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Mobility of the 2-Methylimidazolate Linkers in ZIF-8 Probed by 2H NMR: Saloon Doors for the Guests
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2015-12-01 00:00:00 , DOI: 10.1021/acs.jpcc.5b09312 Daniil I. Kolokolov 1, 2 , Alexander G. Stepanov 1, 2 , Hervé Jobic 3
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2015-12-01 00:00:00 , DOI: 10.1021/acs.jpcc.5b09312 Daniil I. Kolokolov 1, 2 , Alexander G. Stepanov 1, 2 , Hervé Jobic 3
Affiliation
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ZIF-8 is one of the most interesting metal–organic frameworks due to its high stability and unique capabilities for hydrocarbons separation. Its porous network is formed by large almost spherical cavities interconnected by very narrow windows, ∼3.4 Å, which should be too small even for methane. At the same time the direct experimental observations show that ZIF-8 cavities are able to host even such large molecules as benzene. This effect is associated with the flexibility of the cavity widows, arising from dynamical freedom on the 2-methylimidazole (2-mIM) linkers that form the framework. In this work, by means of 2H NMR we show that the 2-mIM linkers of the ZIF-8 are very mobile and their mobility is sensitive to the presence of benzene guest. In contrast with other known MOFs based on linearly bonded carboxylates, in guest-free ZIF-8 the plane of 2-mIM linker exhibits two-site flips within a sector of 2φf = 34° with very low activation barrier (1.5 kJ mol–1) and high rates (∼1012 s–1). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes γlib comparable with the two-site flips (γlib = ±17° above 560 K). This is direct evidence that the ZIF-8 linkers twist notably, thus increasing the aperture of the windows sufficiently to fit very large molecules. Upon benzene loading, the geometry of the observed motions remains similar but the potential barrier of the linkers flipping rises up to 9 kJ mol–1.
中文翻译：
2 H NMR探测ZIF-8中2-甲基咪唑基连接子的迁移性：客用轿车门
ZIF-8由于其高稳定性和独特的碳氢化合物分离能力而成为最有趣的金属有机框架之一。它的多孔网络是由几乎是球形的大空腔构成，这些空腔由非常狭窄的窗口（约3.4Å）互连，即使对于甲烷，该窗口也应该太小。同时，直接的实验观察结果表明ZIF-8空腔甚至可以容纳大分子，例如苯。这种作用与空腔寡妇的柔韧性有关，这种柔韧性是由形成骨架的2-甲基咪唑（2-mIM）接头的动力学自由引起的。在这项工作中，通过21 H NMR表明，ZIF-8的2-mIM连接基非常易移动，并且其迁移率对苯客体的存在敏感。与基于线性键合的羧酸盐等公知的MOF，在无客体- ZIF-8的2-MIM接头展品的平面双位点2φ的扇区内的翻转对比度˚F = 34°具有非常低的活化障碍（1.5千焦耳摩尔- 1）和高速率（〜10 12 s –1）。上述380 k处的接头开始表现出与逐渐增加幅度γ额外的快速librations LIB与两个站点可比翻转（γ LIB=在560 K以上时为±17°）。这是ZIF-8接头显着扭曲的直接证据，从而充分增加了窗口的孔径，可容纳非常大的分子。在苯负载后，观察到的运动的几何形状保持相似，但连接子翻转的势垒增加至9 kJ mol –1。
更新日期：2015-12-01
中文翻译：
2 H NMR探测ZIF-8中2-甲基咪唑基连接子的迁移性：客用轿车门
ZIF-8由于其高稳定性和独特的碳氢化合物分离能力而成为最有趣的金属有机框架之一。它的多孔网络是由几乎是球形的大空腔构成，这些空腔由非常狭窄的窗口（约3.4Å）互连，即使对于甲烷，该窗口也应该太小。同时，直接的实验观察结果表明ZIF-8空腔甚至可以容纳大分子，例如苯。这种作用与空腔寡妇的柔韧性有关，这种柔韧性是由形成骨架的2-甲基咪唑（2-mIM）接头的动力学自由引起的。在这项工作中，通过21 H NMR表明，ZIF-8的2-mIM连接基非常易移动，并且其迁移率对苯客体的存在敏感。与基于线性键合的羧酸盐等公知的MOF，在无客体- ZIF-8的2-MIM接头展品的平面双位点2φ的扇区内的翻转对比度˚F = 34°具有非常低的活化障碍（1.5千焦耳摩尔- 1）和高速率（〜10 12 s –1）。上述380 k处的接头开始表现出与逐渐增加幅度γ额外的快速librations LIB与两个站点可比翻转（γ LIB=在560 K以上时为±17°）。这是ZIF-8接头显着扭曲的直接证据，从而充分增加了窗口的孔径，可容纳非常大的分子。在苯负载后，观察到的运动的几何形状保持相似，但连接子翻转的势垒增加至9 kJ mol –1。
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