Stuyver Thijs, Danovich David, Shaik Sason
Institute of Chemistry , The Hebrew University , Jerusalem 91904 , Israel.
Algemene Chemie , Vrije Universiteit Brussel , Pleinlaan 2 , 1050 Brussels , Belgium.
J Phys Chem A. 2019 Mar 7;123(9):1851-1860. doi: 10.1021/acs.jpca.9b01095. Epub 2019 Feb 20.
Few concepts are more familiar to chemists than the concept of acidity strength. In almost any undergraduate chemistry textbook, one can find lists of factors affecting the acidities of organic and inorganic molecules. The factors, invoked to explain trends in the acidity strength through series of compounds, rely on concepts such as hybridization, delocalization, inductive effects, and electronegativity. Some of these concepts could be considered somewhat fuzzy, whereas others have a rigorous physical definition, yet together they shape the traditional framework used by chemists for the qualitative assessment of acidity strengths. At the same time, a thermodynamic cycle reveals that the acidity of a H-A bond is dependent on only three unequivocally definable quantities: the bond dissociation energy, the electron affinity of A, and the solvent effects. Here we attempt to answer the following questions: "How are the qualitative factors, found in textbooks, related to these quantities?" and "How can we connect this plethora of factors to the nature of the acidic H-A bond being cleaved heterolytically in an acidic dissociation process?" To do so, we turn to valence bond theory and model a generic acidic dissociation process. Within this model, the quantities, determining the acidity strength of an H-A compound (as revealed through the thermodynamic cycling process), arise naturally and lucidly, thus enabling the evaluation of the effects of the different qualitative factors found in the literature on the bonding situation. Our analysis projects surprising and thought-provoking anomalies, which challenge common chemical knowledge.
对化学家来说，很少有概念比酸度强度的概念更熟悉。在几乎任何一本本科化学教科书中，人们都能找到影响有机和无机分子酸度的因素列表。这些被用来解释一系列化合物酸度强度趋势的因素，依赖于诸如杂化、离域、诱导效应和电负性等概念。其中一些概念可能被认为有些模糊，而另一些则有严格的物理定义，但它们共同构成了化学家用于定性评估酸度强度的传统框架。与此同时，一个热力学循环表明，H - A键的酸度仅取决于三个明确可定义的量：键解离能、A的电子亲和能和溶剂效应。在这里，我们试图回答以下问题：“教科书中发现的定性因素与这些量有何关系？”以及“我们如何将大量这样的因素与在酸解离过程中异裂断裂的酸性H - A键的性质联系起来？”为了做到这一点，我们求助于价键理论并对一个通用的酸解离过程进行建模。在这个模型中，决定H - A化合物酸度强度的量（如通过热力学循环过程所揭示的）自然而清晰地出现，从而能够评估文献中发现的不同定性因素对成键情况的影响。我们的分析揭示了令人惊讶且发人深省的异常情况，这些情况挑战了常见的化学知识。
