Poly Methoxy Phenols in Solution: O−H Bond Dissociation Enthalpies, Structures, and Hydrogen Bonding

Abstract

The effect of methoxy substitution on the phenolic-hydrogen bond dissociation enthalpy has been established by a photoacoustic calorimetric method and by means of density functional theory (DFT) calculations. Experimentally, the relative BDE(O−H) in kcal mol-1 with respect to phenol are found to be as follows: 2-methoxyphenol (−4.0), 4-methoxyphenol (−4.9), 2,6-dimethoxyphenol (−10.6), 2,4-dimethoxyphenol (−9.0), 2,4,6-trimethoxyphenol (−13.6), and ubiquinol-0 (−12.0). The intramolecular hydrogen-bond enthalpy in o-methoxy-substituted phenol is −4.3 kcal mol-1; the intramolecularly hydrogen-bonded molecule forms an additional hydrogen bond with HBA solvents. The low BDE(O−H) for ubiquinol-0 of 78.5 kcal mol-1 and the protective intramolecular hydrogen bond make it a good antioxidant.