ELUCIDATION OF THE MULTIPLE EQUILIBRIA OF MALVIN IN AQUEOUS-SOLUTION BY ONE-DIMENSIONAL AND 2-DIMENSIONAL NMR

One- and two-dimensional NMR were used to characterize the several forms of malvin present in aqueous solution in the pH range 0.3-4.5 and to determine their molar fractions as a function of pH. In addition to the flavylium cation, two hemiacetal forms and both the cis and trans forms of chalcone were firmly identified. The pathways for the interconversion of the different forms were derived and the molar absorbances of the species calculated by coupling NMR and UV/Vis data. Equilibrium constants were determined at different temperatures, and enthalpy and entropy changes were calculated for the interconversion processes. The conclusions are supported by molecular orbital calculations.