Calculations of isotope effects in elimination reactions. New experimental criteria for tunneling in slow proton transfers

Abstract

Isotope-effect calculations have been carried out on the model reaction CCH/sub 2/CH/sub 2/Cl + OH/sup -/ ..-->.. CCH=CH/sub 2/ + Cl/sup -/ + H/sub 2/O. When the bending motions of the nontransferred ..beta..-hydrogen are coupled with the stretching motion of the transferred ..beta..-hydrogen, a tunnel correction to the secondary ..beta..-hydrogen isotope effect is introduced. The kinetic secondary effect can be larger than the equilibrium secondary effect, in agreement with recent experiments. The contribution of tunneling to the secondary kinetic effect decreases with increased mass of the transferred atom. Deviations from the expected relation between deuterium and tritium isotope effects, both primary and secondary, can occur when tunneling is important, and are large enough to observe easily when k/sub H//k/sub D/ (or K/sub H//k/sub T/) calculated from k/sub D//k/sub T/ is compared with directly calculated k/sub H//k/sub D/ (or k/sub H//k/sub T/). Deuterium substitution at the ..beta..-carbon also reduces the contribution of tunneling to ..beta..-carbon isotope effects. Tunneling causes a decrease in the ratio of Arrhenius preexponential factors, A/sub light//A/sub heavy/, not only for primary hydrogen isotope effects but also for secondary hydrogen isotope effects and carbon isotope effects. The ratio increases again when the mass of the transferred atom increases.more » Deviations from the rule of the geometric mean can be expected when isotopic substitution at one position affects the contribution of tunneling to the isotope effect at another. These predictions provide new experimental tests for tunneling in slow proton transfers. 30 references, 7 tables.« less