Lithoxoidou Alexandra T, Bakalbassis Evangelos G
Laboratory of Applied Quantum Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece.
J Phys Chem A. 2005 Jan 20;109(2):366-77. doi: 10.1021/jp0462658.
A PCM continuum model, at the DFT/B3LYP level, is used to study the solvent and substituent effects on the conformers, intramolecular hydrogen bond (HB) enthalpies, (Delta H(intra)s), and O-H bond dissociation enthalpies, (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 biochemical transformations. Seven solvents, differing in their H-bonding ability and polarity, are selected to model different environmental situations. Very good correlations are found between the computed R(O-H) and nu(O-H) values in solution for all non-HB 2-X-ArOH, showing that the former can be used as an universal molecular descriptor for the latter and vice-versa. In all 2-X-ArOH, the HB parent conformer is the most stable in all media, closely matching frequency experimental data in CCl4. However, for all 2-X-ArO*, the most stable conformer 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 intramolecular HB-strength in solution, Delta H(S,intra), varies significantly with the size of the HB ring formed and the nature of the substituents. Reasonable correlations, derived between the two energetic parameters (BDE(aw,sol) and Delta H(S,intra)) and the solvent ( and a), and/or molecular, [R(O-H) and nu(O-H)] ones, allow for an approximate estimation of the two former from the four latter. 2-X(edg) decrease BDEs (hence, increase the antioxidant efficiency of the solute, too) in all media; 2-X(ewg) present an opposite result. Moreover, an isodesmic reactions study affords total stabilization effect (TSE) values (identical to the Delta[BDE(aw)]s), which are mainly governed by the stabilization of the phenolic radical (SPR) than that of the parent molecule (SPP). Quantitative correlations between the two effects in the TSE in both the gas and the liquid phases are also given. Unlike in 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 BDE(sol)s. An effective antioxidant in solution should involve either one of the two edg in any one of the two latter solvents.
在密度泛函理论/ B3LYP水平下，使用极化连续介质模型（PCM）来研究液相中2-取代苯酚（2-X-ArOH）的构象、分子内氢键（HB）焓（ΔH（intra）s）和O-H键解离焓（BDEs）的溶剂和取代基效应。选择了两个供电子（edg）和三个吸电子（ewg）取代基，它们参与了各种生化转化。选择了七种氢键能力和极性不同的溶剂来模拟不同的环境情况。对于所有非氢键的2-X-ArOH，计算得到的溶液中R（O-H）和ν（O-H）值之间发现了很好的相关性，表明前者可以用作后者的通用分子描述符，反之亦然。在所有2-X-ArOH中，氢键母体构象在所有介质中最稳定，与CCl4中的频率实验数据非常匹配。然而，对于所有2-X-ArO*，由于距离长或空间效应，最稳定的构象要么形成“反向”氢键，要么不形成氢键。观察到一些2-X-ArO*构象在溶液中的稳定性变化。溶液中的分子内氢键强度ΔH（S，intra）随形成的氢键环大小和取代基性质而有显著变化。两个能量参数（BDE（aw，sol）和ΔH（S，intra））与溶剂（和a）和/或分子（[R（O-H）和ν（O-H）]）参数之间得出合理的相关性，从而可以从后四个参数近似估计前两个参数。2-X（edg）在所有介质中都会降低BDEs（因此也会提高溶质的抗氧化效率）；2-X（ewg）则呈现相反的结果。此外，等键反应研究提供了总稳定化效应（TSE）值（与Δ[BDE（aw）]s相同），其主要由酚自由基（SPR）的稳定化而非母体分子（SPP）的稳定化决定。还给出了气相和液相中TSE中两种效应之间的定量相关性。与质子溶剂不同，在非质子、非极性和/或低极性溶剂中，2-X（edg）-ArOH的自由基比母体物种具有更好的稳定性，可以很好地解释其较小的BDE（sol）s。溶液中的有效抗氧化剂应该在上述两种溶剂中的任何一种中包含两个edg中的一个。
