Computational study of oxidation mechanism of mineral green pigments.

The molecular structures, electronic properties, and the UV-vis absorption spectra of the oxides of mineral green pigment (MGP) have been theoretically calculated by using the density-functional theory (DFT) method. Our results reveal that the ground-state structure (isomer-I) of the oxides takes on an O-bridged bond with the polycyclic ring. The chemical stabilities of the oxides (I-V) are gradually reduced, of which the I isomer has the largest gap (2.50 eV). Moreover, the adsorption of the two oxygen atoms tailors the electronic structures of oxides, and the electronic properties are keys to understand the structural stabilization of the complexes. Additionally, the strongest UV-vis absorption band of the I isomer has been assigned; e.g., the crucial excitation originates from a HOMO-17→LUMO (32%) transition at ~ 187.3 nm.