DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of single electron transfer–proton transfer and sequential proton loss electron transfer mechanisms of chroman derivatives antioxidant action

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In this paper, the study of various ortho and meta-substituted chroman-6-ol derivatives is presented. The reaction enthalpies related to the individual steps of two stepwise mechanisms of phenolic antioxidants action, single electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET), for studied compounds have been calculated using DFT/B3LYP method in gas-phase and water. Results reveal that electron-donating substituents induce rise in the proton dissociation enthalpy (PDE) and proton affinity (PA) whereas Electron withdrawing groups cause increase in the ionization potentials (IP) electron transfer enthalpy (ETE). Result indicated substituents in ortho have in comparison with the substituents in meta position significantly greater influence on PA and ETE, on the contrary, substituents in meta position cause considerable effect on IP and PDE rather than substituents in ortho positions. Results reveal that water attenuates the substituent effect on IP, PA and PDE values. Computed results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. From the thermodynamic point of view, SPLET mechanism represents the most promising process in water. Furthermore, results show that calculated enthalpies can be correlated with the length of phenolic C–O and O–H + (bond length of O–H after electron abstraction) bonds and partial charge on the phenoxy radical oxygen atom q(O) of the studied molecules successfully.