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2楼2010-09-02 21:34:45
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4楼2010-09-02 22:40:52
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-------------------------------------------------------------------------------- Completed crystal structures must pass the following tests. 1 The bonds in the model must be chemically reasonable. Similar bonds should have similar geometries and match literature values. 2 There should be no atoms with displacement parameters that are non-positive definite, npd. The displacement parameters should be checked for signs of systematic error. For example, ellipsoids of several heavy atoms aligned in one direction may indicate the need for a better absorption correction. Nonspherical or large ellipsoids suggest that the model may need to include disorder. 3 The structure should be refined to convergence, that is the maximum shift/error ratio should be < 0.05. All non-hydrogen atoms should be refined with anisotropic displacement parameters provided that there are at least 10 data per parameter. Lower data-to-parameter ratios indicate that either the data were not collected to a high enough scattering angle, or that Friedel-related (or equivalent) data were not collected for a structure in a noncentrosymmetric space group. 4 Noncentrosymmetric space groups should be refined with the correct absolute structure. The sample must contain atoms with sufficient differences in anomalous scattering to yield a meaningful absolute structure result. For example, the absolute structure cannot be determined under either of the following conditions, only one type of atom (besides hydrogen) and any type of radiation, or only first row atoms while using Mo Kα radiation. Also the data must be collected with sufficient quality (sufficient count times) to produce large (>10) mean F2/σ. The best test for absolute structure is the Flack test.6 5 The weighting scheme should be adjusted so as to produce nearly constant values for the variances as functions of intensity and resolution. Doing this will also make the goodness of fit, S, have a value around 1.0. 6 There should be no peaks with strong intensities in a list of "worst-fitting data." 7 The final difference map should have no abnormally high peaks or low valleys. 8 The final R1 and wR2 should be reasonably low for the quality of data. Good small molecule crystal structures usually have R1 < 0.05. Acceptable small molecule crystal structures typically have R1 < 0.10. -------------------------------------------------------------------------------- References 1 G. M. Sheldrick, SHELXTL Reference Manual, 1997, Bruker-AXS, Inc., Madison, WI 2 A. J. C. Wilson, Acta Cryst., 1976, A32, 994-996. 3 H. D. Flack & D. Schwarzenbach, 1988 Acta Cryst., 1988, A44 499-506. 4 W. C. Hamilton, Acta Cryst., 1965, 18, 502-510. 5 D. Rogers, Acta Cryst., 1981, A37, 734-741. 6 H. D. Flack, Acta Cryst., 1983, A39 876-881. 7 E. Parthe & L. M. Gelato, Acta Cryst., 1984, A40,169-183. 8 C. Bernardinelli & H. D. Flack, Acta Cryst., 1985, A41, 500-511  |
6楼2010-09-03 08:46:25
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7楼2010-09-03 09:01:28
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8楼2010-09-03 19:42:58
