Term: primary isotope effect
https://doi.org/10.1351/goldbook.P04831

Definition:
A kinetic isotope effect attributable to isotopic substitution of an atom to which a bond is made or broken in the rate-controlling step or in a pre-equilibrium step of a specified reaction. The corresponding isotope effect on the equilibrium constant of a reaction in which one or more bonds to isotopic atoms are broken, is called a 'primary equilibrium isotope effect'.

Related Terms:
1) secondary isotope effect (http://doi.org/10.1351/goldbook.S05523).
2) kinetic isotope effect (http://doi.org/10.1351/goldbook.K03405).
3) isotope effect (http://doi.org/10.1351/goldbook.I03327-1).
4) equilibrium constant (http://doi.org/10.1351/goldbook.E02177).
5) equilibrium isotope effect (http://doi.org/10.1351/goldbook.I03327-2).
6) rate-controlling step (http://doi.org/10.1351/goldbook.R05139).
7) pre-equilibrium (http://doi.org/10.1351/goldbook.P04810).

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)

Citation: 'primary isotope effect' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.P04831

License: 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.

Disclaimer: 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.