Charge-density distribution and electrostatic flexibility of ZIF-8 based on high-resolution X-ray diffraction data and periodic calculations.
Charge-density distribution and electrostatic flexibility of ZIF-8 based on high-resolution X-ray diffraction data and periodic calculations.
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DOI：
10.1021/ic5028256
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发表时间：
2015-02
影响因子：
4.6
通讯作者：
S. Novakovic;G. Bogdanović;C. Heering;Gamall Makhloufi;D. Francuski;C. Janiak
中科院分区：
文献类型：
--
作者：
S. Novakovic;G. Bogdanović;C. Heering;Gamall Makhloufi;D. Francuski;C. Janiak
采用高分辨率x射线衍射数据和Invariom细化相结合的方法，获得了典型多孔配位聚合物ZIF-8的电子密度分布。此外，周期性量子化学计算已用于描述相同几何形状（m1）的ZIF-8的理论密度特征，以及以甲基咪唑酸酯连接剂的构象变化为特征的“高压”形式的ZIF-8 （m2）。对ZIF-8的两种极限结构形式的电子和静电特性进行了彻底的比较，提出了在框架内发生的扩散和吸附过程的其他方面。β笼的四元孔（FM）和六元孔（SM）的尺寸可由总电子密度分布可靠地确定。分析表明，m2中的FM在尺寸上与SM孔径竞争，应考虑小分子和阳离子的扩散。Bader的拓扑分析（分子中原子的量子理论）显示了两种ZIF-8形式的相似性质。另一方面，对它们的静电性能分析显示出巨大的差异。该研究表明，ZIF-8框架具有特殊的静电灵活性，其中小的构象变化会导致显著不同的静电势（EP）分布，这一特征可能对ZIF-8框架的功能和动力学非常重要。m1的空腔表面包含38个不同的区域，在空腔体积中有中度正、负或中性EP和弱正EP。与m1相比，m2形式只显示了两个不同EP的区域，其中正EP占据了整个腔面，而强负EP则完全局限于FM孔内。腔体积内的电位也比m1内的电位更正。在ZIF-8结构中，连接体重定向对电位的显著影响与系统的高对称性和近端排列的结构片段的协同效应对静电特性的放大有关。
The electron-density distribution in a prototypical porous coordination polymer ZIF-8 has been obtained in an approach combining high-resolution X-ray diffraction data and Invariom refinement. In addition, the periodic quantum-chemical calculation has been used to describe the theoretical density features of ZIF-8 in the same geometry (m1) and also in a "high-pressure" form of ZIF-8 (m2) characterized by conformational change with respect to the methylimidazolate linker. A thorough comparison of the electronic and electrostatic properties in two limiting structural forms of ZIF-8 proposes additional aspects on diffusion and adsorption processes occurring within the framework. The dimensions of the four-membered (FM) and six-membered (SM) apertures of the β cage are reliably determined from the total electron-density distribution. The analysis shows that FM in m2 becomes competitive in size to the SM aperture and should be considered for the diffusion of small molecules and cations. Bader's topological analysis (quantum theory of atoms in molecules) shows similar properties of both ZIF-8 forms. On the other hand, analysis of their electrostatic properties reveals tremendous differences. The study suggests exceptional electrostatic flexibility of the ZIF-8 framework, where small conformational changes lead to a significantly different electrostatic potential (EP) distribution, a feature that could be important for the function and dynamics of the ZIF-8 framework. The cavity surface in m1 contains 38 distinct regions with moderately positive, negative, or neutral EP and weakly positive EP in the cavity volume. In contrast to m1, the m2 form displays only two regions of different EP, with the positive one taking the whole cavity surface and the strong negative one localized entirely in the FM apertures. The EP in the cavity volume is also more positive than that in m1. A pronounced influence of the linker reorientation on the EP of the ZIF-8 forms is related to the high symmetry of the system and to an amplification of the electrostatic properties by cooperative effects of the proximally arranged structural fragments.