Influence of intramolecular hydrogen bond on the antioxidant activity of hindered phenols

Quantum chemistry method based on density functional theory was applied to calculate the bond dissociation energy (EBD) of the O-H bond of different types of hindered phenols. The antioxidant activity of hindered phenols in lubricating base oil was verified by using pressurized differential scanning calorimeter. The relationship between the EBD of hindered phenols and their antioxidant activity was investigated, and the influences of structure factors on the EBD of hindered ortho-bisphenols were analyzed. Results indicated that activity conformations of hindered phenols with lower EBD had higher antioxidant activity in lubricating base oil. The intramolecular hydrogen bond had a significant impact on the EBD of hindered ortho-bisphenol. For hindered ortho-bisphenols, their lowest energy conformations had higher EBD and were not antioxidant activity conformations due to the formation of two O-H┅π bonds, while the local minimum energy conformations with a O-H┅O hydrogen bond and much lower EBD were their antioxidant activity conformations. In addition, the lower intramolecular rotation energy barrier played a main role in lowering EBD of methene bridged ortho-bisphenol, which was the main reason for its excellent antioxidant performance.