Deductions from Small Primary Deuterium Isotope Effects

Discussion is given on the use of primary deuterium isotope effects of small magnitude in conjunction with calculations to show whether the transition state is product-like or reactant-like. The primary deuterium isotope effect can be of nearly any magnitude, depending on the force constants of the transition state relative to those of the reactant. If the transition state has force constants exactly like those of the reactants, the kinetic isotope effect k/sub H/ /k/sub D/ is 1.00. If the transition state has force constants exactly like those of the products, k/sub H//k/sub D/ is the same as the equilibrium isotope effect K/sub H//K/sub D/ for the reaction. If the XH and HY force constants of the transition state are equal, k/sub H//k/sub D/ has some max value of 7 or more. It was concluded that the kinetic isotope effect must always be greater than K/ sub H//K/sub D/ if the transition state structure is on the product side of symmetrical. Therefore, if K/sub H//K/sub D/ were, say, 1.5 and k/sub H//k/sub D/ were 1.2, the transition state for the reaction must in reality be closer to reactants than to products. If, on the other hand, K/sub H//K/sub D/ were less than 1.00 and k/sub H//k/sub D/ were als o much less than 1.00, the transition state must be closer to products than to reactants. The concept was examined by application of the experimental data in 2 cases involving the abstraction of H atoms from toluene and deuterotoluenes by Cl atoms. (P.C.H.)