Schalken Martin J, Chantler Christopher T
School of Physics, University of Melbourne, Australia.
J Synchrotron Radiat. 2018 Jul 1;25(Pt 4):920-934. doi: 10.1107/S1600577518006549. Epub 2018 May 30.
Accurate experimental XAFS (X-ray absorption fine-structure) data including uncertainties are required during analysis for valid comparison of results and conclusions of hypothesis testing on structural determinations. Here an approach is developed to investigate data without standard interpolation of experimental data and with minimal loss of information content in the raw data. Nickel coordination complexes bis(i-n-propylsalicylaldiminato)nickel(II) (i-pr) and bis(N-n-propylsalicylaldiminato)nickel(II) (n-pr) are investigated. The additional physical insight afforded by the correct propagation of experimental uncertainty is used to determine newly refined structures for the innermost co-ordination shell. Two sets of data are investigated for each complex; one optimized for high point accuracy and one optimized for high point density. Clearly both are important and in this investigation the quality of the physical insight from each is directly provided by measured and propagated uncertainties to fairly represent the relevant accuracies. The results provide evidence for an approximate tetrahedral geometry for the i-pr Ni complex that is more symmetric than previously concluded, with our high point accuracy data yielding ligand lengths of 2.017 ± 0.006 Å and 2.022 ∓ 0.006 Å for Ni-N and Ni-O bonds, respectively, and an even more skewed square-planar (i.e. rhombohedral) arrangement for the n-pr complex with corresponding bond lengths of 2.133 ± 0.004 Å and 1.960 ∓ 0.003 Å. The ability to distinguish using hypothesis testing between the subtle differences in XAFS spectra arising from the approximate local tetrahedral and square-planar geometries of the complexes is also highlighted. The effect of standard interpolation on experimental XAFS spectra prior to fitting with theoretical model structures is investigated. While often performed as a necessary step for Fourier transformation into position space, this will nonetheless skew the fit away from actual data taken, and fails to preserve the information content within the data uncertainty. The artificial effects that interpolation imposes on χ are demonstrated. Finally, a method for interpolation is introduced which locally preserves the χ and thus information content, when a regular grid is required, e.g for further analysis in r-space.
在对结构测定进行假设检验的结果和结论进行有效比较的分析过程中，需要准确的包含不确定性的实验XAFS（X射线吸收精细结构）数据。本文开发了一种方法，用于在不对实验数据进行标准插值且原始数据信息损失最小的情况下研究数据。对镍配位络合物双（异正丙基水杨醛亚胺基）镍（II）（i-pr）和双（N-正丙基水杨醛亚胺基）镍（II）（n-pr）进行了研究。通过正确传播实验不确定性所提供的额外物理见解，用于确定最内层配位壳层的新优化结构。对每种络合物研究了两组数据；一组针对高点精度进行了优化，另一组针对高点密度进行了优化。显然，两者都很重要，在本研究中，每组物理见解的质量直接由测量和传播的不确定性提供，以公平地表示相关精度。结果为i-pr镍络合物的近似四面体几何结构提供了证据，该结构比先前得出的结论更对称，我们的高点精度数据得出Ni-N和Ni-O键的配体长度分别为2.017±0.006 Å和2.022∓0.006 Å，而n-pr络合物的平面正方形（即菱形）排列更为倾斜，相应的键长为2.133±0.004 Å和1.960∓0.003 Å。还强调了使用假设检验区分络合物近似局部四面体和平面正方形几何结构引起的XAFS光谱细微差异的能力。研究了在使用理论模型结构进行拟合之前，标准插值对实验XAFS光谱的影响。虽然这通常作为傅里叶变换到位置空间的必要步骤进行，但这仍会使拟合偏离实际采集的数据，并且无法保留数据不确定性内的信息内容。展示了插值对χ施加的人为影响。最后，引入了一种插值方法，当需要规则网格时，例如在r空间中进行进一步分析时，该方法可以局部保留χ，从而保留信息内容。
