Quantum-chemical and correlation study of quinizarine protonation

Fine structure of quinizarine πl,π*-absorption in sulfuric acid is defined by isomerism of its protonated products. The monocation is an isomeric mixture, the isomers differ by the charge localization on various carbon atom: 1,4,9-trihydroxy-10-oxo-10H-anthracen-1-onium, 1,9,10-trihydroxy-4-oxo-4H-anthracen-1-onium, and 4,9,10-trihydroxy-10-oxo-10H-anthracen-1-onium. For the dication the structures found were exclusively the conformers of 1,4,9,10-tetrahydroxyanthrqacene 1,10-dication containing 0 to 2 intramolecular hydrogen bonds. The protonation leads to a shift of the tautomeric and conformational equilibria characteristic of quinizarine.