Wiberg Kenneth B
Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA.
J Org Chem. 2002 Jul 12;67(14):4787-94. doi: 10.1021/jo020100i.
To investigate the origin of substituent effects on the acidity of benzoic acids, the structures of a series of substituted benzoic acids and benzoates have been calculated at the B3LYP/6-311+G* and MP2/6-311+G* theoretical levels. The vibrational frequencies were calculated using B3LYP/6-311+G* and allowed corrections for the change in zero-point energies on ionization, and the change in energy on going from 0 K (corresponding to the calculations) to 298 K. A more satisfactory agreement with the experimental values was obtained by energy calculations at the MP2/ 6-311++G* level using the above structures. The resulting Delta H(acid) values agree very well with the experimental gas-phase acidities. The energies of compounds with pi-electron-accepting or -releasing substituents, rotated to give the transition state geometries, provided rotational barriers that could be compared with those found for the corresponding substituted benzenes. Isodesmic reactions allowed the separate examination of the substituent effects on the energies of the acids and on the anions. Electron-withdrawing groups stabilize the benzoate anions more than they destabilize the benzoic acids. Electron-donating groups stabilize the acids and destabilize the anions by approximately equal amounts. The gas-phase acidities of meta- and para-substituted benzoic acids are linearly related. This is also found for the acidities of substituted phenylacetic acids and benzoic acids. Since direct pi-electron interactions are not possible with the phenylacetic acids, this indicates that the acidities are mainly controlled by a field effect interaction between the charge distribution in the substituted benzene ring and the negative charge of the carboxylate group. The Hammett sigma(M) and sigma(P) values are also linearly related for many small substituents from NO(2) through the halogens and to OH and NH(2). Most of the other substituents fall on a line with a different slope
为了研究取代基对苯甲酸酸性影响的起源，在B3LYP/6 - 311 + G和MP2/6 - 311 + G理论水平上计算了一系列取代苯甲酸和苯甲酸盐的结构。使用B3LYP/6 - 311 + G计算振动频率，并对电离时零点能的变化以及从0 K（对应于计算值）到298 K时能量的变化进行校正。使用上述结构在MP2/6 - 311++G水平进行能量计算，与实验值得到了更令人满意的吻合。所得的ΔH(酸)值与实验气相酸度非常吻合。对于具有π电子接受或释放取代基的化合物，旋转以给出过渡态几何结构，其提供的旋转势垒可与相应取代苯的旋转势垒进行比较。等键反应允许分别考察取代基对酸和阴离子能量的影响。吸电子基团使苯甲酸盐阴离子稳定的程度大于使苯甲酸不稳定的程度。供电子基团使酸稳定和使阴离子不稳定的程度大致相等。间位和对位取代苯甲酸的气相酸度呈线性关系。对于取代苯乙酸和苯甲酸的酸度也发现了这种情况。由于苯乙酸不可能有直接的π电子相互作用，这表明酸度主要由取代苯环中的电荷分布与羧酸盐基团的负电荷之间的场效应相互作用控制。对于从NO(2)到卤素以及OH和NH(2)的许多小取代基，哈米特σ(M)和σ(P)值也呈线性关系。大多数其他取代基落在一条斜率不同的直线上
