Exner Otto, Böhm Stanislav
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Praha 6, Czech Republic.
J Org Chem. 2002 Sep 6;67(18):6320-7. doi: 10.1021/jo020172+.
Fundamental model compounds for the Hammett equation, meta- and para-substituted benzoic acids, were investigated by the density functional theory at the B3LYP/6-311+G(d,p) level. Energies of 25 acids and of their anions were calculated in all possible conformations and from them the energies of the assumed mixture of conformers. Relative acidities correlated with the experimental gas-phase acidities almost within the experimental uncertainty, much more precisely than in the case of previous calculations at lower levels. Dissection of the substituent effects into those operating in the acid molecule and in the anion was carried out by means of isodesmic reactions starting from monosubstituted benzenes. Both effects are cooperating in the resulting effect on the acidity; those in the acid molecule are smaller but not negligible. They are also responsible for some deviations from the Hammett equation (through-resonance of para donor substituents) and for the weaker resonance in the acid molecule in meta derivatives; in the anions the inductive and resonance effects are almost equal. On the other hand, the cooperation of effects in the acid and in the anion makes the relative acidity more sensitive to electron withdrawing and is probably one of the reasons why the Hammett equation is so generally valid.
运用密度泛函理论在B3LYP/6 - 311 + G(d,p)水平上研究了哈米特方程的基本模型化合物——间位和对位取代苯甲酸。计算了25种酸及其阴离子在所有可能构象下的能量，并由此得出假定构象混合物的能量。相对酸度与实验气相酸度的相关性几乎在实验不确定度范围内，比之前较低水平的计算更为精确。通过从单取代苯开始的等键反应，将取代基效应分解为在酸分子和阴离子中起作用的效应。这两种效应共同作用于对酸度的最终影响；在酸分子中的效应较小但不可忽略。它们还导致了一些与哈米特方程的偏差（通过对位供电子取代基的贯穿共振）以及间位衍生物中酸分子内较弱的共振；在阴离子中，诱导效应和共振效应几乎相等。另一方面，酸和阴离子中效应的协同作用使相对酸度对吸电子作用更敏感，这可能是哈米特方程如此普遍有效的原因之一。
