X-ray diffraction data collecting

Otwinowski Z, Minor W. Processing of X-ray diffraction data collected in oscillation mode. Methods Enzymol 1997 276 307-26. 

Pflugrath, J. W. (1999). The finer things in X-ray diffraction data collection. Acta Crystallogr. D 55, 1718-1725. 

Garman, E. F (1991). Modern methods for rapid X-ray diffraction data collection from crystals of macromolecules. In Methods in Molecular Biology, vol. 56, Crystallographic Methods and Protocols, Jones, C., MuUoy B. and Sanderson, M. R., eds. Humana Press. 

Otwinowski, Z. and Minor, W. (1997). Processing of X-ray Diffraction Data Collected in the Oscillation Mode. Method Enzymol. 276A, 307-326. 

As anticipated, lower temperature increases the number of observations from an X-ray diffraction data collection (at constant radiation dose). This is however just one of the advantages that could improve a structure solution or a refinement. In fact, a reduced thermal motion usually implies a more reliable standard model, given that for smaller atomic displacements the harmonic approximation is more appropriate and less correlation is found between variables within a least squares refinement. This returns higher precision of the parameters calculated from those variables (for example bond distances, bond angles, etc.). 

One of the most relevant and fruitful areas of structural investigation for synthetic oiganic chemistry during the past decade has been the crystal structure determinations of a variety of enolate and closely related carbanions. Although these species have been considered only as transient reactive intermediates, a number of these enolates can be crystallized out of solution at subambient temperature and stabilized under a stream of cold, dry nitrogen gas during the 24-48 h necessary for X-ray diffraction data collection. A systematic review of these structures known to date begins with the ketone enolates. 

This second edition is vastly improved over the first. Numerous small errors present in the original were rooted out and banished, both in the text and the figures. The clarity of numerous figures was improved, and new tables were added and mathematical nomenclature made more uniform. This second edition includes two new chapters, one on macromolecular crystallization, and a second on X-ray diffraction data collection. Refinement and anomalous dispersion phasing are treated somewhat more extensively. More than 35 new figures have been incorporated. 

Bragg s equation gives an easy way to understand XRPD. Powder X-ray diffraction data collected on crystalline samples gives information about peak intensities and peak positions. Peak intensities are determined by the contents of unit cells, and peak positions are closely related to the cell constants. Interplanar spacing is a function of Miller indices and cell constants. Therefore, if the cell constants are known for a crystalline compound, peak positions corresponding to Miller indices can be obtained from the Bragg s equation the wavelength. A, is machine-specific. The determination of cell parameters in structure determination of XRPD pattern is a reverse process to find cell constants from peak positions. Here, note that the cell constants for a unit cell are not affected by the contents in the unit cell. The contents in the unit cell have effects on the peak intensities. 

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