Structural investigation of mMNi(II) complex isomers using transmission XAFS: the significance of model development

Abstract

High-accuracy transmission XAFS determined using the hybrid technique has been used to refine the geometries of bis(N-n-propyl-salicylaldiminato) nickel(II) (n-pr Ni) and bis(N-i-propyl-salicylaldiminato) nickel(II) (i-pr Ni) complexes which have approximately square planar and tetrahedral metal coordination. Multiple-scattering formalisms embedded in FEFF were used for XAFS modelling of the complexes. Here it is shown that an IFEFFIT-like package using weighting from experimental uncertainty converges to a well defined XAFS model. Structural refinement of (i-pr Ni) was found to yield a distorted tetrahedral geometry providing an excellent fit, χr(2) = 2.94. The structure of (n-pr Ni) is best modelled with a distorted square planar geometry, χr(2) = 3.27. This study demonstrates the insight that can be obtained from the propagation of uncertainty in XAFS analysis and the consequent confidence which can be obtained in hypothesis testing and in analysis of alternate structures ab initio. It also demonstrates the limitations of this (or any other) data set by defining the point at which signal becomes embedded in noise or amplified uncertainty, and hence can justify the use of a particular k-range for one data set or a different range for another. It is demonstrated that, with careful attention to data collection, including the correction of systematic errors with statistical analysis of uncertainty (the hybrid method), it is possible to obtain reliable structural information from dilute solutions using transmission XAFS data.

Accurate X-ray Absorption Spectra of Dilute Systems: Absolute Measurements and Structural Analysis of Ferrocene and Decamethylferrocene

“…This has recently yielded significant structural insights into the environment of coordination complexes. 6 Notwithstanding these significant successes of XAFS, the detail and reliability of structural information obtained from XAFS is often limited due to the relative nature of many standard measurements, poor characterization of experimental uncertainties, and ill-defined refinement statistics from standard theoretical approaches. 7 High absolute accuracy modern measurements such as those using the X-ray extended range technique (XERT) 8 have shown success in quantifying XAFS spectra with uncertainties less than 0.1%.…”

High-accuracy measurement of mass attenuation coefficients and the imaginary component of the atomic form factor of zinc from 8.51 keV to 11.59 keV, and X-ray absorption fine structure with investigation of zinc theory and nanostructure

“…X-ray absorption fine-structure (XAFS) spectra display the energy-dependent fine structure in the mass attenuation coefficients, sensitive to many structural details of materials (Rehr & Albers, 2000). Thus, X-ray mass attenuation coefficients, particularly across absorption edge regions, are widely used in many techniques and fields including biological (Coulthard & Sham, 1996;Streltsov et al, 2018), molecular, condensed matter and solid state fields (Palmberg & Rhodin, 1968;Bohic et al, 2001) to study structural parameters including bond lengths, thermal parameters, oxidation number, coordination number and electron inelastic mean free paths of materials and molecules (Ignatov et al, 2001;Han et al, 2002;Islam et al, 2015Islam et al, , 2016. Attenuation coefficients and atomic form factors are important in the study of the ISSN 1600-5775 density of electron states (Joly et al, 1999;Hossain et al, 2005), roughness of sample surfaces (Glover et al, 2009), dynamics of atomic processes, elastic scattering and other scattering of X-ray interactions with matter (Hopersky et al, 2004) and self-absorption of materials (Trevorah et al, 2019).…”

Rotation of X-ray polarization in the glitches of a silicon crystal monochromator

“…The special design of I20-scanning has allowed the performance of EXAFS measurements on samples as dilute as 10 micromolal (Bowron & Diaz-Moreno, 2014). Dilute samples cannot easily be examined by measuring their X-ray absorp-tion as a function of incident photon energy in transmission because the absorption of the matrix dominates the EXAFS near the absorption edge of the absorber, although such studies have been successful for millimolar samples when great care was taken to remove systematic errors from the data collection (Chantler et al, 2015;Islam et al, 2015). Instead, the fluorescence yield of the absorber in the sample is measured using a large-area fluorescence detector.…”