Sedghiniya Sima, Soleimannejad Janet, Janczak Jan
School of Chemistry, College of Science, University of Tehran, Teharan 14155-6455, Iran.
Institute of Low Temperature and Structure Research, Polish Academy of Science, Okólna 2, 50-950 Wrocław, Poland.
Acta Crystallogr C Struct Chem. 2019 Apr 1;75(Pt 4):412-421. doi: 10.1107/S2053229619003127. Epub 2019 Mar 13.
At one extreme of the proton-transfer spectrum in cocrystals, proton transfer is absent, whilst at the opposite extreme, in salts, the proton-transfer process is complete. However, for acid-base pairs with a small ΔpK (pK of base - pK of acid), prediction of the extent of proton transfer is not possible as there is a continuum between the salt and cocrystal ends. In this context, we attempt to illustrate that in these systems, in addition to ΔpK, the crystalline environment could change the extent of proton transfer. To this end, two compounds of salicylic acid (SaH) and adenine (Ad) have been prepared. Despite the same small ΔpK value (≈1.2), different ionization states are found. Both crystals, namely adeninium salicylate monohydrate, CHN·CHO·HO, I, and adeninium salicylate-adenine-salicylic acid-water (1/2/1/2), CHN·CHO·2CHN·CHO·2HO, II, have been characterized by single-crystal X-ray diffraction, IR spectroscopy and elemental analysis (C, H and N) techniques. In addition, the intermolecular hydrogen-bonding interactions of compounds I and II have been investigated and quantified in detail on the basis of Hirshfeld surface analysis and fingerprint plots. Throughout the study, we use crystal engineering, which is based on modifications of the intermolecular interactions, thus offering a more comprehensive screening of the salt-cocrystal continuum in comparison with pure pK analysis.
在共晶体的质子转移光谱的一个极端情况下，不存在质子转移，而在相反的极端情况下，即在盐中，质子转移过程是完全的。然而，对于ΔpK（碱的pK - 酸的pK）较小的酸碱对，由于在盐和共晶体两端之间存在连续体，因此无法预测质子转移的程度。在这种情况下，我们试图说明在这些体系中，除了ΔpK之外，晶体环境也可能改变质子转移的程度。为此，制备了水杨酸（SaH）和腺嘌呤（Ad）的两种化合物。尽管具有相同的小ΔpK值（≈1.2），但发现了不同的电离状态。两种晶体，即一水合腺嘌呤水杨酸盐，CHN·CHO·HO，I，和腺嘌呤水杨酸盐 - 腺嘌呤 - 水杨酸 - 水（1/2/1/2），CHN·CHO·2CHN·CHO·2HO，II，均已通过单晶X射线衍射、红外光谱和元素分析（C、H和N）技术进行了表征。此外，基于Hirshfeld表面分析和指纹图谱，对化合物I和II的分子间氢键相互作用进行了详细的研究和量化。在整个研究过程中，我们使用基于分子间相互作用修饰的晶体工程，因此与纯pK分析相比，能够对盐 - 共晶体连续体进行更全面的筛选。
