Structures Solutions and Refinements

Both structures were determined using direct methods included in the Multan 78 version 

The refinement includes an anisotropic vibration for all the non-hydrogen atoms. Difference-Fourier syntheses phased by the phosphorus, nitrogen, sulfur, oxygen and carbon atoms revealed the hydrogen atoms in their expected positions. 

In the last refinement cycles the hydrogen atoms were positioned geometrically following the indications concerning the bonding of such atoms in aziridinyl groups, i.e. (C—H) = 0.97 A and H—C H = 116° according to Dermer and Ham and 

In the final stages of refinement, an overall extinction parameter was calculated 0.257 X 10 for SOAz (I) and 0.129 x 10 for SOAz (II). 

The final R values were 0.051 and 0.035, corresponding in both cases to final difference maps exhibiting peaks lower than 0.4 eA .  

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Recent developments and prospects of these methods have been discussed in a chapter by Schneider et al. (2001). It was underlined that these methods are widely applied for the characterization of crystalline materials (phase identification, quantitative analysis, determination of structure imperfections, crystal structure determination and analysis of 3D microstructural properties). Phase identification was traditionally based on a comparison of observed data with interplanar spacings and relative intensities (d and T) listed for crystalline materials. More recent search-match procedures, based on digitized patterns, and Powder Diffraction File (International Centre for Diffraction Data, USA.) containing powder data for hundreds of thousands substances may result in a fast efficient qualitative analysis. The determination of the amounts of different phases present in a multi-component sample (quantitative analysis) is based on the so-called Rietveld method. Procedures for pattern indexing, structure solution and refinement of structure model are based on the same method. 

An absorption correction was applied which resulted in transmission factors ranging from 0.891 to 0.991. Reflections with 20 < 50 were used for structure solution and refinement. 

Sheldrick, G. M. SHELXTL-PLUS Program for Crystal Structure Solution and Refinement University of Gottingen Gottingen, Germany, 1997. 

The data needed for structure solution and refinement are moduli of the structure factors Fhki- To obtain these from the raw intensity measmements, we need to... 

Yu, R. C., Yakimansky, A V., Kothe, H., Voigt-Martin, I. G., Schollmeyer, D., Jansen, J., Zandbergen, H. and Tenkovtsev, A. V. (2000). Strategies for structure solution and refinement of small organic molecules from electron diffraction data and limitations of the simulation approach. Acta Crystallogr. A, 56, 436-50. 

Structural Solution and Refin ent. The structure was solved by analysis of similar structuies and expanded using Fourier maps. All atoms were refined anisotropically. The final cycle of full-matrix least-squares refinement was based on 555 observed reflections (7 > 3.00o(7)) and 70 variable parameters and converged (largest parameter shift was 0.00 times its esd) with unweighted and weighted agreement foctors of... 

Structure Solution and Refinement. The structure was solved by direct methods. Infonnadon on the collection of data and the refinement are given in Table SV. All atoms refined anisotro-pically. A correction for secondary extinction was applied (coefficient... 

Figure 4.1. The flowchart illustrating common steps employed in a structural characterization of materials by using the powder diffraction method. It always begins with the sample preparation as a starting point, followed by a properly executed experiment both are considered in Chapter 3. Preliminary data processing and profile fitting are discussed in this chapter in addition to common issues related to phase identification and analysis. Unit cell determination, crystal structure solution and refinement are the subjects of Chapters 5,6, and 7, respectively. The flowchart shows the most typical applications for the three types of experiments, although any or all of the data processing steps may be applied to fast, overnight and weekend experiments when justified by their quality and characterization goals.

Crystallographic Structure Visualization During Structure Solution and Refinement... 

Table 17.33 Software for crystallographic structure visualization during structure solution and refinement.

After successful structure solution and refinement, the final and very important phase completes the structural analysis. This is the detailed inspection of the structure looking for the intramolecular and intermolecular structural features of the system studied. Traditionally a plot of the molecule or a representative part of the structure (in a case continuous structures, like metallo-organic or organic frameworks, MOF or OF, respectively) with thermal ellipsoids viz. anisotropic displacement parameters) is presented. Such a plot drawn using program ORTEP [23] for compound 1 is presented in Fig. 9.12 (top). 

In this chapter diffraction and in particular X-ray and neutron diffraction will be described in general, with an emphasis on powder diffraction techniques. The specific properties of X-ray and neutron diffraction and a description of sources and instruments for powder diffraction studies will be presented. Furthermore the use of powder diffraction data, from the simple use for phase identification to structure solution and refinements with the Rietveld methods, will be described. Two examples showing the potential for powder... 

Powder diffraction techniques have developed enormously since 1990. This is partly due to improved X-ray and neutron sources and instrumentation and partly because of improved methods, algorithms and software for structure solution and refinements of experimental data. The more effective computers are also important. 

Finally, there has been significant progress in software and methods for structure solution and refinements during the last years. Based on the steady improvement in computer performance, it is expected that algorithms and software codes will be further improved and strengthen in the coming years. 

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