Term: transition species
https://doi.org/10.1351/goldbook.T06467

Definition:
Any intermediate species in an elementary reaction. An activated complex is an example of a transition species, but the latter term also includes other intermediate species. Transition species have also been called transition states, but there is then danger of confusion with a true activated complex, which is also commonly known as a transition state.

Related Terms:
1) elementary reaction (http://doi.org/10.1351/goldbook.E02035).
2) activated complex (http://doi.org/10.1351/goldbook.A00092).
3) transition state (http://doi.org/10.1351/goldbook.T06468).

Source: PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 190 (https://doi.org/10.1351/pac199668010149)

Citation: 'transition 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.T06467

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