DFT/B3LYP study of the substituent effect on the reaction enthalpies of the individual steps of sequential proton loss electron transfer mechanism of phenols antioxidant action: Correlation with phenolic C [sbnd]O bond length

Abstract

The reaction enthalpies related to the individual steps of sequential proton loss electron transfer (SPLET) mechanism, for phenol and 30 meta- and para-substituted phenols (ArOH) were calculated using DFT/B3LYP method. This newly discovered mechanism represents an alternative way to the extensively studied hydrogen atom transfer (HAT) mechanism. Except the comparison of calculated reaction enthalpies with available experimental and theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the proton affinities of phenoxide anions ArO − (reaction enthalpy of the first step). Electron-withdrawing groups induce the increase in the reaction enthalpy of the second step of SPLET where electron from ArO − is abstracted. Found results indicate that dependences of reaction enthalpies on Hammett constants of the substituents are linear. Moreover, we found, that proton affinities and electron transfer enthalpies from ArO − can be successfully correlated with the length of phenolic C O bonds of the studied substituted phenol molecules.