBSTITUENTS ON KINETIC ISOTOPE EFFECTS

The E2 reactions of methyl- and fluoro-substituted ethyl chlorides wit h CH3-, NH2-, OH-, F-, SH-, and Cl- have been investigated at the MP2/ 6-31+G(d) level of theory for probing whether kinetic isotope effects are related to the geometry of the transition stucture. The primary ki netic isotope effects only display the expected maximum for the most s ymmetric transition structure when corrections for equilibrium isotope effects are taken into account. Such corrections,are in general not p ossible from experimental data alone. Secondary kinetic isotope effect s at the CH2Cl end of ethyl chloride correlate qualitatively with the carbon hybridization, although a quantitative relationship is lacking, even for closely related systems. The corresponding isotope effects a t the carbon where a hydrogen is abstracted show little correlation wi th the geometry of the transition state.