Mdssbauer studies under

The Mossbauer effect is the emission and resonant absorption of nuclear y-rays studied under conditions such that the nuclei have negligible recoil velocities when y-rays are emitted or absorbed. This is only achieved by working with solid samples in which the nuclei are held rigidly in a crystal lattice. The energy, and thus the frequency of the y-radiation involved, corresponds to the transition between the ground state and the short-lived excited state of the nuclide concerned. Table 2.4 lists properties of several nuclei which can be observed using Mdssbauer spectroscopy. 

The catalyst prepared above was characterized by X-ray diffraction, X-ray photoelectron and Mdssbauer spectroscopic studies. The catalytic activities were evaluated under atmospheric pressure using a conventional gas-flow system with a fixed-bed quartz reactor. The details of the reaction procedure were described elsewhere [13]. The reaction products were analyzed by an on-line gas chromatography. The mass balances for oxygen and carbon beb een the reactants and the products were checked and both were better than 95%. 

Four Fe/fS -like clusters appear to be present in [FeMo] and these have been designated as P-clusters( 5). The P clusters are, however, by no means ordinary Fe S clusters. If Indeed they are Fe S clusters at all. P clusters are conspicuous in UV-VIS and especially, MCD and Mossbauer spectra(6,13). The observed spectra are clearly not conventional. The clusters have a very high spin In their oxidized forms, probably S - 7/2 from recent(28a) EPR studies, and have decidedly inequivalent Fe populations(28). This implies that the putative Fe S/j clusters are highly distorted, presumably by unsymmetrlcal coordination from the protein. Moreover, the P clusters do not appear to behave Identically to each other under many circumstances. There is open disagreement as to the redox behavior of this set(6,2, 22). Furthermore, an additional Mdssbauer signal called S may in fact be part of the P-cluster signal(28). 

The Mossbauer spectra of iron in numerous minerals have been studied, but a few examples will serve to illustrate this technique. In the rockforming silicate minerals, Mdssbauer spectroscopy has been used to study the oxidation state, spin state and coordination of iron, and its distribution between different sites in a structure. Thus, Fig. 2.46 (after Williams et al., 1971) shows the spectrum of an augite [essentially (Ca,Mg,Fe)2Si20J, the structure of which contains two kinds of sixfold-coordinate sites that may be occupied by iron (the Ml and M2 sites). The Mdssbauer spectrum can be fitted to three quadrupole doublets peaks 1 and 1 have parameter characteristic of Fe (in both Ml and M2 sites), peaks A and A have parameters characteristic of Fe in Ml, and peaks C and C of Fe + in the M2 sites. These assignments, based chiefly on comparisons with endmember compositions and related species, also enable estimates of site populations to be made on the basis of the areas under the peaks. Studies of the variation in site populations as a function of composition and thermal treatment have led to important advances in understanding intercrystalline order-disorder equilibria, as pioneered in the work of Virgo and Hafner (1970). 

Optical methods of spectrometer calibration include laser interferometric methods and Moire fringe counting techniques. Since such methods depend on the accurately known wavelength of light (for example, 6328.1983 A for a He-Ne laser at 293 K under standard conditions), they are independent of any assumptions made in the iron foil technique and, thus, are intrinsically more reliable. Moreover, such methods do not require the acquisition of57 Co Mdssbauer sources and reference materials for iron absorber studies and may thus be attractive... 

This protein is a regulator of the events that lead to the switch from aerobic to anaerobic metabolism in E. coli. It is only active in bacterial cells under conditions of oxygen deprivation. Previous studies showed that FNR is a DNA-binding protein, which is active only in the dimeric form with one [4Fe-4S] cluster per monomer. Recent data now demonstrate that its [4Fe-4S] cluster is converted to a [2Fe-2S] cluster upon exposure of the protein to oxygen. A breakthrough Mdssbauer in vivo study seems to support this observation as discussed by Munck and collaborators in Chapter 9. 

Catalytic properties of Y zeolite containing iron species were studied in a separate series of combined catalytic and in-situ Mdssbauer measurements [3]. Even at 720 K the extra-framework iron species could not be reduced with hydrogen to the metallic state. Nevertheless, in presence of hydrogen a significant catalytic conversion of carbon monoxide was observed at 670 K. It could be evidenced that x-iron carbide directly formed from Fe(ll) ions under the experimental conditions of this catalytic reactions was the catalytically active component. 

Another useful physical method is resonance Raman spectroscopy. It is found that if the exciting radiation in a Raman experiment is near an absorbtion feature of the metal ion in the electronic spectrum, then Raman scattering involving bonds in the immediate vicinity of the metal is greatly enhanced. This selectivity for the vicinity of the active site is very useful in bioinorganic studies because the absorbtions from the active site would otherwise be buried under the multitude of absorbtions from the rest of the protein. For iron proteins, Mdssbauer... 

As shall be discussed in sections 4 and 5, the change in electronic structure properties under the application of external pressure gives important information. It may lead to novel phenomena and in addition the volume dependence is a crucial parameter in testing theoretical models. For example, certain intermetallics may be driven from a localized to an itinerant magnetic state with reduced volume. Mdssbauer spectroscopy is well suited for such studies. 

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