Mossbauer spectrum discussion

As discussed in Sects. 3.1.1-3.1.3, successful acquisition of Mossbauer spectra depends on accurate knowledge of the relative velocity of the source and sample. External vibrations that impart differential velocity components to the source and sample would degrade the quality of the Mossbauer spectrum. This degradation... 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

JJor chemists interested in modem theories of chemical bonding, the most useful data obtainable by the Mossbauer technique are the magnitude and sign of the electric quadrupole field gradient tensor and the magnitude of the shift, 8, (which we prefer to call the chemical isomeric. Cl, shift), of the center of the Mossbauer spectrum relative to some standard absorber. Although a considerable amount of chemical and structural information is potentially available from quadrupole data on iron compounds, relatively little use has been made of such data in the literature, and we will not discuss this parameter here. We will instead restrict ourselves to two main points review of the explanations put forth to explain Cl shift data in iron compounds, and a survey of some of the correlations and generalizations which have been found. 

The Fe(III)-NO complex of NPl is EPR silent (Fig. 3) because it contains an odd-electron (ferriheme) center bound to the odd-electron diatomic NO 24), which creates a FeNO center. The NMR spectrum of NPl Fe(III)-NO is that of a diamagnetic protein 85). However, whether the electron configuration is best described as Fe(II)-NO+ or antiferro-magnetically coupled low-spin Fe(III)-NO- is not completely clear, even though the infrared data 49) discussed earlier (Fig. 7) are consistent with the former electron configuration. Thus, as a prelude to planned detailed studies of the Mossbauer spectra of the nitrophorins and their NO complexes, we have reported the Mossbauer spectrum of the six-coordinate complex of OEPFe(III)-NO 86). 

It should be noted here that in addition to collecting in situ Mossbauer spectra (as described above), it may be advantageous to perform dynamic experiments in the Mossbauer spectroscopy cell, i.e., the simultaneous collection of the Mossbauer spectrum and the measurement of the catalytic reaction rate over the sample. This point has recently been discussed by Dumesic et at. 102a), and simple cells for this purpose have been described elsewhere 102a, 102b). 

Carbonyl and alkene complexes of ruthenium have been previously reviewed. The syntheses of [Ru(CO)(NH3)5]2+ are summarized in Scheme 3. The dependence of vc=0 upon counterion and upon solvent has been discussed. 88 The Mossbauer spectrum of [Ru(CO)(NH3)5 2+ has been reported.27 120 The replacement of HzO by isn in /razw-[Ru(L)(NH3)4(OH2)]2+ has been assessed, the lability increasing in the order L = CO = N2 < isn < py < imid (N bound) az NH3 < OH- < CN-< SO2- < imid (C bound).189 For n acid ligands good correlation between specific rate and k-l/2 values for the ruthenium(II)/(III) couple was observed.189... 

The relative area of the Mossbauer spectrum varies with pressure also primarily through changes in the recoil-free fraction of the absorber (Eqn. 13). The volume dependence of the recoil-free fraction is similar to that of 6sod, since both are related to the value of m (Eqns. 2 and 15). The recoil free fraction therefore increases with pressure, and similar to the temperature variation discussed above, a phase transformation could cause either a discontinuity or a change in slope in the variation of recoil-free fraction with pressure. 

The preparation of europium metasilicate hydrate from solutions of europium bromide and sodium metasilicate has been described.The kinetics and mechanism of the reduction of thiocyanato- and isothiocyanato-penta-amminecobalt(iii) ions by europium(ii) in acid solution have been discussed in terms of europium(ii) attack on the ambidentate bridging ligand at the end remote from the cobalt centre. The differences in the activation enthalpies for the reduction of the complexes were attributed to (a) differences in enthalpy of formation of the precursor complex Eu - X - Co (NH3)5, and (b) ease of stretching of the Co—S or Co—N bond in the precursor complex. The low-temperature Mossbauer spectrum of EUH2 suggested the covalent transfer of electron density. to the metal 6s orbital to be more marked in EuH2 than EuO. ... 

The structure of the ferricenium tetrachloroferrate has been a matter for discussion, since the Mossbauer spectrum consisting of a singlet 544> and the IR data 542> lend support to (41). 

Figure 9-3B shows a Mossbauer spectrum, recorded in the presence of a weak applied magnetic field, of a sample of E. coli whole cells, anaerobically grown, in which FNR has been over-expressed (FNR+). For comparison. Figure 9-3A shows again the quadrupole doublet of the isolated protein, discussed in Section 9.1, shown here to emphasize that this feature represents the dominant contribution in the spectrum of the anaerobic whole cells containing over-expressed FNR. As indicated by the dotted vertical lines, the position of the central doublet in Figure 9-3B coincides with that of the doublet in Figure 9-3A. Analysis of the spectrum of Figure 9-3B suggests that the [4Fe-4S] + cluster of the FNR transcription factor represents about 20% of the total iron in this anaerobic sample.

Molecular rotation in the cavity has been discussed for many compounds using nmr(8,9), esr(lO) and Mossbauer spectroscopy(4,5). A complete description of the Mossbauer spectrum for such a case has been given by Gibb(4) for that of the 3 1 clathrate of thiocarbonyl diamide and bis( -cyclopentadienyl)iron(II). Evidence for conformational isomerism has been presented recently based on the ir spectra of y-cyclopentadienylmetal carbonyl complexes such as CpFe(CO) SiCl Me(ll), ip -MeC H Mn(CO) (P(OMe) )(12) and others(13), and on the Mossbauer... 

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