Quantum-Chemical Calculations for the Electronic Absorption Spectra of Certain Anthocyanidins

Abstract

Semi-empirical PM3 calculations are made for the electronic absorption spectra of colored forms of nonmethylated anthocyanidins (aurantinidin, pelargonidin, cyanidin, and delphinidin). The position of the absorption band peak is shown to depend on the orientation of hydroxyl groups in the aglycone structure. The observed tendency toward a bathochromic shift after ОН groups are added to the ring generally corresponds to experimental data. The results for the uncharged quinonoid forms of pelargonidin (a hypsochromic shift of absorption bands) do not in this case agree with the prevailing opinion of specialists about the bathochromic shift. While this opinion is confirmed by DFT calculations, there are still no reliable experimental data on their validity. The formation of single-charged (negatively) quinonoid structures is clearly the reason for the bathochromic shift of absorption bands, which is consistent with the experimental data. When the charge multiplicity increases (to −2), a hypsochromic shift of absorption bands to the experimentally observed yellow color of anthocyanins in alkaline solutions is again predicted.