Coordination compounds absorption spectroscopy

The ligation of Ln3+ by DTPA, DTPA-BPA, DTPA-BGLUCA, and DTPA-BEN-GALAA has been derived from Nd3+-C distances as evaluated from Nd3+-induced 13C relaxation rate enhancements [5-8]. The distances obtained are in agreement with those observed in crystal structures of this class of compounds. H NMR studies on various paramagnetic [Ln(DTPA)]2 complexes [9,10] and 13C relaxation rate measurements on the diamagnetic [La(DTPA)] 2 confirm this mode of coordination [11]. A detailed X-ray absorption spectroscopy study of [Gd(DTPA)2-] also showed that the local structure around the Gd3+ ion in aqueous solution is similar to that in crystals [12]. 

An X-Ray absorption spectroscopy study has been carried out on a carbon-supported Co and Co-Mo sulfide catalyst, the latter consisting of a fully sulfided Co-Mo-S (type II) phase. Detailed information on the structure of the cobalt sulfide phase is obtained by comparing the EXAFS and XANES spectra of the catalysts with those of CogS8 and CoS2 reference compounds. It is shown that the cobalt atoms in the Co-Mo-S phase have an octahedral-like sulfur coordination while the cobalt atoms in the sulfided Co/C catalyst have a larger fraction of octahedral cobalt than Co Sg. On the basis of these results, the high HDS activity of a sulfided Co/C catalyst can be understood since it appears that the structure of the cobalt sulfide phase in Co/C is in agreement with that in Co-Mo/C. In the Co-Mo-S phase, approximately one cobalt atom is in contact with one molybdenum atom at a distance of 2.85 A. 

Extended X-ray absorption fine structure (EXAFS) and K edge absorption spectroscopy have been used to study vanadium in coaI7). In contrast to the common belief that vanadyl porphyrins are the major compounds present, these studies demonstrate that in the high vanadium content samples examined, the metal is tetravalent and coordinated to oxygen ligands. Similar investigations have been carried out on the vanadocytes of aci-dians to identify the form of vanadium in tunicate blood cells22 (vide infra). 

Very recently, a photochemical synthesis of an Fe(VI) compound has been reported by the Wieghardt group. The octahedrally coordinated iron in [(Me3Cy-ac)FeN](PF6)2 has a Fe=N triple bond of 1.57 A as shown by X-ray absorption spectroscopy. Mossbauer spectroscopy in a field of 7 T shows that the iron is diamagnetic and that the sign of A g is positive (Figure 16). 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

Li+ salts with conservation of the rotaxanated topography. This was proved when Cu1 was nearly quantitatively reincorporated into the polyrotaxane. An X-ray absorption spectroscopy (XAS) study confirmed that the Cu1 is in a similar environment to model compounds, but that the symmetry of the coordination sphere is reduced due to steric constraints of the polymer matrix.71 Attempts to replace Cu1 with Co11 or Zn11 using the Li+ intermediate, or by electropolymerizing rotaxane 93 with Co11 or Zn11 in the place of Cu1 failed.72... 

X-ray absorption spectroscopy particularly, structural studies by EXAFS and XANES spectroscopy of coordination compounds in the solid state or in solution, glasses... ... 

Lead is not spectroscopically silent. The absorption spectroscopy, photoelectron spectroscopy, and NMR spectroscopy of Pb(II) compounds have all been extensively studied. These techniques not only provide useful insights into the electronic structure and stmctures of Pb(II) compounds, but have also proven useful in the characterization of lead coordination environments in complex samples (e.g., samples of biological or environmental origin). 

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