Title: microscopic diffusion control
Long Title: IUPAC Gold Book - microscopic diffusion control
DOI: 10.1351/goldbook.M03912
Status: current

Definition
The observable consequence of the limitation that the rate of a bimolecular chemical reaction in a homogeneous medium cannot exceed the rate of encounter of the reacting molecular entities. If (hypothetically) a bimolecular reaction in a homogeneous medium occurred instantaneously when two reactant molecular entities made an encounter, the rate of reaction would be an encounter-controlled rate, determined solely by rates of diffusion of reactants. Such a hypothetical 'fully diffusion controlled rate' is also said to correspond to 'total microscopic diffusion control', and represents the asymptotic limit of the rate of reaction as the rate constant for the chemical conversion of the encounter pair into product (or products) becomes large relative to the rate constant for separation (or dissociation) of the encounter pair. 'Partial microscopic diffusion control' is said to operate in a homogeneous reaction when the rates of chemical conversion and of separation are comparable. (The degree of microscopic diffusion control cannot usually be determined with any precision.)

Related Terms
- bimolecular: https://goldbook.iupac.org//terms/view/M03989
- chemical reaction: https://goldbook.iupac.org//terms/view/C01033
- diffusion: https://goldbook.iupac.org//terms/view/D01716
- dissociation: https://goldbook.iupac.org//terms/view/D01801
- encounter: https://goldbook.iupac.org//terms/view/E02086
- encounter pair: https://goldbook.iupac.org//terms/view/E02090
- encounter-controlled rate: https://goldbook.iupac.org//terms/view/E02089
- mixing control: https://goldbook.iupac.org//terms/view/M03946
- molecular entities: https://goldbook.iupac.org//terms/view/M03986
- precision: https://goldbook.iupac.org//terms/view/P04799
- rate constant: https://goldbook.iupac.org//terms/view/O04322
- rate of reaction: https://goldbook.iupac.org//terms/view/R05156

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1140 (https://doi.org/10.1351/pac199466051077)

Related Reference
- PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 173 (https://doi.org/10.1351/pac199668010149)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/M03912/html
- json: https://goldbook.iupac.org/terms/view/M03912/json
- xml: https://goldbook.iupac.org/terms/view/M03912/xml

Citation: Citation: 'microscopic diffusion control' 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.M03912
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.
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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.
Accessed: 2026-04-29T09:36:14+00:00