kinetic isotope effect

https://doi.org/10.1351/goldbook.K03405
The effect of isotopic substitution on a rate constant is referred to as a kinetic isotope effect. For example, in the reaction:
K03405-1.png
the effect of isotopic substitution in reactant A is expressed as the ratio of rate constants \(\frac{k^{\text{l}}}{k^{\text{h}}}\), where the superscripts \(\text{l}\) and \(\text{h}\) represent reactions in which the molecules A contain the light and heavy isotopes, respectively. Within the framework of transition state theory in which the reaction is rewritten as:
K03405-2.png
and with neglect of isotopic mass on tunnelling and the transmission coefficient, \(\frac{k^{\text{l}}}{k^{\text{h}}}\) can be regarded as if it were the equilibrium constant for an isotope exchange reaction between the transition state [TS] and the isotopically substituted reactant A, and calculated from their vibrational frequencies as in the case of a thermodynamic isotope effect. Isotope effects like the above, involving a direct or indirect comparison of the rates of reaction of isotopologues, are called 'intermolecular', in contrast to intramolecular isotope effects, in which a single substrate reacts to produce a non-statistical distribution of isotopomeric product molecules.
See also: isotope effect
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1130 (https://doi.org/10.1351/pac199466051077)