Term: radical pair (geminate pair)
https://doi.org/10.1351/goldbook.R05074

Definition:
The term is used to identify two radicals in close proximity in solution, within a solvent cage. They may be formed simultaneously by some unimolecular process, e.g. peroxide decomposition, or they may have come together by diffusion. While the radicals are together, correlation of the unpaired electron spins of the two species cannot be ignored: this correlation is responsible for the CIDNP phenomenon. A radical pair is called geminate provided that each radical partner is a descendant of the same parental pair.

Related Terms:
1) geminate recombination (http://doi.org/10.1351/goldbook.G02603).
2) radicals (http://doi.org/10.1351/goldbook.R05066).
3) cage (http://doi.org/10.1351/goldbook.C00769).
4) unimolecular (http://doi.org/10.1351/goldbook.M03989).
5) cidnp (http://doi.org/10.1351/goldbook.C01079).
6) decomposition (http://doi.org/10.1351/goldbook.D01547).
7) diffusion (http://doi.org/10.1351/goldbook.D01716).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1156 (https://doi.org/10.1351/pac199466051077)

Citation: 'radical pair (geminate pair)' 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.R05074

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