Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion
Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion
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DOI：
10.1021/jacs.1c09455
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发表时间：
2021-12-15
影响因子：
15
通讯作者：
Beves, Jonathon E.
中科院分区：
文献类型：
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作者：
Fillbrook, Lucy L.;Guenther, Jan-Philipp;Beves, Jonathon E.
据报道，反应过程中小分子自扩散的变化归因于“流动性增强”。我们证明了顺磁性离子浓度变化对核磁共振（NMR）信号强度的关键作用，这导致了扩散系数的错误测量。我们提出了解决这个问题的简单方法。使用混洗梯度幅度可以实现精确的扩散核磁共振测量，即使顺磁离子浓度变化引起的随时间变化的弛豫率也是如此。添加顺磁弛豫剂可以在单次实验中准确测定扩散系数和反应动力学。我们分析了铜催化的叠氮化物-炔环加成“点击”反应，据称该反应可以提高迁移率。通过我们的方法，我们准确测量了溶剂、起始材料和产物的扩散行为，发现反应过程中扩散系数没有整体增加。我们使用替代还原剂克服了 NMR 信号重叠，以提高扩散测量的准确性。由于在催化循环期间与铜催化剂结合，随着反应的进行，炔反应物的扩散速度减慢。该中间体的形成通过互补核磁共振技术和密度泛函理论计算得到证实。我们的工作对最近关于分子在反应过程中主动推进或游动的说法提出了质疑，并证实时间分辨扩散核磁共振测量可以为反应机制提供有价值的见解。
The reported changes in self-diffusion of small molecules during reactions have been attributed to "boosted mobility". We demonstrate the critical role of changing concentrations of paramagnetic ions on nuclear magnetic resonance (NMR) signal intensities, which led to erroneous measurements of diffusion coefficients. We present simple methods to overcome this problem. The use of shuffled gradient amplitudes allows accurate diffusion NMR measurements, even with time-dependent relaxation rates caused by changing concentrations of paramagnetic ions. The addition of a paramagnetic relaxation agent allows accurate determination of both diffusion coefficients and reaction kinetics during a single experiment. We analyze a copper-catalyzed azide-alkyne cycloaddition "click" reaction, for which boosted mobility has been claimed. With our methods, we accurately measure the diffusive behavior of the solvent, starting materials, and product and find no global increase in diffusion coefficients during the reaction. We overcome NMR signal overlap using an alternative reducing agent to improve the accuracy of the diffusion measurements. The alkyne reactant diffuses slower as the reaction proceeds due to binding to the copper catalyst during the catalytic cycle. The formation of this intermediate was confirmed by complementary NMR techniques and density functional theory calculations. Our work calls into question recent claims that molecules actively propel or swim during reactions and establishes that time-resolved diffusion NMR measurements can provide valuable insight into reaction mechanisms.