The literature (1,2) lists the C=O stretching mode for substances with two quinonoid carbonyls in the same ring between 1660 and 1680 cm−1. The spectra of some 3,3',5,5'-tetraalkyl-4,4'-diphenoquinones have been recorded and it was found that in such compounds the position of the strongest absorption, and hence what is considered to be the carbonyl band, is 1599 ± 9 cm−1. The band position is apparently insensitive to the effects of physical state, mass, and vibrational coupling because its location is the same in solution and solid spectra as shown in Table I. Diphenoquinone itself has an exceptionally low C=O band (1,2) at about 1630 cm−1, but the alkyl substitution discussed here lowers the value even more.
J. M. Gordon and J. W. Forbes, "Carbonyl Frequency and Stability of Some Diphenoquinones," Appl. Spectrosc. 15, 19-19 (1961)