Vanadium, K-edge

Vanadium K-edge XANES measured in transmission mode at 15 K showed that the attached V complex (2) maintained its square pyramidal conformation with a V=0 bond (Figure 2.2a). Curve-fitting analysis of V K-edge EXAFS Fourier transforms (Figure 2.2b) provided local structure information on the supported V complex (2) with an unsaturated conformation, which differs from that of the V-monomer precursor (1). The EXAFS curve-fitting was performed in the R-space with two shells short V=0 and long V—O bonds. A V=0 bond was observed at 0.157 0.001 nm,... 

Christmann U, Dau H, Haumann M, Kiss E, Liebisch P, Rehder D, Santoni G, Schulzke C (2004) Substrate Binding to Vanadate-Dependent Bromoperoxidase from Ascophyllum nodosum A Vanadium K-edge XAS Approach. Dalton Trans 2534... 

Vanadium K-edge X-ray absorption spectroscopy has provided the most detailed information on the environment of V in the VFe proteins. Data for the VFe proteins of A. chroococcum and A. vinelandii are very similar (33, 34). The edge and near-edge structures of these spectra, which reflect coordination geometry and probable oxidation level of the absorbing atom, are both similar to the complex VFe3S4Cl3[HCON(CH3)2]3. The relatively low intensity of the pre-... 

Fig, 3, Vanadium K-edge and XANES of the VFe protein of A. chroococcum and a VFe3S4 cubane cluster, (a) Dithionite-reduced VFe protein (b) [VFe3S4Cl3(DMF)3). ... 

Structure information on the FeVco of vanadium nitrogenases has been obtained from iron and vanadium K-edge X-ray absorption spectroscopy (XAS) (for background information, see Section 3.6.), in particular so from the extended (EXAFS) region. 

This approach has been applied in a,condensed system for the determination of the V—O distance in a silica glass. The overall shape of the vanadium k-edge spectrum... 

In Fig. 18 we. report the vanadium K-edge XANES spectra of a spontaneously dehydrated sol-gel sample 1.6 H O. This sample is formed of oriented sheets separated by about 10 A of water molecules. Each sheet consists of square plane bipyramids with the base on the xy plane and a short double bond of 1.58 A on one... 

P. Chaurand, J. Rose, V. Briois, M. Salome, O. Proux, V. Nassif, L. Olivi, J. Susini, J.-L. Hazemann, J.-Y. Bottero, New methodological approach for the vanadium K-edge X-ray absorption near-edge structure interpretation application to the speciation of vanadium in oxide phases from steel slag. J. Phys. Chem. B 111(19), 5101-10 (2007)... 

Frank, P., Hodgson, K.O., Kustin, K., and Robinson, W.E. (1998) Vanadium K-edge X-ray absorption spectroscopy reveals species differences wifriin the same ascidian genera. /. Bid. Chem., 279, 24498-24503. 

Frank, P., Carlson, R.M.K., Carlson, E.J., and Hodgson, K.O. (2003a) Medium-dependence of vanadium K-edge X-ray absorption spectra with application to blood cells from phlebobranch tunicates. Coord. Chem. Rev., 237, 31—39. 

Figure 24.1 NEXAFS and AES analysis of VC/V(110). Open circles NEXAFS uptake curve obtained by measuring the fluorescence-yield of carbon K-edge feature at 285.5 eV following the formation of carbide on V(110) at 600 K. Closed circles the carbon/vanadium atomic ratios as a function of carbide formation as derived from the AES C(KLL)/V(LMM) peak-to-...

Fjo. 8. Oxidation state versus energy positions of various absorption features in the V K edge XANES spectra of various vanadium oxides. From Wong era/. (3/2). 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

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). 

Based on the studies of the XANES (X-ray absorption near edge structure) spectra of LiVMoOe and the standard samples (such as V2O5, VO2, M0O3, M0O2, etc) at the V and Mo K-edges, we can determine the valence of the vanadium ion is about 4.5 while that of molybdenum ion is 6. 

Vanadyl porphyrins, VOP, mentioned above [see (5)] have been used as precursors for nonoxo vanadium(rv) porphyrins, [VPX2] (P = OEP, TPP, TMTP, TPTP X = Cl or Br). EXAFS at the Br K-edge has been used to coirfirm the trans configuration of X atoms in the complexes. The bare atom renders the compounds extremely reactive and one theory is that they could act as precursors in the synthesis of complexes formed during coal conversion. ... 

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