Absorption spectra of caffeic acid, caffeate and their 1: 1 complex with Al(III):: Density functional theory and time-dependent density functional theory investigations

The UV-visible absorption spectra of caffeic acid, caffeate and of the predominant complex obtained in the presence of aluminum ion ( 1: 1 stoichiometry) have been simulated by using the time-dependent density functional theory (TD-DFT) technique, taking into account solvent effects. Whereas the use of the B3LYP hybrid XC functional with the 6-31+G(d,p) basis set allows us to reproduce fairly well the essential features of the experimental spectra of caffeic acid and caffeate, it is necessary to introduce an effective core potential to properly describe the aluminum ion and its environment and to obtain a good agreement between theoretical and experimental spectra of the 1: 1 complex. The ligand presents two potential complexing sites in competition. The results of our calculations show that the aluminum ion coordinates preferentially at the level of the catecholate group, and the [Al(H2O)(4)(CA)], [Al(H2O)(3)(OH)(CA)](-) and [Al(H2O)(4)(HCA)](+) complexed forms could coexist in aqueous solution at pH = 5.