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 Phenols Antioxidant Action

Abstract

The reaction enthalpies related to the individual steps of two phenolic antioxidants action mechanisms, single electron transfer-proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET), for 30 meta and para-substituted phenols (ArOH) were calculated using DFT/B3LYP method. These mechanisms represent the alternative ways to the extensively studied hydrogen atom transfer (HAT) mechanism. Except the comparison of calculated reaction enthalpies with available experimental and/or theoretical values, obtained enthalpies were correlated with Hammett constants. We have found that electron-donating substituents induce the rise in the enthalpy of proton dissociation (PDE) from ArOH+* radical cation (second step in SET-PT) and in the proton affinities of phenoxide ions ArO- (reaction enthalpy of the first step in SPLET). Electron-withdrawing groups cause the increase in the reaction enthalpies of the processes where electron is abstracted, i.e., in the ionization potentials of ArOH (first step in SET-PT) and in the enthalpy of electron transfer from ArO- (second step in SPLET). Found results indicate that all dependences of reaction enthalpies on Hammett constants of the substituents are linear. The calculations of liquid-phase reaction enthalpies for several para-substituted phenols indicate that found trends hold also in water, although substituent effects are weaker. From the thermodynamic point of view, entering SPLET mechanism represents the most probable process in water.

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

“…Moreover, the calculated value is very close to the value assigned by McMahon and Kebarle (1977) to carboxylic COOH group deprotonation (1394 kJ mol -1 ), which did not follow the overall trend. In comparison to nonsubstituted phenol PA (Klein and Lukeš, 2006a), the COOH group located in the para position to the OH group resulted in a 62 kJ mol -1 decrease in its PA. If the two groups are mutually in the meta position, a 30 kJ mol -1 drop in the phenolic OH group PA was found.…”

“…However, it should be noted that available experimental values were obtained from the solution phase experiments. Gas phase values are usually close, but not necessarily identical, to the solution phase BDEs (Bakalbassis et al, 2003;Klein and Lukeš, 2006a;Rimarčík et al, 2011;Vagánek et al, 2014). Therefore, usual confrontation of calculated gas phase BDEs with solution phase experimental data may not be fully correct.…”

“…For phenols (Klein and Lukeš, 2006a), calculated PAs were in the range from 1346 kJ mol -1 (p-NO 2 ) to 1466 kJ mol -1 (p-NH 2 ). Only for the COMe, SO 2 Me, and NO 2 groups in the para position, PAs of the corresponding phenols are lower than values obtained for benzoic acids.…”

Theoretical Study of Primary Antioxidant Action Thermodynamics

“…For para-and meta-substituted phenols (Klein and Lukeš 2006a, 2006b, 2006c, Klein et al 2009) and thiophenols (Rimarčík et al 2011, Rottmannová et al 2012, we have quantified the effect of various electron-donating and electron-withdrawing substituents on the enthalpies of the homolytic and heterolytic O-H and S-H bonds cleavage in the gas-and solution-phase. Usually, benzene and water were chosen as typical models of polar and non-polar solvent.…”

“…The reaction enthalpy of the second step has been denoted as electron transfer enthalpy, ETE (Klein and Lukeš, 2006a). Flavonoids may also act as secondary antioxidants via chelation of copper and iron cations (Craft et al 2012, Gülçin 2012, Galano et al 2016, Dimitrić Marković et al 2011, Leopoldini et al 2011.…”