Title: encounter complex
Long Title: IUPAC Gold Book - encounter complex
DOI: 10.1351/goldbook.E02087
Status: current

Definition
A complex of molecular entities produced at an encounter-controlled rate, and which occurs as an intermediate in a reaction mechanism. When the complex is formed from two molecular entities it is called an 'encounter pair'. A distinction between encounter pairs and (larger) encounter complexes may be relevant in some cases, e.g. for mechanisms involving pre-association.

Related Terms
- complex: https://goldbook.iupac.org//terms/view/C01203
- encounter: https://goldbook.iupac.org//terms/view/E02086
- encounter-controlled rate: https://goldbook.iupac.org//terms/view/E02089
- molecular entities: https://goldbook.iupac.org//terms/view/M03986
- pre-association: https://goldbook.iupac.org//terms/view/P04794
- reaction mechanism: https://goldbook.iupac.org//terms/view/M03804

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1112 (https://doi.org/10.1351/pac199466051077)

Related Reference
- PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2239 (https://doi.org/10.1351/pac199668122223)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/E02087/html
- json: https://goldbook.iupac.org/terms/view/E02087/json
- xml: https://goldbook.iupac.org/terms/view/E02087/xml

Citation: Citation: 'encounter complex' 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.E02087
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Accessed: 2026-06-12T14:52:36+00:00