Parsons Simon, Flack Howard D, Wagner Trixie
EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JJ, Scotland.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2013 Jun;69(Pt 3):249-59. doi: 10.1107/S2052519213010014. Epub 2013 May 17.
Several methods for absolute structure refinement were tested using single-crystal X-ray diffraction data collected using Cu Kα radiation for 23 crystals with no element heavier than oxygen: conventional refinement using an inversion twin model, estimation using intensity quotients in SHELXL2012, estimation using Bayesian methods in PLATON, estimation using restraints consisting of numerical intensity differences in CRYSTALS and estimation using differences and quotients in TOPAS-Academic where both quantities were coded in terms of other structural parameters and implemented as restraints. The conventional refinement approach yielded accurate values of the Flack parameter, but with standard uncertainties ranging from 0.15 to 0.77. The other methods also yielded accurate values of the Flack parameter, but with much higher precision. Absolute structure was established in all cases, even for a hydrocarbon. The procedures in which restraints are coded explicitly in terms of other structural parameters enable the Flack parameter to correlate with these other parameters, so that it is determined along with those parameters during refinement.
使用Cu Kα辐射收集的单晶X射线衍射数据，对23个不含有比氧重的元素的晶体，测试了几种绝对结构精修方法：使用倒易孪生模型的常规精修、在SHELXL2012中使用强度商数进行估计、在PLATON中使用贝叶斯方法进行估计、在CRYSTALS中使用由数值强度差异组成的约束进行估计以及在TOPAS-Academic中使用差异和商数进行估计，其中这两个量均根据其他结构参数进行编码并作为约束实施。常规精修方法得到了准确的弗拉克参数值，但标准不确定度范围为0.15至0.77。其他方法也得到了准确的弗拉克参数值，但精度要高得多。在所有情况下都确定了绝对结构，甚至对于一种烃类也是如此。那些根据其他结构参数明确编码约束的程序，使得弗拉克参数能够与这些其他参数相关联，从而在精修过程中与那些参数一起被确定。
