Extremely high kinetic isotope effects and tunneling

Extremely high kinetic isotope effects and tunneling 

It has been found that the measured kinetic isotope effects (primary and secondary) are sometimes significantly higher than the expected values. Maximum isotope effects will be observed when there is practically no difference between the zero-point energies of isotopically labeled substrates in the reaction transition state (eq. 1.14.6). Then, as already pointed out, the difference between the energies of activation E - E.  

We should bear in mind that Arrhenius pre-exponential factors (A = FZ, where P is the steric factor and Z is collision number) are almost equal for all isotopically labeled compounds. Namely, the change of the number of neutrons in nuclei does not significantly influence the value of PZ, because the charge of the species does not change. Investigating different reaction models it was found that A /Aj) was always between 0.7 and 1.2 (for the temperature range 20-2000 K) the absolute minimum was O.S. Thus, we can write  

Secondary a-deuterium kinetic isotope effect and tunneling 

In reactions where the secondary a-deuterium kinetic isotope effect (a-D KIE) was substantial (20% per one D, or more) the mechanism was found to be dissociative (Sj.jl), with the transition state similar to the carbonium ion. Small a-KIE were found in nucleophilic substitutions of the associative type (8 2). In these substitutions amounted to about 1.00 (no isotope 

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