Modelhyphen;reaction calculations are used to show that relative14Csngbnd;13C kinetic isotope effects, defined asrequals;lnlpar;k12sol;k14rpar;sol;lnlpar;k12sol;k13rpar;, should, within the framework of the presently accepted statisticalhyphen;thermodynamic theory of such effects in the harmonic approximation, generally be restricted to the range1.8le;rle;2.0. Significant deviations from this range may occur when the individual14C and13C kinetic isotope effects are of unusually small magnitude and/or are associated with easily detectable temperaturehyphen;dependence anomalies. Reported cases of experimental values ofrsignificantly outside the 1.8ndash;2.0 range are examined with regard to the modelhyphen;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.
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