Validation of Quantum Chemistry Predictions. Quinoidal-Base Tautomers in Anthocyanins and Related Compounds

Using suitable monoprotic model compounds, we deduce the relative mole fraction distribution of tautomers in anthocyanins and related polyprotic molecules. For species bearing OH at positions C7 and C4', only the C7 tautomer is relevant. When OH groups occupy positions C7, C5, and C4', the C7 tautomer is the major with C5 as the minor. The apigeninidin absorption spectrum is reproduced by a model comprising the tautomers with OH at C7 (78%), C5 (21%), and C4' (1%).