Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination.
Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination.
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DOI：
10.1107/s1600576714022985
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发表时间：
2015-02-01
影响因子：
6.1
通讯作者：
Stalke D
中科院分区：
文献类型：
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作者：
Krause L;Herbst-Irmer R;Sheldrick GM;Stalke D
对Ag Kα和Mo Kα微源（IµS）单晶衍射数据进行了详细比较，结果表明，当吸收显著时，Ag Kα单晶衍射数据更好。用于校正吸收的经验校正也可以很好地校正高度聚焦的IµS光束的影响。用银和钼微源比较了六种吸收因子范围广的模型化合物的衍射数据质量。实验在两个30 W风冷Incoatec IµS多层光学微聚焦光源上进行，光源安装在带有SMART APEX II CCD探测器的Bruker D8测角仪上。所有数据均使用标准Bruker软件进行分析、处理和细化。结果表明，Ag - k - α辐射在重元素参与下是有益的。基于晶体面位置和指数的数值吸收校正对于高度聚焦的微源光束的作用有限，可能是因为晶体完全沐浴在均匀强度的（顶帽形）光束中的假设不再有效。幸运的是，在SADABS中实施的经验校正，虽然最初是为了校正吸收，也很好地校正了晶体辐照有效体积的变化。在研究的三个案例中（两个Ag和一个Mo），在应用SADABS实施的经验校正后，最终SHELXL R1对所有数据的校正低于1%。由于这样的修正是为了优化通过晶体的不同路径但具有相同的Bragg 2θ角的等效反射强度的一致性，因此需要对吸收的2θ依赖进行进一步的修正。为此，SADABS使用自定义值为μr的球形晶体的透射系数（其中μ为线性吸收系数，r为晶体的有效半径）；当r偏向于最小的晶体尺寸时，得到最好的结果。本文的结果表明，IUCr关于强吸收晶体必须采用数值吸收校正的出版物要求需要修改。
A detailed comparison of single-crystal diffraction data collected with Ag Kα and Mo Kα microsources (IµS) indicates that the Ag Kα data are better when absorption is significant. Empirical corrections intended to correct for absorption also correct well for the effects of the highly focused IµS beams. The quality of diffraction data obtained using silver and molybdenum microsources has been compared for six model compounds with a wide range of absorption factors. The experiments were performed on two 30 W air-cooled Incoatec IµS microfocus sources with multilayer optics mounted on a Bruker D8 goniometer with a SMART APEX II CCD detector. All data were analysed, processed and refined using standard Bruker software. The results show that Ag Kα radiation can be beneficial when heavy elements are involved. A numerical absorption correction based on the positions and indices of the crystal faces is shown to be of limited use for the highly focused microsource beams, presumably because the assumption that the crystal is completely bathed in a (top-hat profile) beam of uniform intensity is no longer valid. Fortunately the empirical corrections implemented in SADABS, although originally intended as a correction for absorption, also correct rather well for the variations in the effective volume of the crystal irradiated. In three of the cases studied (two Ag and one Mo) the final SHELXL R1 against all data after application of empirical corrections implemented in SADABS was below 1%. Since such corrections are designed to optimize the agreement of the intensities of equivalent reflections with different paths through the crystal but the same Bragg 2θ angles, a further correction is required for the 2θ dependence of the absorption. For this, SADABS uses the transmission factor of a spherical crystal with a user-defined value of μr (where μ is the linear absorption coefficient and r is the effective radius of the crystal); the best results are obtained when r is biased towards the smallest crystal dimension. The results presented here suggest that the IUCr publication requirement that a numerical absorption correction must be applied for strongly absorbing crystals is in need of revision.