X-ray absorption fine structure data

The task in using these simple expressions (7) and (8) lies in finding the number of ligands n, the force constants Xt./o> and values for the bond length difference Ar. The values of n and Arare obtained from X-ray crystallographic or extended X-ray absorption fine structure data. The force constants /, and fs are obtained from available vibrational spectroscopic data using the equation. 

High-resolution crystallographic studies of type 1 copper proteins supplemented by EXAFS (extended X-ray absorption fine structure) data and analyses of sequence conservation show that the archetypal type 1 copper site has certain constant features. There exist several excellent reviews. For reference, the copper sites in plastocyanin, azurin, tomato plantacyanin, and cucumber stellacyanin are shown in Figure 6. 

The discovery of the phenomenon that is now known as extended X-ray absorption fine structure (EXAFS) was made in the 1920s, however, it wasn t until the 1970s that two developments set the foundation for the theory and practice of EXAFS measurements. The first was the demonstration of mathematical algorithms for the analysis of EXAFS data. The second was the advent of intense synchrotron radiation of X-ray wavelengths that immensely facilitated the acquisition of these data. During the past two decades, the use of EXAFS has become firmly established as a practical and powerfiil analytical capability for structure determination. ... 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

Quantitative data on local structure can be obtained via an analysis of the decaying slope next to the absorption edge. The absorption of an X-ray photon boosts a core electron up into an unoccupied band of the material which, in a metal, is the conduction band above the Fermi level. Electrons in such a band behave as if nearly free and no fine structure would be expected on the absorption tail . However, fine structure is observed up to 500 to 1000eV above the edge (see Figure 2.73(b)). The ripples are known as the Kronig fine structure or extended X-ray absorption fine structure (EX AFS). 

Another noteworthy example is x-ray absorption fine structure (EXAFS). EXAFS data contain information on such parameters as coordination number, bond distances, and mean-square displacements for atoms that comprise the first few coordination spheres surrounding an absorbing element of interest. This information is extracted from the EXAFS oscillations, previously isolated from the background and atomic portion of the absorption, using nonlinear least-square fit procedures. It is important in such analyses to compare metrical parameters obtained from experiments on model or reference compounds to those for samples of unknown structure, in order to avoid ambiguity in the interpretation of results and to establish error limits. 

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). 

Concurrently with the X-ray crystallographic studies, extended X-ray absorption fine structure (EXAFS) studies confirmed many of the bond distances proposed for nitrogenase s FeMoco cluster. The EXAFS data of reference 25 indicate short Fe-Fe distances of 2.61, 2.58, and 2.54 A for M+, M (resting state), and M forms, respectively. The authors believe that the short M center bond lengths indicate Fe-Fe bonds in this cluster. In another study using dithionite-reduced MoFe-protein Fe-S, Fe-Fe, Fe-Mo distances of 2.32, 2.64, and 2.73 A, respectively, were found in the 1 to 3 A region and Fe-Fe, Fe-S and Fe-Fe distances of 3.8, 4.3, and 4.7 A, respectively, were found in the 3 to 5 A region.30... 

DAF-1, butene diffusion in, 42 36 Dangling bonds, 34 138 Data analysis, in extended X-ray absorption fine structure studies, 35 31-33 Dawson structure... 

Lithium intercalation in VeOis has been studied by Stallworth et al. ° Variable-temperature Li NMR indicated considerable mobility for Li+ in the intercalated materials. The Li NMR data were compared with ESR spectra and near-edge X-ray absorption fine structure (NEXAFS) data on the same materials, and a correlation between vanadium oxidation state (from NEXAFS data) and NMR shift was observed. The authors explained the shifts in terms of different coupling mechanisms between the and shifts. The shifts were, however, extracted from static NMR experiments, and it is possible that some of the different local environments, typically revealed in a MAS spectrum, were not seen in this study. 

Extended X-ray absorption fine structure (EXAFS) studies on the Fe/Mo/S aggregate in nitrogenase have made available structural data that are essential in the design of synthetic analog clusters. Analyses of the Mo K-edge EXAFS of both the Fe-Mo protein and the FeMoco (9) have shown as major features 3-4 sulfur atoms in the first coordination sphere at 2.35 A and 2-3 iron atoms further out from the Mo atom at 2.7 A. The Fe EXAFS of the FeMoco (10,11) shows the average iron environment to consist of 3.4 1.6 S(C1) atoms at 2.25(2) A, 2.3 +0.9 Fe atoms at 2.66(3) A, 0.4 0.1 Mo atoms at 2.76(3) A and 1.2 1.0 0(N) atoms at 1.81(7) A. In the most recent Fe EXAFS study of the FeMoco (11) a second shell of Fe atoms was observed at a distance of 3.75 A. 

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