spin label

Also contains definition of: spin probe
https://doi.org/10.1351/goldbook.S05868
A
stable
paramagnetic
group
(typically a nitryl radical) that is attached to a part of a
molecular entity
whose microscopic environment is of interest and may be revealed by the electron spin
resonance
(ESR) spectrum of the spin
label
. When a simple
paramagnetic
molecular entity is used in this way without covalent
attachment
to the molecular entity of interest it is frequently referred to as a 'spin probe'.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1166 [Terms] [Paper]