Theoretical study of phenolic antioxidants properties in reaction with oxygen-centered radicals

A dominant transfer mechanism [hydrogen atom transfer (HAT) or proton-coupled electron transfer (PCET)] in the reaction of phenols with certain types of oxygen-centered radicals was selected by examining the conformations, singly occupied molecular orbitals (SOMOs), charge separation and spin density in optimized transition structures (TSs) such as (CH3)3-C-O3-O2⋯H⋯O1-Ar and (CH3)3-C-O2⋯H⋯O1-Ar. The change in charge on the hydrogen (ΔH) and the SOMO conformations in the TS (CH3)3-C-O3-O2⋯H⋯O1-Ar or (CH3)3-C-O2⋯H⋯O1-Ar were used as criteria for determining the dominant H-atom transfer mechanism. Increased electron density on the O2 and O3 oxygens in the PCET-TS selectively stabilizes the TS by providing greater binding energy for H+ transfer. Thus, the (O2+O3−O1) charges, Δ(O2+O3−O1) charges, and (O2+O3) spin densities in the PCET-TS were well correlated with the experimental antioxidative activity (k1). The spin densities on the radical oxygens (O2+O3) in the HAT-TS were highly negatively correlated with k1, while the spin densities on the ring in the HAT-TS showed good positive correlation with k1 values. In the TS of phenols containing O7(para) in the structure, the spin densities on O7(para) and on the ring were correlated well with k1. Since the reactivity of phenols with an alkylperoxy radical does not strongly depend on the radical structure, the procedure presented here can be used to estimate phenol reactivity with any alkylperoxy radical (R-O-O).