Charge-Density Distribution and Electrostatic Flexibility
of ZIF‑8 Based on High-Resolution X‑ray Diffraction
Data and Periodic Calculations
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Abstract
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 (<b>m1</b>) and also in a “high-pressure”
form of ZIF-8 (<b>m2</b>) 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 <b>m2</b> 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 <b>m1</b> contains
38 distinct regions with moderately positive, negative, or neutral
EP and weakly positive EP in the cavity volume. In contrast to <b>m1</b>, the <b>m2</b> 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 <b>m1</b>. 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