DFT calculations of the IR and Raman spectra of anthraquinone dyes and lakes

The IR and Raman spectra of alizarin, purpurin, kermesic acid, and carminic acid have been computed performing density functional theory calculations at the B3LYP/6-31G(d) level of theory. The computational approach adopted has been validated on the basis of a series of calculations with different level of theory on the anthraquinone molecule, the common moiety of these dyes. The computed vibrational frequencies have been adopted as a guideline to propose the vibrational assignment of the dyes and to obtain useful information on the Raman spectra of their aluminum complexes.