Structure determination EXAFS

The violet hexaaquo ion [Cr(OH2)6] is a regular octahedron with Cr-0 distances in the range 191.5-199.1 pm, depending on the particular compound used for the structure determination. EXAFS measurements on dilute solutions have provided information about the second hydration shell it appears to contain an average of 13.5 water molecules at a Cr-0 distance of 402(2) pm. ... 

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 a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

EXAFS data are in agreement with the structure determined by NMR with two carbons at relatively short distances (not resolved at 1.79 A), corresponding to the (= CHtBu) and (= CtBu) ligands, another carbon and an... 

Good images indicating nearly uniform clusters of other metals are lacking, but evidence from EXAFS spectroscopy, combined with IR spectroscopy and extraction of clusters into solution, has provided a basis for structure determination of a number of small metal carbonyl clusters and clusters formed by their decarbonylation. Compilations of these are reported elsewhere [6,12,26]. 

Elder, R.C., Ludwig, K, Cooper, J.N. and Eidsness, M.K (1985) EXAFS and WAXS structure determination for an antiarthritic drug, sodium gold(l) thiomalate. Journal of the American Chemical Society, 107, 5024—5026. 

EXAFS analysis is a powerful spectroscopic method for structural analysis which has been extensively applied to the problem of structure determination in nanoparticles, and especially bimetallic nanoparticles [170-172]. The X-ray absorption spectrum of an element contains absorption edges corresponding to the excitation of electrons from various electronic states at energies characteristic of that element, i.e., K edges arise from the excitation of electrons from Is states, and LI, II, III edges from excitations from 2s, 2p 1/2, and 2p3/2 states. When the X-ray energy is increased above an edge, oscillations (fine... 

The applications of polarized x-ray absorption spectroscopy (PXAS) for structure determination in inorganic and bioinorganic systems are discussed. PXAS studies of oriented samples add angular detail to the information obtained from x-ray absorption edges and from EXAFS. In some cases, PXAS can be used to determine molecular orientation. In other cases, PXAS can be used to infer the details of electronic structure or of chemical bonding. Some of the potential future applications of PXAS are discussed. 

Structure Determination. In general, good agreement was found between the observed EXAFS and the known structure in all... 

The long Cu-S(Met) bond (2.9 A) does not contribute to the EXAFS fit for PCu(II) under a variety of conditions, including studies on orientated single crystals at liquid He temperatures [38]. From fitting procedures reported the two Cu-N(His) bonds at 1.97 A are 0.1 A less and the Cu-S(Cys) distance 2.11 A (0.02 A smaller) than those obtained from the X-ray crystal structure determination [16]. 

NMR investigations [129, 132, 133], EXAFS and XANES studies [134-136], and theoretical calculations [127, 137, 138] performed on higher-order cyanocuprates strongly suggested that the cyanide anion was not bound to copper in these R2Cu(CN)Li2 species. Additional evidence was provided by the first X-ray crystal structure determinations of higher-order cyanocuprates ](C(5H4CH2NMe2-2)2 Cu(CN)Li2] [139] (Fig. 1.34) and [(tBu)2Cu(CN)Li2] [130] (Fig. 1.35). 

Data analysis procedures have developed substantially over the last few years. In particular, use of least square refinement methods have been developed. Recent progress with theoretical development for the treatment of multiple scattering has resulted in Ugand group refinement such as an imidazole. We can expect further development in this area which ought to lead us to restrained least square refinement procedures for EXAFS data analysis. This type of restrained refinement is commonly used for macromolecular crystallographic structure determination where a similar problem of imderdeterminancy exists... 

Di Cicco, A., and Minicucci, M., Solid and liquid short-range structure determined by EXAFS multiple-scattering data analysis, /. Synchrotron Rad., 6, 255-257,1999. 

As an example, Fig. 5.6 depicts a typical diffraction spectrum. It is evident that long range order does not exist in our chalcogenide samples. However, the broad difffactrogram peak centered at 20 = 42.5° has the characteristic of a nanodivided ruthenium metal [22]. This points out that the active center in this chalcogenide materials is essentially of metallic nature. The material, either in powder or colloidal form, was analyzed by the EXAFS technique [11]. The local range order of this technique allowed for some structural determination of our samples. Thus, for example, the co-ordination distances for ruthenium-selenium and ruthenium-ruthenium are R(RU-se) = 2.43 A y R(ru.rU) = 2.64 A, respectively. The metal-metal co-ordination distance is of the same order of magnitude as that of well known cluster based materials such as the Chevrel phase [35, 37], cf. Fig. 5.2b. This testifies that the used chemical route leads to the formation of cluster-like materials. 

The methods of structure determination of supported nanoclusters are essentially the same as those mentioned previously for supported metal complexes. EXAFS spectroscopy plays a more dominant role for the metal clusters than for the complexes because it provides good evidence of metal-metal bonds. Combined with density functional theory, EXAFS spectroscopy has provided much of the structural foundation for investigation of supported metal clusters. EXAFS spectroscopy provides accurate determinations of metal-metal distances ( 1-2%), but it gives only average structural information and relatively imprecise values of coordination numbers. EXAFS spectroscopy provides structure data that are most precise when the clusters are extremely small (containing about six or fewer atoms) and nearly uniform (Alexeev and Gates, 2000). 

Other techniques used in surface structure determination include extended X-ray absorption fine structure (EXAFS), ion scattering, electron forward focusing, and helium atom diffraction. 

---