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A verdazyl radical is a chemical compound that contains an unpaired electron that is shared over the four nitrogen atoms of the heterocyclic core. It differs from the traditional view of radicals in that it is stable.

History
In 1963, Kuhn and Trischmann reported the synthesis of a paramagnetic material upon the attempted methylation of a formazan and subsequent oxidation using atmospheric oxygen: The resulting verdazyl radical has a CH2 (methylene) group at the bridge position and is termed a Kuhn verdazyl. Since this time, many variations on the structure of the verdazyl radical have been done. This has included the replacement of the bridge position by CO (carbonyl, "6-oxoverdazyl") and CS (thiocarbonyl, "6-thioverdazyl"). In addition, the groups at the various positions of the molecule have been replaced.

Properties
Due to the unpaired electron, the compounds are electron paramagnetic resonance active molecules. Verdazyl radicals exhibit a 9-line pattern with hyperfine values of ~5,6. This indicates an equal sharing of the spin on each of the 4 nitrogen atoms. The molecules are highly coloured and have been explored by UV-vis spectroscopy, the Kuhn verdazyls exhibit a green colour (λmax=600nm). While the 6-oxoverdazyl radicals have a red colour (λmax=500nm).