Abstract: Quantum chemical studies of five kinds of phenolic antioxidants, such as gallic acid, caffeic acid, protocatechuic acid, vanillic acid and ferulic acid were reported. Based on the optimization of molecular geometry and energy calculation, optimal structures, frontier orbital energies, dissociation enthalpies (BDE) and hydroxyl ionization potential (IP) were achieved. The results showed that antioxidant properties of five phenolic acids derived from dehydrogenation capacity of hydroxyl group from phenolic acids and the effect of ortho substitution on benzene ring. In non-polar solvents, the antioxidant activity of five phenolics was found to be ranked from high to low in the following sequence: caffeic acid ＞ gallic acid ＞ ferulic acid ＞ protocatechuic acid ＞ vanillic acid. In polar solvents, the decreasing sequence of antioxidant activity of five phenolic acids was gallic acid ＞ caffeic acid ＞ ferulic acid ＞ protocatechuic acid ＞ vanillic acid.

Key words: phenolic acid, antioxidant activity, quantitative calculation