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

► The alizarin intramolecular proton transfer takes place in the ground and excited states. ► The equilibrium between the normal form of alizarin and its tautomer form depends on the temperature. ► A value of 1.2 kcal mol−1 has been calculated for the free enthalpy of the reaction. ► At room temperature, we estimate that about 10% of alizarin is under the tautomer form.