Why Static Molecular Parameters Cannot Characterize the Free Radical Scavenging Activity of Phenolic Antioxidants

Abstract

The static molecular parameters including O–H bond length, O–H Mulliken population, O–H charge difference, O–H bond stretching force constant (k) and highest occupied molecular orbital energy level (EHOMO) for 11 para substituted phenols were calculated by density functional theory B3LYP on the basis set of 6-31G**. In combination with the O–H bond dissociation enthalpies (BDE) for the phenols, it was found that there were poor correlationships between O–H BDE and static O–H bond parameters, i.e., bond length, Mulliken population and k. Taking into consideration that O–H BDE correlates well with the logarithm of free radical scavenging rate constant for phenolic antioxidant, the ineffectiveness of these static O–H bond parameters in characterizing antioxidant activity can be understood. Although O–H charge difference and EHOMO can measure the O–H BDE for simple phenols, they are invalid when the phenols possess intramolecular hydrogen bond. Hence, to predict natural phenolic antioxidant activity, we will have to calculate the O–H BDE, despite it is a time-consuming process.