Theoretical Study of the Substituent and Solvent Effects on the Reaction Enthalpies of the Antioxidant Mechanisms of Tyrosol Derivatives

Abstract In this paper, the study of various ortho- and meta-substituted tyrosol derivatives is presented. The reaction enthalpies related to three antioxidant action mechanisms HAT, SET-PT, and SPLET for substituted tyrosols have been calculated using the DFT/B3LYP method in gas phase and water. Calculated results show that electron-withdrawing substituents increase the bond-dissociation enthalpy (BDE), ionization potential (IP), and electron-transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton-dissociation enthalpy (PDE) and proton affinity (PA). In the ortho-position, substituents show larger effect on reaction enthalpies than at the meta-position. In comparison to the gas phase, water attenuates the substituent effect on all reaction enthalpies. In the gas phase, BDEs are lower than PAs and IPs, i.e., HAT represents the thermodynamically preferred pathway. On the other hand, SPLET mechanism represents the thermodynamically favored process in water. Results show that calculated enthalpies can be correlated with Hammett constants (σm) of the substituted tyrosols successfully. Furthermore, calculated IP and PA values for substituted tyrosols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO).