Spectroscopy EXAFS

EXAFS spectroscopy was already discussed for metal carbonyl dusters in Section 4.2.2.S. Since metal dusters are simpler in composition than metal carbonyl dusters, the EXAFS method [72, 73, 137] can provide more information about the former than about the latter. In particular, information about the structure of the 

Similar data are listed in Tkble 4-9 for dusters formed from [Ire(CO)i(] in the NaY zeolite. The first shell data indicate an Ir-Ir coordination number of 3.6 ( 20%), which is consistent with the presence of Ir octahedra, but does not rule out other structures in addition to these. It was suggested that the Ir dusters are stabilized by the rigid environment of the zeolite cages so that migration and entering occur less rapidly than on a support with larger pores. 

The initial application of EXAFS spectroscopy to thiolate-hgated heme systems was a combined study of iron porphyrin model complexes, ferric cytochrome P-450, and ferric chloroperoxidase by Cramer et al. [109]. This study reported the development of methods of EXAFS spectral analysis for iron porphyrin model complexes. Analysis of the EXAFS spectral data for a variety of structurally defined model iron porphyrins demonstrated that a fit of the EXAFS spectra using 3 waves (Fe-Np, Fe-Ca, and Fe-X J led to determination of axial Fe-X distances with an accuracy of better than 0.025 A. Fe-Np bond distances were determined with even greater accuracy. 

The procedure used for the model complexes was then extended to the proteins. For low-spin ferric cytochrome P-450, state 1, the results were consistent with an Fe-S bond distance of 2.19 + 0.03 A [109]. The results for high-spin ferric chloroperoxidase, state 2, indicated an Fe-S bond distance of 2.30+0.03 A. An acceptable fit to the EXAFS spectra of ferric P-450 and chloroperoxidase could not be obtained without inclusion of a sulfur atom. Thus, this report was the first direct observation of sulfur donor ligation to the heme iron in each of these two proteins. EXAFS was also the first technique to yield quantitative information about Fe-S bond lengths in these proteins. The Fe-S distance determined for native high-spin chloroperoxidase is 0.024 A shorter than the known Fe-S bond distance in the high-spin model complex, Fe -(PPIXDME) (SC6H4-PNO2) [110]. 

EXAFS data were obtained by curve fitting. Abbreviations TPP, tetraphenylporphyrin PPIXDME, protoporphyrin IX dimethyl ester. OEP, octaethylporphyrin SPr, n-propanethiolat SEt, ethanethiolate PrSH, n-propanethiol THT, tetrathiophene MeSSMe, dimethyldisulfide TpPP, meso-tetrakis (a,o,a,a-o-pivalamidophenyl)porphyrin  

Analysis of low temperature data using parameters derived from the study of model complexes at room temperature may result in high N values due to Debye-Waller effects (see Ref. 116) Fe-0(dioxygen)= 1.77 A N(0 )= 1.3  

An additional observation reported in the initial EXAFS study was that the trough at 9 A could be used as an indicator of the geometry of the heme system [109], This spectral feature is dominated by the interference between the Fe-Np and Fe-C components, and is relatively insensitive to Fe-X. For porphyrins having the iron in-plane (P-450 state 1 in this study), this trough was observed at fe 9 A . However, for systems in which the iron is out-of-plane (chloroperoxidase state 2), it was seen at fe 9 A . This correlation has also been seen in more recent studies (see Figs. 13, 15 and 16). 

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Figure 8.39 Fourier transformed Fe extended X-ray absorption fine structure (EXAFS) and retransformation, after applying a 0.9-3.5 A filter window, of (a) a rubredoxin, (b) a plant ferredoxin and (c) a bacterial ferredoxin, whose structures are also shown. (Reproduced, with permission, Ifom Teo, B. K. and Joy, D. C. (Eds), EXAFS Spectroscopy, p. 15, Plenum, New York, 1981)...

The trimetaUic uranyl cluster (U02)3(C03) 3 has been the subject of a good deal of study, including nmr spectroscopy (179—182) solution x-ray diffraction (182), potentiometric titration (177,183,184), single crystal x-ray diffraction (180), and exafs spectroscopy in both the soHd and solution states (180). The data in this area have consistendy led to the proposal and verification of a trimeric (U02)3(C03) 3 cluster (181,182,185). 

EXAFS Spectroscopy Techniques and Applications. (B. K. Teo and D. C. Joy, eds.) Plenum, New York, 1981. Contains historical items and treatments of EXELFS, the electron-scattering counterpart of EXAFS. 

