G.A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, 1 Akademicheskaya Street, Ivanovo, 153045, Russia.
J Mol Model. 2023 Jul 5;29(8):230. doi: 10.1007/s00894-023-05624-2.
Proton transfer in acid-base systems is not well understood. Some acid-base reactions do not proceed to the extent that is expected from the difference in the pK values between the base and acid in aqueous solutions, yet some do. In that regard, we have computationally studied the process of proton transfer from the acids of varying strength (benzenesulfonic acid (BSu), methansulfonic acid (MsO), and sulfuric acid (SA)) to the amines with different numbers of propyl substituents on the nitrogen atom (propylamine (PrA), dipropylamine (DPrA), and tripropylamine (TPrA)) upon complexation. Density functional theory calculations were used to thoroughly examine the energetic and structural aspects of the molecular complexes and/or ionic pairs resulting from the acid-base interaction. The potential energy curves along the proton transfer coordinate in these acid-amine systems were analyzed. The change in free energies accompanying the molecular complexes and ionic pair formations was calculated, and the relationship between the energy values and the ΔРА parameter (difference in proton affinity of the acid anion and amine) was established. The larger ΔРА values were found to be unfavorable for the formation of ionic pairs. Using structural, energy, QTAIM, and NBO analyses, we determined that the hydrogen bonds in the molecular complexes PrA-MsO and PrA-BSu are stronger than those in their corresponding ionic pairs. The ionic pairs with the TPrA cation possess the strongest hydrogen bonds of all the ionic pairs being studied, regardless of the anion. The results showed that hydrogen bonding interactions in the molecular complexes contribute significantly to the energies of the acid-base interaction, while in the ionic pairs, the most important energy contribution comes from Coulomb interactions, followed by hydrogen bonding and dispersion forces. The ionic pairs with propylammonium, dipropylammonium, and tripropylammonium cations have stronger ion-ion interactions than tetrapropylammonium (TetPrA)-containing ionic pairs with the same anions. This effect rises with the order of the cation: TetPrA → TPrA → DPrA → PrA, and the sequence of anions is SA → BSu → MsO. The results obtained here expand the concept of acid-base interaction and provide an alternative to experimental searches for suitable acids and bases to obtain new types of protic ionic liquids.
All quantum-chemical calculations were carried out by using the DFT/B3LYP-GD3/6-31++G(d,p) level as implemented in the Gaussian 09 software package. For the resulting structures, the electron density distribution was analyzed by the "atoms in molecules" (QTAIM) and the natural bond orbital (NBO) methods on the wave functions obtained at the same level of theory by AIMAll Version 10.05.04 and Gaussian NBO Version 3.1 programs, respectively.
质子在酸碱系统中的转移尚未得到很好的理解。有些酸碱反应并没有进行到预期的程度，即碱基和酸在水溶液中的 pK 值之间的差异，但有些反应确实进行了。在这方面，我们通过计算研究了质子从不同强度的酸（苯磺酸（BSu）、甲烷磺酸（MsO）和硫酸（SA））向氮原子上带有不同数量丙基取代基的胺（丙胺（PrA）、二丙胺（DPrA）和三丙胺（TPrA））转移的过程。使用密度泛函理论计算彻底研究了酸碱相互作用产生的分子复合物和/或离子对的能量和结构方面。分析了这些酸-胺系统中质子转移坐标上的势能曲线。计算了伴随分子复合物和离子对形成的自由能变化，并建立了能量值与 ΔРА 参数（酸阴离子和胺的质子亲和能差）之间的关系。发现较大的 ΔРА 值不利于离子对的形成。通过结构、能量、QTAIM 和 NBO 分析，我们确定了分子复合物 PrA-MsO 和 PrA-BSu 中的氢键比其相应的离子对中的氢键更强。无论阴离子如何，与研究的所有离子对相比，TPrA 阳离子的离子对具有最强的氢键。结果表明，分子复合物中的氢键相互作用对酸碱相互作用的能量有显著贡献，而在离子对中，最重要的能量贡献来自库仑相互作用，其次是氢键和色散力。带有丙铵、二丙铵和三丙铵阳离子的离子对具有比含有相同阴离子的四丙铵（TetPrA）离子对更强的离子-离子相互作用。这种效应随阳离子的顺序升高：TetPrA→TPrA→DPrA→PrA，阴离子的顺序为 SA→BSu→MsO。这里得到的结果扩展了酸碱相互作用的概念，并为实验寻找合适的酸和碱以获得新型质子离子液体提供了替代方案。
所有量子化学计算均使用 DFT/B3LYP-GD3/6-31++G(d,p) 水平作为在 Gaussian 09 软件包中实现的，对所得结构，通过 AIMAll Version 10.05.04 和 Gaussian NBO Version 3.1 程序分别在同一理论水平上获得的波函数上分析电子密度分布的“分子中的原子”（QTAIM）和自然键轨道（NBO）方法。
