Valence bond structure 58 Zeolites

Fig. 4 Proposed defect cluster model in as-made zeolites with quaternary ammonium cations as structure directing agents (SDAs) hydrogen bond distances of 1.68 A are determined experimentally from the H NMR chemical shift of 10.2 ppm X and Y are atoms not further specified in the SDA the interaction between the SDA and the SiO- group is assumed based on bond valence arguments (see text)...

Besides X-ray powder diffraction, infrared and Raman spectra are important practical tools in the characterization of solid materials. However, the interpretation of the absorption spectra requires theoretical input. Therefore, the calculation of vibrational properties plays an important role in understanding the structure of complex solids. The vibrational properties of ionic materials can be described surprisingly well by force fields which include only pair potentials in the form of electrostatic interactions and a term describing the repulsion as atoms get close to each other. In the case of zeolites, addition of bond angle terms in the spirit of valence force fields provides additional improvements. However, the description of covalent and metallic solids usually requires quantum mechanical approaches. 

Given the correlation between bond valence and electron density, it appears tempting to compare also what electron density maps and maps of the BVSE predict as ion transport pathways. Hirshfeld surface analysis has been explored to characterize intermolecular interactions in molecular crystals [47,48]. This analysis is based on the procrystal, which is obtained from superposition of spherical atomic electron densities placed at the crystal structure positions, a quantity that can readily be calculated from the structure using software tools such as CrystalExplorer [49]. The approach was also explored as a tool to map out voids in porous crystals such as metal organic framework materials and zeolites [50]. 

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