X-ray scattering, anomalous

Application. Anomalous X-ray diffraction (AXRD), anomalous wide-angle X-ray scattering (AWAXS), and anomalous small-angle X-ray scattering (ASAXS) are scattering methods which are selective to chemical elements. The contrast of the selected element with respect to the other atoms in the material is enhanced. The phase problem of normal X-ray scattering can be resolved, and electron density maps can be computed. 

Consider a polystyrene-ffej-polybutadiene star block copolymer with four arms coupled by a central Si-atom. Or consider a metal catalyst (e.g., Au) supported in activated carbon. Then the scattering of only the selected element (Si, Au, respectively) can be extracted [242]. Even the distribution of the elements in the material can be mapped based on ASAXS data. A concise review of the ASAXS method in combination with AXRD and AWAXS has been published by Goerigk et al. [243]. 

Technical Requirements. AXS requires an X-ray source with easily tunable, monochromatic photon wavelength. This means that a respective device can only be operated at a synchrotron. In general a 2D detector is used. 

Procedure. After the element of interest has been chosen, an X-ray absorption edge of the element that is inside the tuneable range of the synchrotron is selected. In general this is a K-edge or an L-edge. For the conversion between energy, E in keV, and the wavelength of radiation, X in nm, the relation 

For Comparison Notions of Normal Scattering. As the electron density 

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Murthy, H. M., et al. (1988). Crystal structure of Qostrid-ium addi-urici ferredoxin at 5-A resolution based on measurements of anomalous X-ray scattering atmultiple wavelengths. J. Biol Chem. 263,18430-18436. 

Blume, M., in Resonance Anomalous X-ray Scattering. Theory and Applications, G. Materlik,... 

In 1998, Matsubara, Utimoto and coworkers reported a general procedure for the preparation of bis(iodozincio)methane, which was obtained as a THF solution (equation 6)9. The detailed structural study of the solution by SANS (small angle neutron scattering), AXS (anomalous X-ray scattering) and EXAFS (extended X-ray absorption fine structure) implied that the g< wt-dizinc species prepared from diiodomethane and zinc is the monomeric bis(iodozincio)methane (3). Formation of polymethylene zinc 4 via Schlenk... 

An anomalous X-ray scattering study of (+(-tubocurarine dibromide (73 4MeOH) confirmed the structure and absolute configuration earlier assigned. The N-N distance is 10.66 A (47,48). A purified sample of (+(-tubocurarine chloride (73a), prepared by selective quatemization of (+(-tubocurine (74) (Section V,B,1), gave the anomalous MS behavior previously ascribed to thermal disproportionation or the presence of impurities. In particular, the MS has mle 594 [17%, (M - 15)+, loss of Me] but also 608 (3.7%) and 622 (0.5%). The higher peaks seem to be due to a thermal Hofmann elimination followed by recombination with methyl radicals. Similar behavior is observed with the tertiary bases (—)-curine (75) and (+(-tubocurine (74), and may be general for alkaloids of this skeletal type (49). 

Absolute configurations can be determined by X-ray crystallography, either by the method of anomalous X-ray scattering or by that of internal comparison with a reference of known absolute configuration. 

Anomalous X-ray scattering (Section 2.2.2) is a specific property, but generally it increases with X and with atomic number Z. The difference may be significant if an absolute configuration of the compound is to be determined. With Mo radiation, it is hopeless to attempt such determinations unless atoms with Z > 15 are present in the sample. With Cu radiation, even anomalous scattering of oxygen is sometimes sufficient to effect discrimination. 

Cox DE, Wilkinson AP (1994) Powder diffraction studies using anomalous dispersion. In Materlin G, Spurhs CJ, Fischer K (eds) Resonant Anomalous X-ray Scattering Theory and Applications. North Holland, Amsterdam, p 13... 

New evidence for the persistence of H3O2 bridging in concentrated solution, corroborating our earlier evidence , was obtained by a Differential anomalous x-ray scattering study of solutions of a trinuclear tungsten cluster . ... 

Scattering patterns can be translated by standard diffraction techniques to give these pairwise distribution functions. This is difficult to do with X-ray scattering patterns, although modem techniques such as anomalous X-ray scattering have helped. It is relatively easy to determine these using the nuclear diffraction isotope substimtion technique. 

Nuclear spin dependent neutron scattering relies on polarisation variation, i.e. on a change of the order of the nuclear spin system. Resonant (or anomalous) X-ray scattering is a concomitant feature of core electron ionisation (Eq. (2)). The dispersion of the resonant coherent scattering is very small compared to the rise of absorption (and fluorescence) due to electronic excitation. It typically amounts to... 

Tidswell I M, Lucas C A, Markovic N M and Ross P N 1995 Surface structure determination using anomalous x-ray scattering Underpotential deposition of copper on Pt(111) Phys. Rev. B 51 10 205-8... 

Kortright J, Warburton W, Bienenstock A (1983) Anomalous X-ray scattering and its relationship to EXAFS. Springer Ser Chem Phys, Vol. 27, Springer-Verlag, Berlin, p 362-372... 

Saito M, Waseda Y (2000). Anomalous X-ray scattering for determining the partial structural functions of binary liquids. J Synchrotron Radiat 7 152-159... 

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