Metal complexation of protocatechuic acid and its derivatives: Determination of the optimal computational conditions for the simulation of electronic spectra

The most stable conformations of protocatechuic acid (PCAH) and its mono-deprotonated form (PCA−) have been determined. The ground and low-lying excited electronic states of PCAH in aqueous solution have been studied using DFT. Molecular and electronic properties were calculated using the B3LYP functional, while excited states were examined using TD-DFT methodology. For PCAH, the poor agreement between experimental and theoretical spectra observed with B3LYP and various basis sets leads us to test different functionals in order to improve the accuracy of the calculated electronic transitions. The VSXC and a modified B3LYP with the 6-31G(d,p) basis set have allowed to fairly reproduce the experimental data. These same functionals used with the 6-31+G(d, p) give for the anion very satisfying results. The validation of the methods is illustrated by the determination of the fixing site of Al(III) on PCAH.