DFT and experimental investigation of catecholate derivatives of benzoic acid and pyridine

DFT calculations, at the B3LYP/TZVP level of theory for pyrocatechuic acid (2,3-dihydroxybenzoic acid, 2,3-DHBA), 2,3-dihydroxy-pyridine 2,3-DHPY and their ionized and oxidized forms, have been performed, in combination with experimental data. ¹H, ¹³C, 2D COSY NMR, IR and electronic spectra were coupled to the theoretical calculations. The geometrical parameters were checked by reported crystallographic data. The neutral form of pyrocatechuic acid is the most stable, regarding its ionized (mono-, di- or tri-anions) and oxidized ([2,3-DHBA-sqH]⁻, [2,3-DHBA-sq]²⁻, [2,3-DHBA-q]⁻) species. The most stable conformer 2,3-DHBA-H₃ displays the COOH– group co-planar to the catechol ring, hydrogen bonded with OH(2). In the [2,3-DHBA-H₂]⁻ the stable conformer shows the presence of protonated COOH, while OH(2) is ionized. The tri-anion is the form of 2,3-DHBA with the highest energy. Among the protonated semiquinone radical forms [2,3-DHBA-sqH]⁻, more stable is the OH(3)-oxidized, cited 21.3 kcal/mol lower in energy from the OH(2)-oxidized; in this latter the COO⁻ group lies perpendicular to the benzene ring. The same calculation procedure fitted on the oxygenated [2,3-DHBA-H-O₂]²⁻ shows a weak π-bonding between O(2) and dioxygen, strongly H–bonded to OH(3), while the C(2)–O bond order increases. The different way of 2,3-DHBA oxidation parallels the different, from 3,4-isomer, degradation products. Our DFT calculations show that the keto/enol tautomeric forms of the neutral 2,3-DHPY-H₂ differ by 5.02 kcal/mol. Both species give, upon ionization, the [2,3-DHPY-H]⁻ with the OH(2) deprotonated. The electronic density distribution of [2,3-DHPY-q] justifies further reactions (degradation or dienic addition) as experimentally observed.