The performance of different density functional tight binding (DFTB) methods for the description of six increasingly complex metal−organic framework (MOF) compounds have been assessed. In particular the self-consistent charge density functional tight binding (SCC DFTB) approach utilizing the 3ob and matsci parameter sets have been considered for a set of four Zn-based and two Al-based MOF systems. Moreover, the extended tight binding for geometries, frequencies, and noncovalent interactions (GFN2-xTB) approach has been considered as well. In addition to the application of energy minimizations of the respective unit cells, molecular dynamics (MD) simulations at constant temperature and pressure conditions (298.15 K, 1.013 bar) have been carried out to assess the performance of the different DFTB methods at nonzero thermal conditions. In order to obtain the XRD patterns from the MD simulations, a flexible workflow to obtain time-averaged XRD patterns from (in this study 5000) individual snapshots taken at regular intervals over the simulation trajectory has been applied. In addition, the comparison of pair-distribution functions (PDFs) directly accessible from the simulation data shows very good agreement with experimental reference data obtained via measurements employing synchrotron radiation in case of MOF-5. The comparison of the lattice constants and the associated X-ray diffraction (XRD) patterns with the experimental reference data demonstrate, that the SCC DFTB approach provides a highly efficient and accurate description of the target systems.
As a consequence of the accelerated climate change, solutions to capture, store and potentially activate carbon dioxide received increased interest in recent years. Herein, it is demonstrated, that the neural network potential ANI-2x is able to describe nanoporous organic materials at approx. density functional theory accuracy and force field cost, using the example of the recently published two- and three-dimensional covalent organic frameworks HEX-COF1 and 3D-HNU5 and their interaction with CO2 guest molecules. Along with the investigation of the diffusion behaviour, a wide range of properties of interest is analyzed, such as the structure, pore size distribution and host-guest distribution functions. The workflow developed herein facilitates the estimation of the maximum CO2 adsorption capacity and is easily generalizable to other systems. Additionally, this work illustrates, that minimum distance distribution functions can be a highly useful tool in understanding the nature of interactions in host-gas systems at the atomic level.
By utilizing an expedient anion exchange protocol and starting with an improved synthesis of 2‐azido‐1,3‐dimethoxyimidazolium hexafluorophosphate, a series of six metathetical salts, including the energetic nitrate and perchlorate as well as the halides, was prepared. We focused on the crystallographic and thermal characterization of these compounds, representing intrinsically reactive hybrid organic / inorganic salts, and conducted a small comparative study with the related 2‐methyl‐1,3‐dimethoxyimidazolium salts. The latter compounds were expected to thermally disintegrate less sluggishly since the covalent azido‐group as intrinsic blasting initiator is missing. Both 2‐azido‐ and 2‐methylimidazolium nitrate exhibit a very sharp thermal onset of complete disintegration. In contrast, the respective perchlorates display a continuous thermogravimetric weight loss over a broad temperature range. Additionally, X‐ray single crystal structure determinations were performed for eight of the new compounds as well as for two 2‐azido‐1,3‐dimethoxyimidazolium salts with a mixed anion composition. Phase purity was also checked for five of the congeners using powder X‐ray diffraction (Pawley fitting). Notably, in the 2‐azido‐imidazolium cation, the intra‐ring bonds of the N1−C2 type were found to be significantly shorter than N1−C4 type bonds, and a characteristic asymmetry between the two N1−C2−N(azido) bond angles was observed. The experimental cation geometry was reproduced by DFT calculations on selected compounds.
At multianvil high-pressure/high-temperature conditions of 10 GPa and 1273 K, the first ternary tungsten tellurate WTe2O7 is formed, starting from a stoichiometric mixture of WO3 and TeO2. The compound crystallizes...
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