Poly methoxy phenols in solution: O-H bond dissociation enthalpies, structures, and hydrogen bonding

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-l 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.