Energetics of Hydroxybenzoic Acids and of the Corresponding Carboxyphenoxyl Radicals. Intramolecular Hydrogen Bonding in 2-Hydroxybenzoic Acid

The energetics of the phenolic O−H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O−H- - -O−C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Δ_f (3-OHBA, cr) = −593.9 ± 2.0 kJ·mol^-1 and Δ_f (4-OHBA, cr) = −597.2 ± 1.4 kJ·mol^-1, by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K:  Δ_sub (2-OHBA) = 94.4 ± 0.4 kJ·mol^-1, Δ_sub (3-OHBA) = 118.3 ± 1.1 kJ·mol^-1, and Δ_sub (4-OHBA) = 117.0 ± 0.5 kJ·mol^-1. From the obtained Δ_f (cr) and Δ_sub values and the previously reported enthalpy of formation of monoclinic 2-OHBA (−591.7 ± 1.3 kJ·mol^-1), it was possible to derive Δ_f (2-OHBA, g) = −497.3 ± 1.4 kJ·mol^-1, Δ_f (3-OHBA, g) = −475.6 ± 2.3 kJ·mol^-1, and Δ_f (4-OHBA, cr) = −480.2 ± 1.5 kJ·mol^-1. These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC₆H₄O^•, g) = −322.5 ± 3.0 kJ·mol^-1 Δ_f (3-HOOCC₆H₄O^•, g) = −310.0 ± 3.0 kJ·mol^-1, and Δ_f (4-HOOCC₆H₄O^•, g) = −318.2 ± 3.0 kJ·mol^-1. The O−H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 ± 3.3, 383.6 ± 3.8, and 380.0 ± 3.4 kJ·mol^-1, respectively. Finally, by using the ortho−para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ·mol^-1, which is ca. 6−9 kJ·mol^-1 above the one estimated in its parent (2-OHBA), viz. 20.2 kJ mol^-1 (theoretical) or 17.1 ± 2.1 kJ·mol^-1 (experimental).