Studies of the tautomeric forms of alizarin in the ground state by electronic spectroscopy combined with quantum chemical calculations

Intramolecular proton transfer of alizarin in the first excited state has been reported previously. In this study, we show that this transfer is also present in the ground state. In addition to the absorption of the normal form of alizarin (form A), electronic spectra recorded in methanol exhibit a small absorption band at 560 nm, which is due to the tautomeric form of alizarin (form B). This is confirmed by time-dependent DFT calculations. At room temperature, we estimate that about 10% of alizarin is under the form B. The Gibbs free energy of the equilibrium between the two tautomers is evaluated to 1.2 kcal mol−1.