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

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Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol ( 1 ), hydroxytyrosol ( 1 OH) molecules and the corresponding radicals 1 rad • and 1 O rad • are reported in this work. The experimental values of the gas-phase enthalpy of formation, Δ f H m 0 (g), in kJ·mol –1, of 1 (−302.4 ± 3.4) and 1 OH (−486.3 ± 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite ab initio 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 H m 0 (g) of radicals 1 rad • (−152.3 ± 4.4 kJ·mol –1 ) and 1 O rad • (−370.6 ± 3.8 kJ·mol –1 ), (iv) estimate a reliable O–H bond dissociation enthalpy, BDE of 1 (368.1 ± 5.6 kJ·mol –1 ) and of 1 OH (333.7 ± 5.6 kJ·mol –1 ), and (v) corroborate—using “BDE criteria”—than 1 OH is a more effective antioxidant than 1 .