The 3,3′,5,5′-tetraalkyl-4,4′-diphenoquinones exhibit their most intense IR band near 1600 cm−1, and is assigned to a complex mode involving out-of-phase (C=O)2 stretching. A combination tone assigned as in-phase (C=0)2 stretching and out-of-phase stretching suggests that the frequency separation between the two (C=O)2 stretching modes is small. Other characteristic group frequencies are also discussed.
