Structure and properties of alizarin complex formed with alkali metal hydroxides in methanol solution.

Quantum chemical computations were used for prediction of the structure and color of alizarin complex with alkali metal hydroxides in methanolic solutions. The color prediction relying on the single Gaussian-like band once again proved the usefulness of the PBE0 density functional due to the observed smallest color difference between computed and experimentally derived values. It was found that the alkali metal hydroxide molecules can bind to the two oxygen atoms of both hydroxyl groups of alizarin or to one of these atoms and the oxygen atom from the keto group in a complex with three methanol molecules. This means that two electronic transitions need to be taken into account when considering the spectra of the studied complexes. The resulting bond lengths and angles are correlated with the properties of the alkali metal atoms. The molar mass, the atomic radius, and the Pauling electronegativity of studied metals are quite accurate predictors of the geometric properties of hydroxide complexes with alizarin in methanol solution. Graphical abstract The spectra of the neutral and monoanionic form of alizarin together with color changes resulting from addition of different metal hydroxides and represented in CIE color space.