A Quantum-Chemical and Correlation Study of the Ionization of Purpurin

Anthraquinoid tautomers participate in the ionization of purpurin. The tautomerism takes place in both ground and excited states of the molecules; the excited state is more sensitive to the tautomerism. The solvation and ionization shift the tautomeric equilibria. In the experimental absorption spectra of purpurin, the major bands correspond to the 9,10-, 1,4-, and 1,10-anthraquinoid tautomers; anions with the 9,10-anthraquinoid structure are not detected. The position and intensity of the π,π* bands, and also the quantum-chemical parameters linearly correlate with the degree of ionization of purpurin.