Departamento de Química, Facultad de Ciencias, Universidad de Los Andes, La Hechicera, Merida-5101, Venezuela.
J Phys Chem A. 2012 Jul 19;116(28):7523-30. doi: 10.1021/jp300160g. Epub 2012 Jul 5.
In this work, we have explored the validity of the hypotheses on which rest the Hammett's approach to quantify the substituent effect on a reaction center, by applying two DFT energy decomposition schemes. This is performed by studying the change in the total electronic energy, ΔΔE, associated with a proton transfer isodesmic equilibrium. For this reaction, two sets of substituted benzoic acids and their corresponding benzoate anions have been considered. One of these sets contains para- and meta-substitutions, whereas the other one includes ortho-substituted benzoic acids. For each case, the gas phase change in the total electronic energy has been calculated, and two DFT energy decomposition schemes have been applied. The experimental σ(X) was found to be nearly proportional to the computed ΔΔE. The results for the para- and meta-substituted benzoic acids lead to the conclusion that it is possible to treat separately and, in an additive manner, the electrostatic and steric contributions; and also that the Hammett constant depends mainly on the electronic contributions to the free energy, while the steric contribution is negligible. However, the results for the ortho-substituted cases lead to the conclusion, as was assumed by Hammett, that there are significant qualitative differences between the effects on a reaction site of substituents in the meta- and para-positions and those in the ortho-position.
在这项工作中，我们通过应用两种 DFT 能量分解方案，探索了哈米特方法用于量化反应中心取代基效应的假设的有效性。这是通过研究与质子转移等电子平衡相关的总电子能变化ΔΔE 来实现的。对于该反应，我们考虑了两组取代苯甲酸及其相应的苯甲酸阴离子。其中一组包含对位和间位取代基，另一组则包含邻位取代苯甲酸。对于每种情况，我们都计算了气相中总电子能的变化，并应用了两种 DFT 能量分解方案。实验得到的 σ(X) 值与计算得到的ΔΔE 值几乎成正比。对于对位和间位取代苯甲酸的结果表明，可以分别以加和的方式处理静电和立体贡献，而且哈米特常数主要取决于自由能的电子贡献，而立体贡献可以忽略不计。然而，对于邻位取代基的情况，结果与哈米特的假设一致，即取代基在间位和对位与邻位对反应位点的影响存在显著的定性差异。
