Gallic acid complexation with Al3+ under acidic condition – 27Al NMR and DFT study –

The complexation behavior of the Al3+- Gallic acid (C6H2(OH)3COOH, GA) system under acidic condition was determined through a combination of titration experiments, 27Al NMR, 1H NMR and DFT calculations. The 27Al NMR measurements revealed that Al3+ and GA formed 1:1 monodentate and bidentate complexes. In the monodentate complex, the binding site was the carboxylic group in GA, and the chemical shift in the 27Al NMR spectrum was very similar to that observed in the monocarboxylic acid system. DFT calculations well reproduced the structure of the 1: 1 bidentate complex and the 27Al NMR chemical shift values (exp: 15.7 ppm, calc.: 14.7 ppm) well, and the results revealed that the 1: 1 bidentate complex was formed by the reaction between two phenolic OH groups and Al3+. The titration curve was accurately fitted based on the amount of H+ ions released by the formation of the monodentate and bidentate complexes. The average conditional formation constants of each complex calculated from the 27Al NMR experiments were −3.87 (log K1) (1: 1 monodentate) and −1.15 (log K2) (1: 1 bidentate).