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

Abstract

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-O-3-O-2-(HO1)-H-...-O-...-Ar and (CH3)(3)-C-O-2 (...) H (...) O-1-Ar. The change in charge on the hydrogen (Delta H) and the SOMO conformations in the TS (CH3)(3)-C-O-3-O-2 (....) H (...) O-1-Ar or (CH3)(3)-C-O-2(....) H-.... O-1-Ar were used as criteria for determining the dominant H-atoin transfer mechanism. Increased electron density on the O-2 and O-3 oxygens in the PCET-TS selectively stabilizes the TS by providing greater binding energy for H+ transfer. Thus, the (O-2+O-3-O-1) charges, Delta(O-2+O-3-O-1) charges, and (O-2+O-3) spin densities in the PCET-TS were well correlated with the experimental antioxidative activity (k(1)). The spin densities on the radical oxygens (O-2+O-3) in the HAT-TS were highly negatively correlated with k(1), while the spin densities on the ring in the HAT-TS showed good positive correlation with k(1) values. In the TS of phenols containing O-7(para) in the structure, the spin densities on O-7(para) and on the ring were correlated well with ki. 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-').