EXAFS, definition

EXAFS, definition, 12-1 to 4 EXELFS, definition, 12-1 to 4 Expansion coefficient... 

The low Ti content (up to 3 wt % in Ti02) makes the extraction of vibrational, energetic, and geometric features specific to Ti04 moieties a difficult task as the experimental data are dominated by the features of the siliceous matrix. This is the reason why the structure of the local environment around Ti(IV) species inside TS-1 was only definitively assessed more than 10 years after the discovery of the material, when the atomic selectivity of X-ray absorption spectroscopies (both XANES and EXAFS) were used [58-60]. 

Figure 7.20 A comparison of the filtered EXAFS data (1.0-2.6 A) for redox-poised samples of hydrogenases. Spectra are separated by redox level, with line types indicating different bacterial sources (see Figure 7.19 caption for linetype definitions). Reprinted with permission from Gu, et o/. (1996) and the American Chemical Society.

One of these, electron transfer, actually occurs in the ideal definitional sense. It applies to the few overworked redox reactions where there is no adsorbed intermediate. The ion in a cathodic transfer is located in the interfacial region and receives an electron (ferric becomes ferrous) without the nucleus of the ion moving. Later (perhaps as much as 10-9 s later), a rearrangement of the hydration sheath completes itself because that for the newly produced ferrous ion in equilibrium differs (in equilibrium) substantially from that for the ferric. Now (even in the electron transfer case) the ion moves, but the definition remains intact because it moves after electron transfer. The amounts of such small movements (changes in the ion-solvent distance for Fe2+ and Fe3+ ions in equilibrium) are now known from EXAFS measurements. 

EXAFS has provided detailed information about the local environment of the active Co and Ni sites and the Mo atoms to which they are attached in terms of the types of atoms within two atomic shells away from the atom being characterized. Cobalt and nickel were shown to be definitely bonded to the surfaces of small MoS2 crystallites. Representative structures for the environments of Mo and Co are illustrated in the following diagram. In such structures, Mo has a coordination number (CN) of 6, with six nearneighbor sulfur atoms, three nearby Mo atoms, and one nearby Co or Ni atom. Co-S configurations were either CN = 5 (square pyramidal) or CN = 6 (octahedral), with either one nearby Mo atom (low HDS activity) or two nearby Mo atoms (high HDS activity) (62). 

A cofactor can be extracted from the iron-molybdenum protein, using Af-methylformamide. This cofactor (called FeMoCo) has many spectroscopic properties in common with the native protein, especially the EXAFS spectrum, and activates the inactive large protein derived from Azobacter vinelandii UW45 mutant which cannot incorporate molybdenum. The cofactor contains no protein or peptide, but does contain molybdenum, iron, and sulfur in atomic ratios of 1 6-8 4-9. It is believed to contain the dinitrogen-binding site (presumably molybdenum) but there is no definitive proof of this. 

Work on supported gold catalysts for CO oxidation was also reported by Overbury et al. (2006) the catalyst was Au/Ti02. Although this EXAFS work does not fit into the strict definition being used in this section, it... 

The available data do not permit an unambiguous definition of either the OEC structure or the mechanism of water oxidation. It is possible, however, to construct a model consistent with both the known chemistry of Mn and the available data and to use this model to make testable predictions concerning OEC structure. The present structural results are consistent with, if not proof of, the dimer-of-dimers model of the OEC (11). Within this model, we attribute the 2.7-A feature to the intradimer Mn-Mn distances and the 3.3-A feature to an interdimer Mn-Mn distance. The EXAFS finding that there are two to three (per 4 Mn) 2.7-A Mn-Mn distances (65) is consistent with the presence of two Mn(/i-0)2Mn dimers. The 3.3-A feature, which disappears on hy-droquinone treatment (see Figure 10), is attributed to an interdimer Mn-Mn distance. This distance is typical of oxo- or oxocarboxylato-... 

The copper chaperone function of Atxl was definitively demonstrated by O Halloran and colleagues in 1997 when they showed using electron paramagnetic resonance (EPR), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS),... 

One of the earliest successful applications of EXAFS to probe a me-talloenzyme was the study of the molybdenum site of nitrogenase. Studies were made on both the C. pasteurianum and A. vinelandii MoFe-proteins and on isolated FeMoco (116). These studies showed definitively that molybdenum is present as part of a polynuclear cluster containing sulfur and iron, with Mo—S and Mo—Fe distances of —2.36 and —2.72 A, respectively. This work inspired the successful development of many chemical systems containing Mo—Fe—S clusters, and XAS studies of these systems strengthened the basis for the interpretation of corresponding data for the natural system. The most accurate picture of the molybdenum site of FeMoco currently available involves a coordination of about three oxygen (or nitrogen), sulfur, and iron atoms at —2.10, —2.37, and —2.70 A, respectively (117). 

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