Term: transient (chemical) species
https://doi.org/10.1351/goldbook.T06451

Definition:
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.

Related Terms:
1) persistent (http://doi.org/10.1351/goldbook.P04516).
2) intermediate (http://doi.org/10.1351/goldbook.I03096).
3) unstable (http://doi.org/10.1351/goldbook.U06569).
4) metastable (http://doi.org/10.1351/goldbook.M03872).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1174 (https://doi.org/10.1351/pac199466051077)

Citation: 'transient (chemical) species' 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.T06451

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