06451 IUPAC Gold Book - transient species 10.1351/goldbook.T06451 T06451 current synonym: transient chemical species 1 Relating to a short-lived reaction intermediate. It can be defined only in relation to a time scale fixed by the experimental conditions and the limitations of the technique employed in the detection of the intermediate. The term is a relative one. Transient species are sometimes also said to be 'metastable'. However, this latter term should be avoided, because it relates a thermodynamic term to a kinetic property, although most transients are also thermodynamically unstable with respect to reactants and products. intermediate https://goldbook.iupac.org/terms/view/I03096 metastable https://goldbook.iupac.org/terms/view/M03872 persistent https://goldbook.iupac.org/terms/view/P04516 unstable https://goldbook.iupac.org/terms/view/U06569 PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1174 (https://doi.org/10.1351/pac199466051077) https://goldbook.iupac.org/terms/view/T06451/html https://goldbook.iupac.org/terms/view/T06451/json https://goldbook.iupac.org/terms/view/T06451/plain Citation: 'transient species' 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.T06451 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. 2026-05-10T14:50:00+00:00