03405 IUPAC Gold Book - kinetic isotope effect 10.1351/goldbook.K03405 K03405 current antonym: inverse kinetic isotope effect related: normal kinetic isotope effect 1 The effect of isotopic substitution on a rate constant is referred to as a kinetic isotope effect. For example, in the reaction: the effect of isotopic substitution in reactant $\ce{A}$ is expressed as the ratio of rate constants \(\frac{k^{\rm{l}}}{k^{\rm{h}}}\), where the superscripts \(\rm{l}\) and \(\rm{h}\) represent reactions in which the molecules $\ce{A}$ contain the light and heavy isotopes, respectively. Within the framework of transition state theory in which the reaction is rewritten as: and with neglect of isotopic mass on tunnelling and the transmission coefficient, \(\frac{k^{\rm{l}}}{k^{\rm{h}}}\) can be regarded as if it were the equilibrium constant for an isotope exchange reaction between the transition state $\ce{[TS]^{\ddagger}}$ and the isotopically substituted reactant $\ce{A}$, 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. equilibrium constant https://goldbook.iupac.org//terms/view/E02177 intermolecular https://goldbook.iupac.org//terms/view/I03098 intramolecular isotope effects https://goldbook.iupac.org//terms/view/I03132 isotope effect https://goldbook.iupac.org//terms/view/I03327 isotope exchange https://goldbook.iupac.org//terms/view/I03328 isotopes https://goldbook.iupac.org//terms/view/I03331 isotopically substituted https://goldbook.iupac.org//terms/view/I03338 isotopologues https://goldbook.iupac.org//terms/view/I03351 isotopomeric https://goldbook.iupac.org//terms/view/I03352 rate constant https://goldbook.iupac.org//terms/view/O04322 thermodynamic isotope effect https://goldbook.iupac.org//terms/view/T06319 transition state https://goldbook.iupac.org//terms/view/T06468 transition state theory https://goldbook.iupac.org//terms/view/T06470 transmission coefficient https://goldbook.iupac.org//terms/view/T06479 tunnelling https://goldbook.iupac.org//terms/view/T06531 PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1130 (https://doi.org/10.1351/pac199466051077) https://goldbook.iupac.org/terms/view/K03405/html https://goldbook.iupac.org/terms/view/K03405/json https://goldbook.iupac.org/terms/view/K03405/plain Citation: 'kinetic isotope effect' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.K03405 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-05-31T05:49:26+00:00