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关于一个手性结构的flack因子
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_refine_ls_abs_structure_Flack 0(4) 这个晶体的绝对构型有问题吗？ |
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2楼2009-07-13 23:42:42
xi2004
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bamamei(金币+1):谢谢参与
waiy2001(金币+1,VIP+0):谢谢,学习了,呵呵 7-14 16:05
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貌似偏差太大, flack参数没有意义. 是不是没有重的原子啊? 先提供一个链接 http://www.iucr.org/__data/iucr/lists/coredmg/msg00003.html 再看这个 The correct absolute structure has been defined by the atomic coordinates if _refine_ls_abs_structure_flack is close to 0.0 (and the s.u. is sufficiently small). If _refine_ls_abs_structure_flack is close to 1.0, the incorrect enantiomer is being modelled and the atomic coordinates should be inverted and refined again. In cases of intermediate values of _refine_ls_abs_structure_flack, and you believe that a merohedral twin or a partial mix of enantiomers is present, choose the configuration that gives the lowest value for _refine_ls_abs_structure_flack. Note that if the s.u. of the Flack parameter is large, e.g. greater than or equal to 0.3, one cannot confidently derive the absolute structure from the data, because, within the 3sigma confidence limits, the full range of possible values of the Flack parameter are plausible. In such cases, even if the Flack parameter itself is close to 0.0, no conclusions about the absolute structure are justified. This will usually be the case if the compound is a weak anomalous scatterer (i.e. no atom heavier than Si is present). If a heavy atom is present, but the s.u. is still large, you should ensure that a significant fraction (ideally all) of the Friedel opposites of the symmetry unique reflections have been included in the data set. If the absolute structure parameter is meaningless because the compound is a weak anomalous scatterer, it is best to remove the absolute structure parameter from the CIF. If the absolute structure parameter is meaningful, please also provide appropriate reference details under _refine_ls_abs_structure_details. For example: _refine_ls_abs_structure_details 'Flack (1983), XXXX Friedel pairs' and replace the XXXX with the actual number of Friedel pairs used in the refinement. [An easy way to determine the number of Friedel pairs is to look at the difference between the number of unique reflections used in SHELXL when a MERG2 and MERG 3 instruction is used (MERG 3 forces Friedel pairs to be merged before use).] |
3楼2009-07-14 09:07:33
4楼2009-07-14 14:43:39
5楼2009-07-22 19:24:35
