Term: ion pair return
https://doi.org/10.1351/goldbook.I03233

Definition:
The recombination of a pair of ions R+ and Z− formed from ionization of RZ. If the ions are paired as a tight ion pair and recombine without prior separation into a loose ion pair this is called 'internal ion-pair return': I03233-1.png  It is a special case of 'primary geminate recombination'. If the ions are paired as a loose ion pair and form the covalent chemical species via a tight ion pair, this is called 'external ion-pair return': I03233-2.png It is a special case of 'secondary geminate recombination'. When the covalent molecule RZ is reformed without direct evidence of prior partial racemization or without other direct evidence of prior formation of a tight ion pair, (e.g. without partial racemization if the group R is suitably chiral) the internal ion-pair return is sometimes called a 'hidden return'. External (unimolecular) ion-pair return is to be distinguished from 'external (bimolecular) ion return', the (reversible) process whereby dissociated ions are converted into loose ion pairs: I03233-3.png 

Related Terms:
1) ionization (http://doi.org/10.1351/goldbook.I03183).
2) loose ion pair (http://doi.org/10.1351/goldbook.I03231-2).
3) geminate recombination (http://doi.org/10.1351/goldbook.G02603).
4) racemization (http://doi.org/10.1351/goldbook.R05030).
5) chiral (http://doi.org/10.1351/goldbook.C01057).
6) unimolecular (http://doi.org/10.1351/goldbook.M03989-1).
7) bimolecular (http://doi.org/10.1351/goldbook.M03989-2).
8) tight ion pair (http://doi.org/10.1351/goldbook.I03231-1).

Images:
1) Not defined (https://goldbook.iupac.org/img/inline/I03233-1.png) 
2) Not defined (https://goldbook.iupac.org/img/inline/I03233-2.png) 
3) Not defined (https://goldbook.iupac.org/img/inline/I03233-3.png) 

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1127 (https://doi.org/10.1351/pac199466051077)

Citation: 'ion pair return' 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.I03233

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