Protonation of 1,4,5-tri-and 1,4,5,8-tetrahydroxyanthraquinones in sulfuric acid: Multistep reaction involving tautomers and conformers

The fi ne structure of the π l,π*-absorption of hydroxyanthraquinones solutions in sulfuric acid arises due to the existence of the protonated forms as equilibrium mixtures of tautomers and conformers distinguished by the positions of the π-bonds, charges, and the number of intramolecular hydrogen bonds. Using quantum-chemical calculations and correlation analysis of the absorption spectra tautomers were identifi ed of mono- and dications of 1,4,5-trihydroxyanthraquinone of 9,10-, 1,4-, and 1,10-isomeric structure. For 1,4,5,8-tetrahydroxyanthraquinone 9,10-, 1,4-, 1,10-, and 1,5-isomeric mono- and dications and their conformers were found. © 2012 Pleiades Publishing, Ltd.