Extended X-ray absorption fine structure EXAFS measurements

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Kinetic studies have proposed evidence for the formation of heterodinuclear intermediate. In order to study the structure of the intermediate in solution as related to the reaction mechanism, extended X-ray absorption fine structure (EXAFS) measurements have been undertaken for the metal-substitution reaction of mercury(ll) porphyrin complex with copper(II) in an acetate buffer (pH = 5.6). 

Mechanistic studies by Haynes et al. demonstrated that the same catalytic cycle operates for both homogeneous and ionically attached systems [107,108]. Reaction of quaternized poly(4-vinylpyridine-co-styrene-co-divinylbenzene) with [Rh(CO)2I]2 generated the ionically attached Rh(I) complex [Rh(CO)2I2] (Scheme 12). IR spectroscopy revealed v(CO) bands of the supported cis-[Rh(CO)2I2] at frequencies close to those observed for this complex in solution. The structure of the supported complex was also confirmed by extended X-ray absorption fine structure (EXAFS) measurements, which showed that the geometry is very similar to that determined by X-ray crystallography for salts of [Rh(CO)2I2] -... 

Visser et al. [41] studied uranyl extraction with CMPO (carbamoylmeth)4phos-phine oxide) and TBP (tri-n-butylphosphate) into [BMIM][PFe] and [OMIM][Tf2N] (TfjN = bis(trifiuoromethanesulfonyl)imide) firom aqueous acidic media and also showed enhancement for uranyl ion extraction into ILs when compared to dode-cane. Extended X-ray absorption fine structure (EXAFS) measurements (Fig. 3.3-2) and UV/vis spectroscopy (Fig. 3.3-3) showed that uranyl-CMPO complexes in [BMIM][PFe] and [OMIM](Tf2N] are similar, but different firom those in dodecane. 

Extended X-ray absorption fine structure (EXAFS) measurements for Pd-Pt systems prepared either by direct redox or refilling method indicated the presence of platinum in the vicinity of palladium for both preparations [13]. Transmission electron microscopy (TEM) coupled with energy dispersive X-ray (EDX) microanalysis was carried out on Pd or Pt modified by Cu, Ag and Au [35], Pd-Sn [38], Pd-Cu [36], Rh-Ge [41], Pt-Ru-Sn and Pt-Ru-Mo (Fig. 9.5) [76], Pd-Sn-Au and Pt-Sn-Au [46] to collect more information on metallic particles in terms of particle size and composition. The catalysts prepared by redox reactions showed that the parent metal is always associated to the additive, but can be found isolated for supports which are able to adsorb the modifier. 

For NMR characterization, some heteroatoms of coordinated ligands are suitable, for example, (7=1/2, 100%), P (7=1/2, 100%) or (7 = 1/2, 1.108%), and (7 = 1/2, 0.37%) especially after enrichment. The local environment of the metal center can be probed by extended X-ray absorption fine structure (EXAFS) measurements. The incorporated metal ions can be quantitatively measured by atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectrometry (ICP-MS), or neutron activation analysis. 

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