Short Intramolecular O···O Contact in Some <i>o</i>‑Dialkoxybenzene Derivatives Generates Efficient Hydrogen Bonding Acceptor Area

Statistical analysis of data extracted from the Cambridge Structural Database (CSD) has been used to investigate the crystal structure properties of <i>o</i>-dialkoxybenzene derivatives, compounds containing two ether oxygen acceptors in <i>ortho</i> positions of benzene ring. It has been shown that in more than 90% of cases, the fragment has predictable geometrical characteristics where the two ether oxygens form short interatomic O···O contact (2.57 Å in average), while O-substitutents take <i>trans</i> position, both approximately coplanar with the benzene ring. Such arrangement of oxygen acceptors produces a large and uniform area of the negative electrostatic potential suitable for multiple hydrogen bonding. The acceptor abilities of the O···O system have been investigated by the statistical CSD analysis. The <i>ab initio</i> estimation of the interaction energy in the dimer of <i>o</i>-dimethoxybenzene (DMB) and H₂O, employed as a model system, is achieved via high-level electron correlation CCSD­(T) calculation with the complete basis set extrapolation. The interaction energy is estimated to be −6.5 kcal/mol. The results indicate the existence of a very flat potential energy surface in the region between methoxy oxygens and that DMB–water is a highly flexible system. The structural role of the O···O acceptor system is particularly interesting considering its ability to form multiple hydrogen bonding

Charge-Density Distribution and Electrostatic Flexibility of ZIF‑8 Based on High-Resolution X‑ray Diffraction Data and Periodic Calculations

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