DFT/B3LYP study of the solvent effect on the reaction enthalpies of homolytic and heterolytic O [sbnd]H bond cleavage in mono-substituted chromans

Abstract

The study of various ortho and meta substituted chroman derivatives was carried out. The reaction enthalpies related to the hydrogen atom transfer (HAT) mechanism and to individual steps of two stepwise mechanisms of phenolic antioxidants action, single electron transfer–proton transfer (SET–PT) and sequential proton loss electron transfer (SPLET) have been calculated using DFT/B3LYP method in solution phase using Polarized Continuum Model (PCM) method. The effects of solvents with various polarities and substituent effect have been investigated. Results show that electron-donating substituents induce a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA), whereas electron-withdrawing groups cause an increase in the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE). It has been found that substituents in ortho positions have significantly greater influence on BDE, PA and ETE in comparison with those in meta position. On the contrary, substituents in meta position show higher effect on IP and PDE than substituents in ortho positions. In comparison to gas phase, studied solvents attenuate the substituent effect on the reaction enthalpies of three studied mechanisms. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in polar solvents like water, DMSO and ethanol, where PA values of studied substituted chromans are considerably lower than BDEs and IPs. However, in benzene, BDEs are lower than PA and IP values, i.e. HAT represents the most probable pathway. Computed results reveal that all dependences of reaction enthalpies on Hammett constants of the substituents in meta position are linear in all solvents.