EXAFS spectroscopy was introduced as a further tool for the in situ investigation of electrode coatings... 

Allen GC, Warren KD (1974) The Electronic Spectra of the Hexafluoro Complexes of the Second and Third Transition Series. 19 105-165 Alonso JA, Baibas LC (1993) Hardness of Metallic Clusters. 80 229-258 Alonso JA, Baibas LC (1987) Simple Density Functional Theory of the Electronegativity and Other Related Properties of Atoms and Ions. 66 41-78 Andersson LA, Dawson JH (1991) EXAFS Spectroscopy of Heme-Containing Oxygenases and Peroxidases. 74 1-40 Antanaitis BC, see Doi K (1988) 70 1-26... 

The two subunits of CODH/ACS have been dissociated to offer a clearer picture of the ACS active site 135). The holoenzyme contains 2 Ni, 12 Fe, and 14 S 120) that are organized into 3 discrete clusters, whereas the isolated a subunit contains only 1 Ni and 4 Fe and has spectroscopic properties similar to those of Cluster A in the native enzyme 186). Based on EXAFS spectroscopy of the a. subunit, the Ni site in Cluster A has been proposed to be coordinated to 2 sulfur ligands at 2.19 A and 2 nitrogen or oxygen ligands at 1.89 A in a distorted square plane 186). 

The goal of this work was to prepare and characterize PtRu/MgO catalysts from cluster A which contained Pt-Ru bonds and compare with that prepared from a mixed solution of Pt(acac)2 and Ru(acac)3. The characterization methods included IR and EXAFS spectroscopy. Ethylene hydrogenation was used to test the catalytic activity of both PtRu/MgO catalysts. 

Intermediates were also observed in the synthesis of a neutral cluster, Ir4(CO)i2, from Ir(CO)2(acac) in the cages of zeohte NaY these were characterized by IR and extended X-ray absorption fine structure (EXAFS) spectroscopies, the latter being a technique ideally suited to investigation of small, highly dispersed species present in small amoimts in sohds. The spectra indicated dimeric intermediates, possibly Ir2(CO)8 [ 16], when the reaction was carried out in the near absence of water in the zeohte in contrast, the reaction in the dehydrated zeolite was faster, and no evidence of intermediates was observed [16]. 

Good images indicating nearly uniform clusters of other metals are lacking, but evidence from EXAFS spectroscopy, combined with IR spectroscopy and extraction of clusters into solution, has provided a basis for structure determination of a number of small metal carbonyl clusters and clusters formed by their decarbonylation. Compilations of these are reported elsewhere [6,12,26]. 

For example, clusters identified by IR spectra and extraction as Ir4(CO)i2 on y-Al203 were found by EXAFS spectroscopy to have an Ir-Ir coordination number of nearly 3, consistent with the tetrahedral structure of the metal frame EXAFS spectroscopy produces the equivalent result for sohd Ir4(CO)i2 [27]. EXAFS spectroscopy is the most appropriate method for determination of framework structures of supported clusters, but it is limited by the errors to clusters with at most about six metal atoms. Thus, it has been used to determine frameworks that are triangular (EXAFS first-shell metal-metal coordination number of 2), tetrahedral (EXAFS first-shell metal-metal coordination number of 3), and octahedral (EXAFS first-shell metal-metal... 

Structural information about the metal-support interface is provided by EXAFS spectroscopy. The EXAFS data provide average structural information and are most informative when the samples are most nearly uniform. 

The longer metal-oxygen distances of about 2.6 A observed by EXAFS spectroscopy for these and related supported metal clusters suggest weak interactions between the metal and surface oxygen atoms these EXAFS contributions are not determined with as much confidence as those characterized by the shorter distances, and the interactions are not well understood. 

Fig. 3 Ir4 cluster supported at the six-ring of zeolite NaX as represented by density functional theory samples were characterized by Extended X-ray absorption fine structure (EXAFS) spectroscopy and other techniques [32]...

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

Metal-metal distances in supported metal clusters (e.g., It4, nearly 2.70 A) determined by EXAFS spectroscopy essentially match those in coordinatively saturated clusters of the same metal (e.g., Ir4(CO)i2). These distances are about O.2-O.3A greater than the metal-metal distances in the free (gas-phase) clusters (e.g., It4, 2.44 A) [32]. Similar results have been determined for supported OS5C [33] and Rhe [28,29]. 

