Mechanistic studies of phenolic antioxidants in reaction with nitrogen- and oxygen-centered radicals

A dominant H-atom transfer mechanism [hydrogen atom transfer (HAT) or proton-coupled electron transfer (PCET)] in the reaction of phenols with certain type of nitrogen-centered radicals [2,2-diphenyl-1-picrylhydrazyl radical (R-N2-N1)] was selected by examining the conformations, singly occupied molecular orbitals (SOMOs), charge separations and spin densities in optimized transition structures (TSs) such as R-N2-N1⋯H⋯O1-Ar. The change in charge on the hydrogen (ΔqH) and the SOMO conformations in the TS (R-N2-N1⋯H⋯O1-Ar) were used as criteria for determining the dominant H-atom transfer mechanism. Strong linear correlations were found between the log (k1) for all analyzed compounds and selected chemical properties, such as the spin densities, molecular orbitals energies and bond dissociation enthalpies. The main objectives of the investigation were to develop theoretical method for determination of the dominant H-atom transfer mechanism (i.e., that with the faster rate) and to select molecular descriptors able to predict total antioxidant activity (k1) of phenols in reaction with particular radicals. Performed QSAR studies has selected the best descriptors of phenols activities against radicals, such as the alkyl-peroxy, alkyloxy, and N-centered radicals.