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Meaning of semiquinone

sem•i•qui•none

Pronunciation: (sem"ē-kwi-nōn', -kwin'ōn, sem"ī-), [key]
— n. Chem.
  1. any of the class of free radicals formed as intermediates in the oxidation of a hydroquinone to a quinone.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.