Acidity changes of p-aminobenzoic acid (pABA) after π-stacking with substituted benzenes (SB) have been investigated using quantum mechanical calculations at the M06-2X/6-311++G(d,p) level of theory. In addition to benzene derivatives that are usually used as a model for aromatic amino acids (AA), two amino acids, tryptophan (TRP) and histidine (HIS), with different cyclic structures were also considered in this work. All substituents enhance the stacking interactions, in which enhancement is higher for electron-withdrawing substituents (EWSs). The stacking interaction (in presence of all substituents) enhances the acidity (3.42 for Br). Natural energy decomposition analysis (NEDA) shows that the polarization interactions (POL) have the largest contribution in the binding energies (ΔE) of neutral and anionic complexes, while the electrostatic effects (ES) are in better correlation with ΔE. The ΔE values are linearly dependent on the sum of electron densities (Σρ BCP ) calculated at the bond critical points (BCPs) between the rings and the Σρ RCP values calculated at the ring critical points (RCPs) in the neutral and anionic SB||pABA complexes. An excellent correlation was found between the ΔE values and a combination of electrostatic (σ meta ), resonance/induction (σ para ), and dispersion/polarizability (molar refractivity, MR) substituent constant terms. There are good relationships between pK a and the global minimum and maximum of electrostatic potential around the O (∑V min-O ) and H (V max-H ) atoms, and the results of natural population analysis (NPA). Therefore, the electrostatic and charge transfer effects play a major role in acidity.

中文翻译：

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π-堆积对对氨基苯甲酸酸容量的影响

已使用量子力学计算在 M06-2X/6-311++G(d,p) 理论水平研究了对氨基苯甲酸 (pABA) 与取代苯 (SB) 进行 π 堆积后的酸度变化。除了通常用作芳香族氨基酸 (AA) 模型的苯衍生物外，本工作还考虑了具有不同环状结构的两种氨基酸，色氨酸 (TRP) 和组氨酸 (HIS)。所有取代基都增强了堆积相互作用，其中吸电子取代基 (EWS) 的增强作用更高。堆积相互作用（在所有取代基的存在下）增强了酸度（Br 为 3.42）。自然能量分解分析 (NEDA) 表明极化相互作用 (POL) 对中性和阴离子复合物的结合能 (ΔE) 的贡献最大，而静电效应 (ES) 与 ΔE 的相关性更好。ΔE 值线性依赖于在环之间的键临界点 (BCP) 处计算的电子密度 (Σρ BCP ) 和在中性和阴离子 SB|| 中的环临界点 (RCP) 处计算的 Σρ RCP 值的总和。 pABA 复合物。在 ΔE 值与静电 (σ meta )、共振/感应 (σ para ) 和色散/极化率 (摩尔折射率，MR) 取代常数项的组合之间发现了极好的相关性。pK a 与O (∑V min-O ) 和H (V max-H ) 原子周围静电势的全局最小值和最大值以及自然种群分析(NPA) 的结果之间存在良好的关系。因此，静电和电荷转移效应在酸度中起主要作用。