Second Shell Coordination Environment

From a structural point of view, the distribution of cations in the second coordination shell of vanadium ions plays an important role to the final properties, for instance 

The comparison of 33LiVP-26 and 33LiVP-48 systems, reported in Fig. 8.7, shows slight differences between the two structures. The V +/Vtot ratio does not effort great changes to the distribution of ions in the second shell. For 33LiVP-48 (Fig. 8.7a), which is characterized by a V +/Vtot around 48 %, it is possible to notice 

Differently, for 33LiVP-26 (Fig. 8.7b and V +A tot 26%) the percentage of ions that surround the other species is higher, as attended due to the higher content in the glass composition. 

Lithium containing systems have a similar Q distribution, as shown in Fig. 8.8, suggesting that the V +/Vtot ratio slightly affects the medium order of phosphorus ions, differently from NaVP systems where the content of V2O5 has strong effects on the glass structures. 

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Figure 8. Calculated spin polarization density map in a Co/Cr—O—Li plane in LiCri/gCoy/gOz from DFT calculations."pj-ie positions of the Cr, Co, Li, and O ions are indicated. Li(l) has a Cr ion as its second cation coordination shell, while Li(2) has a Cr + ion in its first cation coordination shell. The Li spectra of LiCr/ioi-/32 x= 0.05 and 0.1) are shown along with the assignments of resonances corresponding to Li(l) and Li(2) local environments.

Extended X-ray absorption fine structure (EXAFS) studies on the Fe/Mo/S aggregate in nitrogenase have made available structural data that are essential in the design of synthetic analog clusters. Analyses of the Mo K-edge EXAFS of both the Fe-Mo protein and the FeMoco (9) have shown as major features 3-4 sulfur atoms in the first coordination sphere at 2.35 A and 2-3 iron atoms further out from the Mo atom at 2.7 A. The Fe EXAFS of the FeMoco (10,11) shows the average iron environment to consist of 3.4 1.6 S(C1) atoms at 2.25(2) A, 2.3 +0.9 Fe atoms at 2.66(3) A, 0.4 0.1 Mo atoms at 2.76(3) A and 1.2 1.0 0(N) atoms at 1.81(7) A. In the most recent Fe EXAFS study of the FeMoco (11) a second shell of Fe atoms was observed at a distance of 3.75 A. 

Galactose oxidase hinds a single copper ion within Domain 11 on the axis of the wheel. The active site (Fig. 5) is unhke any other biological copper complex, an appropriate distinction for this remarkable enzyme. To explore the site in more detail, the protein environment of the mononuclear copper center may be separated into (A) direcdy coordinated metal hgands (hrst shell, inner sphere interactions) and (B) the extended active site environment (the second shell or outer coordination sphere). 

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]. 

The dielectric continuum models may allow us to predict speciation in aqueous solutions as a function of temperature simply by changing dielectric constant of the polarizing medium. At first glance, this may simply appear to be a return to the Bom-model formalism. However, the inner sphere solvation would be included explicitly. To include temperature effects on the inner solvation shells, we would have to calculate the partition functions of the cluster defining the metal atom and its first and, possibly, second coordination environment. 

The purpose of our work is to show that the local environments of Si, Al, and P in SAPO s can be probed via solid-state NMR and XPS and that their results give a consistent picture of the second coordination shell for all the T-atoms. 

The structural features of the FeMo-cofactor model are generally consistent with the results of analytical and spectroscopic (EXAFS, ENDOR, and Mossbauer) studies of the cofactor. The composition of the nonprotein part of the FeMo-cofactor model, lMo 7Fe 9S l homocitrate, is within the range of values that have been reported (48, 53-55). The absence of protein-bound bridging ligands between the two clusters in FeMo-cofactor is consistent with the ability to extract the intact cofactor from MoFe-protein. EXAFS studies of the Mo environment in both the MoFe-protein and the isolated cofactor indicate that 2-30(N) and 3-5S are directly coordinated to Mo, with 3-4Fe present in the second coordination shell of Mo (104,105) the crystallographic model contains 30(N), 3S, and 3Fe. Studies of Fe EXAFS... 

The redox sensitivity of hemopexin-encapsulated heme to electrolyte composition and pH illustrate the importance of first coordination shell (bis-histidine ligation and heme structure) and second coordination shell (protein structure/folding and environment) effects in these heme proteins. These observations also suggest a possible role for Fe " /Fe redox in hemopexin-mediated heme transport/recycling, as high chloride anion concentration and low pH are known conditions for the endosome where the heme is released. 

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