| Abstract
| - A PCM continuum model, at the DFT/B3LYP level, is used to study the solvent and substituent effects onthe conformers, intramolecular hydrogen bond (HB) enthalpies, (ΔHintras), and O−H bond dissociationenthalpies, (BDEs), in 2-substituted phenols, 2-X-ArOH, in the liquid phase. Two electron-donating (edg)and three electron-withdrawing (ewg) substituents are chosen, involved in a variety of biochemicaltransformations. Seven solvents, differing in their H-bonding ability and polarity, are selected to model differentenvironmental situations. Very good correlations are found between the computed R(O−H) and ν(O−H)values in solution for all non-HB 2-X-ArOH, showing that the former can be used as an universal moleculardescriptor for the latter and vice-versa. In all 2-X-ArOH, the HB parent conformer is the most stable in allmedia, closely matching frequency experimental data in CCl4. However, for all 2-X-ArO•, the most stableconformer either forms a “reverse”-HB or a HB is not formed, due to the long distance or steric effects.Changes in the stability, in solution, are observed for some of the 2-X-ArO• conformers. The intramolecularHB-strength in solution, ΔHS,intra, varies significantly with the size of the HB ring formed and the nature ofthe substituents. Reasonable correlations, derived between the two energetic parameters (BDEaw,sol and ΔHS,intra)and the solvent (and a), and/or molecular, [R(O−H) and ν(O−H)] ones, allow for an approximateestimation of the two former from the four latter. 2-X(edg) decrease BDEs (hence, increase the antioxidantefficiency of the solute, too) in all media; 2-X(ewg) present an opposite result. Moreover, an isodesmic reactionsstudy affords total stabilization effect (TSE) values (identical to the Δ[BDEaw]s), which are mainly governedby the stabilization of the phenolic radical (SPR) than that of the parent molecule (SPP). Quantitativecorrelations between the two effects in the TSE in both the gas and the liquid phases are also given. Unlikein the protic solvents, the better stabilization of the radical than the parent species, derived for the 2-X(edg)-ArOH in the aprotic, apolar, and/or low polar solvents, could account well for their smaller BDEsols. Aneffective antioxidant in solution should involve either one of the two edg in any one of the two latter solvents.
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