Hydrogen atom transfer versus proton coupled electron transfer mechanism of gallic acid with different peroxy radicals

Reactions of gallic acid (GA) with alkyl peroxy radicals (methylperoxy, ethylperoxy, iso-propylperoxy, and tert-butylperoxy) were simulated using density functional theory. The reaction is taking place in the way that hydrogen of hydroxy group of GA is transferred to the oxygen of each of peroxy radical. A newly formed radical is stabilized with delocalization of spin density over entire molecule, while the harmful peroxy radical is neutralized. These simple reactions can occur by two different, non-exclusive mechanisms: hydrogen atom transfer and proton coupled electron transfer. The competition between these mechanisms depends on both the solvent and the nature the free radicals. The main differences of these mechanisms are described, together with corresponding thermodynamic and kinetic consequences. The potency of this antioxidative action was thermodynamically and kinetically estimated for hydrogen atom transfer (HAT) and proton coupled electron transfer (PCET) mechanisms. The first one was estimated by calculating bond dissociation energy (ΔG BDE), while the second one was examined using the activation barriers necessary for this action (transition state theory (TST)), as well as using the zero-curvature tunneling effect (ZCT). Additionally, the analysis of single occupied molecular orbitals (SOMOs) in transition states was used to examine differences between HAT and PCET mechanisms.