Absorption EXAFS

Results from an array of methods, including X-ray absorption, EXAFS, esr and magnetic circular dichroism, suggest that in all ureases the active sites are a pair of Ni" atoms. In at least one urease,these are 350 pm apart and are bridged by a carboxylate group. One nickel is attached to 2 N atoms with a fourth site probably used for binding to urea. The second nickel has a trigonal bipyramidal coordination sphere. 

Multlnuclear clusters of Fe(III) occur hound to the protein when Fe(II), at amounts less than or equal to that required to saturate the protein. Is allowed to oxidize JUi situ. The clusters, predicted from the results of EPR spectroscopy (35,38) and UV-difference spectroscopy (34), were observed and characterized by x-ray absorption (EXAFS) and MOssbauer spectroscopy (Figure 2 Ref. 39). Measurements were made with a complex of Fe(III) and the protein coats of apoferrltln after the binding of 10 Fe(II) atoms/molecule, the admission of air and equilibration for 24 hours. 

The conventional method for determining cation ordering and site populations within a crystal structure is by diffraction techniques using X-ray, electron and neutron sources. For determining site occupancies of transition metal ions, these methods have been supplemented by a variety of spectroscopic techniques involving measurements of Mossbauer, electron paramagnetic resonance (EPR or ESR), X-ray absorption (EXAFS and XANES), X-ray photoelectron (XPS), infrared and optical absorption spectra. 

Seward TM, Henderson CMB, Chamock JM, Dobson BR (1996) An X-ray absorption (EXAFS) spectroscopic study of aquated Ag+ in hydrothermal solutions to 350°C. Geochim Cosmochim Acta 60 2273-2282... 

In the case of metallic adsorbates (metal deposits, underpotentially deposited upd-layers, catalytically active metal deposits), the type of coordination to surface sites (one-, two- or three-fold) and the distance to these sites may be of interest. Vice versa the same type of data may be of importance in the case of adsorbed ions on metal electrodes or about the atomic environment of a given atom/ion in an interphase. Analysis of the fine structure of X-ray absorption (EXAFS, XANES) close to the X-ray absorption edge of the species (atom) of interest will yield this data provided the sample can be prepared in a very thin layer in order to exclude unwanted bulk interference. Otherwise the experiment can be done in reflection (SEXAFS). Information about the distance between the atom of interest and its first and sometimes even second shell of surrounding species can be derived from the spectra [95]. Availability of a suitable light source, generally a synchrotron (for details see p. 15), is an experimental prerequisite. The method has been applied in studies of passive and corrosion layers on various metals [96-102] and of molecular and ionic adsorbates on single crystal surfaces [103]. 

Among the multiple spectroscopic techniques that can provide information about the catalytic active sites under reaction conditions (Raman, IR, UV-vis, X-ray absorption (EXAFS (extended X-ray absorption fine structure)/XANES (X-ray absorption near edge structures)), nuclear magnetic resonance (NMR), electron sprin resonance (ESR), etc.), Raman spectroscopy is the technique of choice because of... 

Despite the presence of organic counterparts and therefore higher phonon energies, Er emission may be observed in OIH as well. As an example, Er -doped diureasils were obtained from different carboxylic acid solvolysis routes [44]. Characterization techniques included X-ray absorption (EXAFS) at the Er Lm-edge, FTIR, and photoluminescence. Judd-Ofelt intensity parameters (Q ) were evaluated from absorption measurements together with the Er Iis/2 radiative lifetime (trad) and the peak emission cross section (cem)- Values so... 

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