Diffraction by single crystals interpretation of results

The final structure, once published, is then also collated in a structural database, such as the Cambridge Stmctural Database (CSD) or the Inorganic Crystal Stmcture Database (ICSD) (Section 2.11.4). 

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Diffraction by single crystals interpretation of results 10.8.1 How good is a structure ... 

Summary The zwitterionic spirocyclic X 5/-silicates 5 and 6 (5/O4C skeletons) and their respective isoelectronic analogs 7 and 8 S1O5 skeletons) were synthesized and structurally characterized by single-crystal X-ray diffraction. In addition, the dynamic behavior of the isoelectronic CH2/O analogs 6 and 8 in solution ([D6JDMSO) was studied by VT H-NMR experiments. The molecular structures of the CH2/O pairs 5/7 and 6/8 in the crystal are very similar, and the dynamic behavior of the CH2/O analogs 6 and 8 in solution (reversible coalescence phenomena) is also characterized by distinct similarities. The results of the VT H-NMR studies can be interpreted in terms of an intramolecular process that leads to an inversion of absolute configuration of the chiral zwitterions (A-6 A-6, AG = 89 2 kJ mok A-8 — A-8, AG = 84.6 0.6 kJ mol ). 

Generally, LEED experiments are conducted on specified faces of single crystals. When this is done, the diffraction pattern produced consists of a series of spots with a location, shape, and intensity that can be interpreted in terms of the surface structure. We focus attention on what can be learned from the location and shape of the spots since the study of intensity is beyond the scope of this book. It is generally assumed that the surface examined by LEED is an extension of an already-known bulk crystal structure. The correctness of this assumption can be tested, and results are often expressed in terms of modifications of the three-dimensional structure at the surface. Before we turn to the LEED patterns below, we must first figure out how they are read. 

Sample purity is a key concern. The NRVS experiment is a bulk technique sampling all Fe nuclei, and impurities that also contain the probe nucleus may confound quantitative data interpretation. Impurities may be introduced during sample preparation or may result from sample instability during measurement. Because of this, care must be taken to ensure purity and reproducibility as judged by Mossbauer (see Mdssbauer Spectroscopy), single-crystal X-ray diffraction, electronic absorption spectroscopy (see Electronic Spectroscopy), Raman spectroscopy or other qnalitative techniqnes. 

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