Energetic and Structural Properties of Two Phenolic Antioxidants: Tyrosol and Hydroxytyrosol

Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol (<b>1</b>), hydroxytyrosol (<b>1</b>OH) molecules and the corresponding radicals <b>1</b>_rad^• and <b>1</b>O_rad^• are reported in this work. The experimental values of the gas-phase enthalpy of formation, Δ_f<i>H</i>_m⁰(g), in kJ·mol^–1, of <b>1</b> (−302.4 ± 3.4) and <b>1</b>OH (−486.3 ± 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite <i>ab initio</i> G3 and G4 levels of theory, provided results that served to (i) confirm the excellent consistency of the experimental measurements performed, (ii) establish that the stabilizing effect of H-bond of hydroxyethyl chain and aromatic ring (OH···π interaction) is smaller in radicals than in parent molecules, (iii) deducecombining experimental data in isodesmic reactionsΔ_f<i>H</i>_m⁰(g) of radicals <b>1</b>_rad^• (−152.3 ± 4.4 kJ·mol^–1) and <b>1</b>O_rad^• (−370.6 ± 3.8 kJ·mol^–1), (iv) estimate a reliable O–H bond dissociation enthalpy, BDE of <b>1</b> (368.1 ± 5.6 kJ·mol^–1) and of <b>1</b>OH (333.7 ± 5.6 kJ·mol^–1), and (v) corroborateusing “BDE criteria”than <b>1</b>OH is a more effective antioxidant than <b>1