Elder, R.C., Tepperman, K.G., Eidsness, M.K., Heeg, M.J., Shaw, C.F. Ill and Schaeffer, N. A. (1983) Gold-based antiarthritic drugs and metabolites. Extended x-ray adsorption fine structure (EXAFS) spectroscopy and x-ray absorption near edge spectroscopy (XANES). ACS Symposium Series, 209, 385-400. 

To be more confident on alloy formation, we performed EDS compositional analysis with an electron probe size of 1 nm FWHM focused on isolated clusters as expected for bimetallic clusters, Au and Fe signals are together present in the spectrum. The measured Au/Fe atomic ratio on the largest clusters is about 1.6 0.2, indicating an Au-enriched alloy. Moreover also preliminary EXAFS spectroscopy on the sample evidenced a clear Au-Fe correlation, unambiguously confirming alloy... 

Spin-state transitions have been studied by the application of numerous physical techniques such as the measurement of magnetic susceptibility, optical and vibrational spectroscopy, the Fe-Mbssbauer effect, EPR, NMR, and EXAFS spectroscopy, the measurement of heat capacity, and others. Most of these studies have been adequately reviewed. The somewhat older surveys [3, 19] cover the complete field of spin-state transitions. Several more recent review articles [20, 21, 22, 23, 24, 25] have been devoted exclusively to spin-state transitions in compounds of iron(II). Two reviews [26, 27] have considered inter alia the available theoretical models of spin-state transitions. Of particular interest is the determination of the X-ray crystal structures of spin transition compounds at two or more temperatures thus approaching the structures of the pure HS and LS electronic isomers. A recent survey [6] concentrates particularly on these studies. 

Andersson, L. A., Dawson, J. H. EXAFS Spectroscopy of Heme-Containing Oxygenases and Peroxidases. Vol. 74, pp. 1-40. 

These studies provide the first advanced EPR studies of a non-Kramers doublet spin system, and they suggest the possibility of investigating similar signals from other proteins in this family. EXAFS spectroscopy does not exhibit Fe- -Fe backscattering in Hred (38). This observation could reflect thermal motion, or it may suggest that, as a consequence of the reduction of Hox to Hred, the Fe-Fe distance has... 

Applications EXAFS spectroscopy is obviously well suited to speciate and quantify the state of heavy metals, e.g. in soils [310]. Similarly, it allows differentiation of ZnO and Zn stearate on the basis of the X-ray absorption structure of zinc. 

Manceau A, Boisset M-C, Sarret G, et al. 1996. Direct determination of lead speciation in contaminated soils by EXAFS spectroscopy. Environ Science Technology 30(5) 1540-1552. 

No single development has influenced the field of EXAFS spectroscopy more than the development of synchrotron radiation sources, particularly those based on electron (or positron) storage rings. These provide a continuum of photon energies at intensities that can be from 103 to 106 higher than those obtained with X-ray tubes,... 

Figure 9. Data reduction and data analysis in EXAFS spectroscopy. (A) EXAFS spectrum x(k) versus k after background removal. (B) The solid curve is the weighted EXAFS spectrum k3x(k) versus k (after multiplying (k) by k3). The dashed curve represents an attempt to fit the data with a two-distance model by the curve-fitting (CF) technique. (C) Fourier transformation (FT) of the weighted EXAFS spectrum in momentum (k) space into the radial distribution function p3(r ) versus r in distance space. The dashed curve is the window function used to filter the major peak in Fourier filtering (FF). (D) Fourier-filtered EXAFS spectrum k3x (k) versus k (solid curve) of the major peak in (C) after back-transforming into k space. The dashed curve attempts to fit the filtered data with a single-distance model. (From Ref. 25, with permission.)...

RhCl(NH3)5]Cl2 exchanged with NaX form a highly active catalyst (RhA) for MeOH carbonylation when used with an organic iodide promoter. Systems prepared from RhCl3 are far less active. EXAFS spectroscopy from the Rh K-edge was used to follow the fate of the Rh... 

Determination of the structures of Grignard reagents continues to be of interest, and reviews on this subject have appeared.126,127 Most of the structure authentications are done on crystalline materials, although solution studies performed with extended X-ray absorption fine structure (EXAFS) spectroscopy are also available. The Grignard compounds MeMgBr and EtMgBr in BunzO were studied at room temperature and —85 °C with EXAFS. At both temperatures, dimers are observed (Mg-(/r-Br) = 2.5 A Mg-O = 2.0 A).128... 

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