Crystal structure analysis absorption correction

De Meulenaar, J., and Tompa, H. The absorption correction in crystal structure analysis. Acta Cryst. 19, 1014-1018 (1965). 

Among 2452 independent reflections in the range 2 <20<7O , 1973 ones with intensities larger than three times their estimated standard deviations (e.s.d. s) were used for the structure analysis Lp-corrected but no absorption and extinction corrections. The crystal data are ... 

A preliminary knowledge of the crystal structure is important prior to a detailed charge density analysis. Direct methods are commonly used to solve structures in the spherical atom approximation. The most popular code is the Shelx from Sheldrick [26] which provides excellent graphical tools for visualization. The refinement of the atom positional parameters and anisotropic temperature factors are carried out by applying the full-matrix least-squares method on a data corrected if found necessary, for absorption and diffuse scattering. Hydrogen atoms are either fixed at idealized positions or located using the difference Fourier technique. 

The single-crystal study of the a-Ni(NCS) (4-ViPy), structure was performed by using a Siemens AED aut omated three-circle diffractometer (filtered MoKa). 5029 independent reflections were measured within 27° of 0 by using the 0) - 20 scan mode, but as little as 950 reflections having 2a(I) have been used for structure analysis. The intensities were corrected for Lorentz-polarization effects but not for absorption. The structure was solved by direct methods, SHELX was used (ref. 5). Full-matrix refinement was made but, in view of low data/parameters ratio, only Ni, thiocyanates and pyridine N atoms were given anisotropic temperature factors. H atoms were included in the refinement at calculated positions. The final R value is 0.056 the weighted = 0.046 (w = 1.7/(a (F) + 0.0002(F) ). 

It will now be clear that the accuracy that may be attained in crystal analysis depends on the number of observed reflections and on the precision with which their intensities can be measured. (We assume that the structure is not complicated by any randomness or disorder, and that the necessary absorption and extinction corrections can be made.) A very useful discussion of the requirements necessary for determining bond lengths to within a limit of error of 0-01 A has been given by Cruickshank (1960). This is, of course, a very ambitious limit, but if it could be achieved it would enable the predictions of the molecular-orbital and valence-bond theories in aromatic hydrocarbons to be distinguished. It is pointed out that at the 0-1% level of significance a bond length difference must be 3-3 times the standard deviation to be accepted as genuine, so the limit of error of 0-01 A would require an e.s.d. (estimated standard deviation) of 0-003 A or better in the bond difference, or a coordinate e.s.d. of 0-0015 A or better. 

X-ray microanalysis is a local analysis, fulfilled by means of microanalyser electron probe, for sample sites of 1-3 pm. The electron probe is formed by electrostatic and magnetic fields to obtain a parallel electron beam with a diameter of --1 pm. The analysis is via primary X-ray sample emission which is spread out into a spectrum by means of X-ray spectrometer. In this method corrections for the atomic number of the element, the absorption of its radiation in the sample, its fluorescence, and the characteristic spectra of other elements contained in the sample must be accounted for. Microanalysis is used for the investigation of two- and three-component systems such as mutual diffusion, crystallization processes, local variations of alloy structure, etc. 

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