Solid stoichiometric adducts of 9,10‐anthraquinone with SbCl₅, ZrCl₄, TiCl₄, SnCl₄, AlCl₃, have been prepared. The very important lowering Δω of the IR. carbonyl frequency, ranging from −175 cm⁻¹for SbCl₅ to −117 cm⁻¹ for SnCl₄ , shows that the acceptor is linked by a dative bond to the carbonyl oxygen atom acting as a donor; an assignment of most of the fundamental IR. frequencies is proposed for anthraquinone and the mentioned adducts.

Similar assignments and interpretation have been made for anthrone and its solid adduct with SbCl₅, ZrCl₄, TiCl₄, SnCl₄, AlCl₃ and ZnCl₂, where the C=O lowerings range from −164 cm⁻¹ for SbCl₅ to −97 cm⁻¹ for ZnCl₂ .