To what extent do bond length and angle govern the 13C and 1H NMR response to weak CH⋯O hydrogen bonds? A case study of caffeine and theophylline cocrystals

IF 1.8 3区 化学 Q4 CHEMISTRY, PHYSICAL
Scott A. Southern , David L. Bryce
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引用次数: 2

Abstract

Weak hydrogen bonds are important structure-directing elements in supramolecular chemistry and biochemistry. We consider here weak CH⋯O hydrogen bonds in a series of cocrystals of theophylline and caffeine and assess to what extent the CH⋯O distance and angle govern the observed 13C and 1H isotropic chemical shifts. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations consistently predict a decrease in the 13C and 1H magnetic shielding constants upon hydrogen bond formation on the order of 2–5 ppm (13C) and 1–2 ppm (1H). These trends are reproduced using the machine-learning approach implemented in ShiftML. Experimental 13C and 1H chemical shifts obtained for powdered samples using one-dimensional NMR spectroscopy as well as heteronuclear correlation (HETCOR) spectroscopy correlate well with the GIPAW DFT results. However, the experimental 13C NMR response only correlates moderately well with the hydrogen bond length and angle, while the experimental 1H chemical shifts only show very weak correlations to these local structural elements. DFT computations on isolated imidazole-formaldehyde models show that the 13C and 1H chemical shifts generally decrease with the C⋯O distance but show no clear dependence on the CH⋯O angle. These results demonstrate that the 13C and 1H response to weak CH⋯O hydrogen bonding is influenced significantly by additional weak contacts within cocrystal heterodimeric units.

Abstract Image

键长和键角在多大程度上控制13C和1H核磁共振对弱CH⋯O氢键的响应?咖啡因和茶碱共晶的个案研究
弱氢键是超分子化学和生物化学中重要的结构导向元素。我们在这里考虑茶碱和咖啡因的一系列共晶中的弱CH⋯O氢键,并评估CH⋯O距离和角度在多大程度上控制观察到的13C和1H各向同性化学位移。包括投影仪在内的增强波密度泛函理论(GIPAW DFT)计算一致预测,氢键形成后13C和1H磁屏蔽常数的下降幅度分别为2-5 ppm (13C)和1-2 ppm (1H)。使用ShiftML中实现的机器学习方法可以再现这些趋势。使用一维核磁共振光谱和异核相关(HETCOR)光谱获得的粉末样品的实验13C和1H化学位移与GIPAW DFT结果具有良好的相关性。然而,实验13C核磁共振响应仅与氢键长度和角度有较好的相关性,而实验1H化学位移与这些局部结构元素的相关性非常弱。对孤立咪唑-甲醛模型的DFT计算表明,13C和1H化学位移通常随着C⋯O距离的减小而减小,但对CH⋯O角没有明显的依赖性。这些结果表明,13C和1H对弱CH⋯O氢键的响应受到共晶异质二聚体单元内附加弱接触的显著影响。
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来源期刊
CiteScore
5.30
自引率
9.40%
发文量
42
审稿时长
72 days
期刊介绍: The journal Solid State Nuclear Magnetic Resonance publishes original manuscripts of high scientific quality dealing with all experimental and theoretical aspects of solid state NMR. This includes advances in instrumentation, development of new experimental techniques and methodology, new theoretical insights, new data processing and simulation methods, and original applications of established or novel methods to scientific problems.
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