Relative 14C–13C Kinetic Isotope Effects

Abstract

Model‐reaction calculations are used to show that relative ¹⁴C–¹³C kinetic isotope effects, defined as $\mathrm{r=ln}{\mathrm{(k}}_{12}{\mathrm{/k}}_{14}\mathrm{)/}\ln {\mathrm{(k}}_{12}{\mathrm{/k}}_{13})$ , should, within the framework of the presently accepted statistical‐thermodynamic theory of such effects in the harmonic approximation, generally be restricted to the range $\mathrm{1.8}\le \mathrm{r}\text{≤2.0}$ . Significant deviations from this range may occur when the individual ¹⁴C and ¹³C kinetic isotope effects are of unusually small magnitude and/or are associated with easily detectable temperature‐dependence anomalies. Reported cases of experimental values of r significantly outside the 1.8–2.0 range are examined with regard to the model‐reaction calculations. It is concluded that, if the reported effects in four of the five such cases are real, additional complications, not considered in the basic theory as usually applied, must be operative. The results of this study are in accord with, and qualify only slightly, a conclusion reached by Bigeleisen almost two decades ago.