Why static molecular parameters cannot characterize the free radical scavenging activity of phenolic antioxidants

The static molecular parameters including O-H bond length, O-H Mulliken pop ulation, O-H charge difference, O-H bond stretching force constant (k) and highest occupied molecular orbital energy level (E-HOMO) for 11 para substi tuted phenols were calculated by density functional theory B3LYP on the bas is 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 w ith the logarithm of free radical scavenging rate constant for phenolic ant ioxidant, the ineffectiveness of these static O-H bond parameters in charac terizing antioxidant activity can be understood. Although O-H charge differ ence and EHOMO can measure the O-H BDE for simple phenols, they are invalid when the phenols possess intramolecular hydrogen bond. Hence, to predict n atural phenolic antioxidant activity, we will have to calculate the O-H BDE , despite it is a time-consuming process.