Oxidation of 4a,4b-Dihydrophenanthrene. IV. Kinetic Isotope Effect of Deuterium in the Propagation and the Initiation Steps

A kinetic isotope effect was found in one of the propagation steps of the reaction of perdeutero-4a,4b-dihydrophenanthrene (PH2) with O2. In this step, PH2+HO2·→PH·+H2O2, the kinetic isotope effect is 7.2 at − 10°C and 9.9 at − 31°C. The theoretical calculation by the previously described method predicts for this step an isotope effect of the correct magnitude. The success of the same method, using a similar theoretical model, for the treatment of reactions involving widely different extents of tunneling is a good indication to the validity of the isotope effect calculations and supports the previous conclusions about the exceptional importance of tunneling in the initiation step. As tunneling is relatively unimportant in the propagation step, the kinetic isotope effect is markedly reduced compared to its value in the initiation step. Hence, the kinetic isotope effect is significantly reduced compared to that in the initiation step. The ratio between the pre-exponential factors in the initiation step for the oxidation of the perhydro and perdeutero compounds is discussed. Bell's criterion for reactions involving tunneling is AD / AH≫1, while in the initiation step the ratio is close to unity. It is concluded that in reactions with a significant tunneling effect the ratio AD / AH may vary within limits, being less than, equal to, or greater than unity.