The role of solvation in proton transfer reactions: implications for predicting salt/co-crystal formation using the ΔpKa rule†

IF 3.1 3区 化学 Q2 CHEMISTRY, PHYSICAL
Aurora J. Cruz-Cabeza, Matteo Lusi, Helen P. Wheatcroft and Andrew D. Bond
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引用次数: 8

Abstract

The ΔpKa rule is commonly applied by chemists and crystal engineers as a guideline for the rational design of molecular salts and co-crystals. For multi-component crystals containing acid and base constituents, empirical evidence has shown that ΔpKa > 4 almost always leads to salts, ΔpKa < ?1 almost always leads to co-crystals and ΔpKa between ?1 and 4 can be either. This paper reviews the theoretical background of the ΔpKa rule and highlights the crucial role of solvation in determining the outcome of the potential proton transfer from acid to base. New data on the frequency of the occurrence of co-crystals and salts in multi-component crystal structures containing acid and base constituents show that the relationship between ΔpKa and the frequency of salt/co-crystal formation is influenced by the composition of the crystal. For unsolvated co-crystals/salts, containing only the principal acid and base components, the point of 50% probability for salt/co-crystal formation occurs at ΔpKa ≈ 1.4, while for hydrates of co-crystals and salts, this point is shifted to ΔpKa ≈ ?0.5. For acid–base crystals with the possibility for two proton transfers, the overall frequency of occurrence of any salt (monovalent or divalent) versus a co-crystal is comparable to that of the whole data set, but the point of 50% probability for observing a monovalent salt vs. a divalent salt lies at ΔpKa,II ≈ ?4.5. Hence, where two proton transfers are possible, the balance is between co-crystals and divalent salts, with monovalent salts being far less common. Finally, the overall role played by the “crystal” solvation is illustrated by the fact that acid–base complexes in the intermediate region of ΔpKa tip towards salt formation if ancillary hydrogen bonds can exist. Thus, the solvation strength of the lattice plays a key role in the stabilisation of the ions.

Abstract Image

溶剂化在质子转移反应中的作用:使用ΔpKa规则†预测盐/共晶形成的意义
ΔpKa规则通常被化学家和晶体工程师作为合理设计分子盐和共晶的指导原则。对于含有酸和碱成分的多组分晶体,经验证据表明ΔpKa >4几乎总是导致盐,ΔpKa <- 1几乎总是导致共晶,- 1和- 4之间的ΔpKa可以是其中之一。本文回顾了ΔpKa规则的理论背景,并强调了溶剂化在决定质子从酸到碱的潜在转移结果中的关键作用。在含有酸和碱成分的多组分晶体结构中,共晶和盐的出现频率的新数据表明,ΔpKa与盐/共晶形成频率之间的关系受晶体组成的影响。对于仅含有主要酸和碱组分的未溶剂化共晶/盐,盐/共晶形成的50%概率点发生在ΔpKa≈1.4,而对于共晶和盐的水合物,这一点转移到ΔpKa≈?0.5。对于可能发生两个质子转移的酸碱晶体,任何盐(单价或二价)与共晶体的总体出现频率与整个数据集相当,但观察到单价盐与二价盐的50%概率点位于ΔpKa,II≈?4.5。因此,在可能发生两个质子转移的地方,平衡是在共晶和二价盐之间,而一价盐远不常见。最后,“晶体”溶剂化所起的总体作用是通过ΔpKa中间区域的酸碱配合物在辅助氢键存在的情况下倾向于形成盐的事实来说明的。因此,晶格的溶剂化强度对离子的稳定性起着关键作用。
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来源期刊
Faraday Discussions
Faraday Discussions 化学-物理化学
自引率
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发文量
259
期刊介绍: Discussion summary and research papers from discussion meetings that focus on rapidly developing areas of physical chemistry and its interfaces
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