DFT study of the structure of hydroxybenzoic acids and their reactions with OH and radicals

B3LYP/6-31++G(d) method was used for the structural study of 3,4,5-trihydroxybenzoic acid (3,4,5-THBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), and 4-hydroxybenzoic acid (4-HBA). Calculated structures agree with available X-ray experimental data within 2%. The phenolic OH bond dissociation enthalpy (BDE) of all sites in each benzoic acid were determined and compared with those of phenol (for 4-HBA) and catechol (for 3,4-DHBA). The consistency between the calculated values and experimental ones are within 5.4% and 9.2%, respectively, for 4-HBA and 3,4-DHBA. The reactions of benzoic acids with and OH radicals were studied and it turns out that benzoic acids react differently with both radicals. We have shown that the reaction of hydroxybenzoic acids with the hydroxyl radical was governed by a phenolic hydrogen (H⁺ + e⁻) transfer from the acid to the radical, while for the superoxide radical, the reaction is governed by a proton (H⁺) transfer from the acid to the radical. The first reaction is evidenced by the delocalization of the radical on the entire quinone moiety, and the second reaction is evidenced by the negative NBO charge on the phenoxide moiety as well as the localization of the radical on the hydroperoxy (O₂H) moiety.