KIE Magnitude and Variation with Reaction Asymmetry

The KIE behavior versus reaction asymmetry for adiabatic PT follows directly [4] from insertion of the isotopic difference between the PER curves described in Eqs. (10.18) and (10.19). The general feature that the KIE is maximal for AGrxn 0 follows from a Bronsted coefficient for a symmetric reaction that is isotope-independent, Sq = 1/2, which reflects the symmetric nature of the electronic structure of the reacting pair at the TS (cf. Eq. (10.12)) [48]. The decrease from the maximum, characterized by a gaussian fall-off with increasing reaction asymmetry, is due to the isotope dependence a ,H 5 oD- As discussed in Section 10.2.2, this isotope dependence is primarily due to the differential rate of change of ZPEl-versus reaction asymmetry between H and D. 

The reaction free energy dependence on the KIE has often been modeled with an expression based on the empirical Marcus PER for PT Eq. (10.6) [9c, 19]. 

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