XANES coordination geometry

Zn and N K-edge XANES have been used to distinguish the coordination geometries in (1,2-ethanediylidene)-bis(5 -methylhydrazonecarbodithionate) zinc complexes (109). The technique distinguished between tetrahedral species, square pyramidal dimers, and square pyramidal monomers, formed when pyridine was present. These studies were in conjunction with spectroscopic characterization and X-ray single-crystal data where possible.53 The results demonstrated the value of this technique when single crystals could not be obtained. 

Figure 2. Effect of coordination geometry on V XANES spectrum V naphthenate (----), (---). ...

XANES contains information of the stereochemical details (coordination geometry and bond angles) and EXAFS gives information about local structures in terms of atomic radial distribution (distances) around the central atom. Some techniques are very deep probes while others are strictly surface oriented e.g., with the EPMA and SIMS, the probe depth is 1 nm with XPS, 2 to 7 nm with EXAFS, 0.5 nm, with XANES, 5 to 50 nm, with AES, 1 to 3 nm, and with ISS, it is only 0.3 nm. 

Difficult samples for investigation are those which present absorbing atomic species in a variety of configurations (heterogeneity). The presence of absorbers in different coordination geometries often leads to featureless XANES. 

Coordination Geometry from XANES Pre-Edge Analysis... 

Fig. 54 shows the XANES and EXAFS spectra of the Cu(l)ZSM-5 zeolite after addition of CO (180). The addition of CO causes a dramatic decrease in the intensity of the band attributed to the Is 4p transition in the XANES of the Cu(I)ZSM-5. In the EXAFS spectra of these catalysts, the Cu-O peak became smaller and shifted to a longer atomic distance (i.e., 1.5 —> 1.8 A), and new peaks attributed to the C atom (Cu-C) and O atom (Cu-C-O) of the adsorbed CO molecule appear due to the addition of CO onto the catalyst. These results indicate that CO molecules adsorb on the isolated Cu ions strongly enough to distort the Cu coordination geometry (180). 

A veritable arsenal of techniques has been mobilized to provide information regarding the structure of redox molecular sieves [9-14], X-Ray powder diffraction (XRD) provides an immediate check for crystallinity and structural type. X-Ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge spectroscopy (XANES) give fiirther insights into coordination geometry and bond lengths. Infiared and Raman spectroscopy have been used to identify characteristic features, e.g. the 960 cm bond attributed to the Si-OTi stretching vibration in TS-1 [9]. 

On the contrary, the structure which extends about 10 eV on each side of the edge, called X-ray Absorption Near Edge Structure (XANES), is strongly affected by multiple scattering effects. From this structure, it is possible to obtain information on the coordination geometry and the effective charge on the absorber. 

The XANES region contains information on the electronic state of the absorbing atom as well as on its coordination geometry. In general, three types of XANES features can be distinguished Pre-edge peaks carrying information on coordi-... 

Zinc. Zinc(II) ions equally adopt both tetrahedral and octahedral coordination geometry with oxide ions as ligands. In this context, it could be expected that the incorporation of the zinc ions into an aluminophosphate framework would proceed easily. The local structure of Zn in ZnAPOs has been studied by EXAFS/XANES, NMR, FTIR, and XRD techniques. The analyses mostly revealed the presence of tetrahedrally coordinated framework Zn + ions (312-322). 

Figure 11.4 (Top) The coordination geometry of Ti atoms in flat surface and 2nm Ti02 particles. (Bottom left) XANES pre-edge spectra of Ti K edge ( = 4.966 keV) for 50 nm Ti02 nanoparticles, 1.9 nm particles before and after modification of ascorbic acid. Al, A2, and A3 stands for preedge peaks. The middle peak intensity can reflect the coordination numbers of Ti atoms. (Bottom right) The bond distances of Ti and O atoms for three kinds of Ti02 above from EXAFS results. 1999 American Chemical Society.

X-ray absorption near edge structure (XANES) spectroscopy is a non-destructive and sensitive probe of the coordination number and geometry as well as of the effective charge of a chosen atom within a molecule and therefore also of the formal oxidation number. Recently, there have been a number of XANES studies at the sulfur K-edge demonstrating the sensitivity of... 

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