spin label

Also contains definition of: spin probe
A @S05900@ @P04404@ @G02705@ (typically a nitryl radical) that is attached to a part of a @M03986@ whose microscopic environment is of interest and may be revealed by the electron spin @R05326@ (ESR) spectrum of the spin @L03424@. When a simple @P04404@ molecular entity is used in this way without covalent @A00512@ to the molecular entity of interest it is frequently referred to as a 'spin probe'.
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1166 [Terms] [Paper]