Crystal structure analysis problematic

Add the remaining part of Ca with Li, Na, and K up to 1.0. If some Ca still remains after this operation, the analysis is suspect. There are also problems if the sum exceeds 1.0. Crystal-chemical data indicate that Na + K occupancies generally vary between 0 and 1, and it is precisely this variability that sometimes renders structural reconstruction problematic. 

Measuring the acidity of the Bronsted and Lewis acid sites is problematic. The adsorption and desorption of various amines has been used, but there is some disagreement about what it means.160 Some workers prefer to use isopropylamine, which desorbs only from Bronsted sites. Solid-state nuclear magnetic resonance (NMR) has also been used in the study of reactions on zeolites.161 Many zeolites crystallize into crystals that are too fine for conventional X-ray analysis. A new method that uses synchrotron X rays on microcrystalline powders promises to make it much easier to determine the structures of zeolites and related materials.162... 

The eudistomins from E. olivaceum extract readily into a non-polar phase, toluene (26-28) or dichloromethane (29), and require repeated chromatographic steps to separate one from another, especially within a related class. Separation in some cases is especially problematic (28, 29) for example, eudistomins N (3) and O (4) proved inseparable at the time they were reported. Structure elucidation relied heavily on interpretation of spectral data, especially 500 MHz NMR and mass spectra (26) several members of the series have now been synthesized (see Section 3). Eudistomin K has been crystallized and subjected to X-ray analysis (32) revealing the absolute configuration of the oxathiazepino-eudistomins. 

An a priori analysis on the reactivity and peculiarities of chemical behavior of molecules is a rather difficult but quite solvable task of theoretical chemistry. If molecules interact with a sohd surface, the complexity of its solution increases repeatedly. This is cause by the circumstances as follows firstly, an interaction occurs between two systems of different nature — molecule and surface that can be considered to be endless at the scale of partner secondly, it is difficult to simulate a surface adequately that is a macrodefect of the crystal periodic structure. Moreover, a definite grade of amorphization of surface layer is a characteristic of even typical crystal [125]. Taking into account probable relaxation and reconstruction of real surface as compared with ideal one, obtaining valid structural information on surface and subsurface layer of solids seems to be rather problematic. A cluster model of solid and its surface that is natural for chemists operating terms of local chemical bonds (despite that it is not quite suitable for the systems with covalent bond) may be considered to be fit for the objects with ionic bonds that are objects of our investigation. 

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