Mdssbauer spectrum iron oxide

Finally, another type of defect one can study is a surface—e.g., the surface of an aluminum oxide catalyst containing iron in the surface layers. Figure 7 shows the Mdssbauer spectrum for " Fe in the surface layers of an aluminum oxide catalyst (6). One sees first of all a quadru-pole splitting which is unusually large for a ferric ion. This is caused by... 

Murad, E. Schwertmann, U. (1984) The influence of crystallinity on the Mdssbauer spectrum of lepidocrocite. Min. Mag. 48 507-511 Murad, E. Schwertmann, U. (1986) Influence of Al substitution and crystal size on the room-temperature Mdssbauer spectrum of hematite. Clays Clay Min. 34 1-6 Murad, E. Schwertmann, U. (1988) Iron oxide mineralogy of some deep-sea ferromanganese crusts. Am. Min. 73 1395-1400 Murad, E. Schwertmann, U. (1993) Temporal stability of a fine-grained magnetite. Clays Clay Min. 41 111-113... 

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). 

Biferrocene 1 (M = Fe) oxidizes in two separate one-electron steps (6, 7) (Ei/2 = 0.31 and 0.64 V), of which only the first is chemically reversible (i.e., the monocation is stable but the dication is not) mild oxidation yields the monocation [l]" (M = Fe) as a mixed-valence Fe(II)Fe(III) species (8-10). Bis(fulvalene)diiron 3 (M = Fe) similarly undergoes two successive one-electron oxidations (the dication is also stable), but in this case the Mdssbauer spectrum of the monocation shows the iron atoms to be equivalent, implying a delocalized electronic structure and an oxidation state of 2.5 for each metal (9,11). The separation of... 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

Mac] Mackenzi, K.J.D., Brown, I.W.M., The Mdssbauer Spectrum and Structure of Iron(III) Aluminium Oxide, FeAlOs , J. Mater. Set Lett., 3, 159-161 (1984) (Crys. Structure, Experimental, 8)... 

Fig. 10. Mdssbauer spectra of reduced (left panels) and oxidized (right panels) PS-I core complex at 80 K (top panels) and 4.2 K (bottom panels), The lower spectrum in each of the left panels was corrected for the unreduced component. The table at top right lists the experimentally determined isomer-shift (IS) and quadrupole-splitting (QS) parameters for oxidized and reduced core complexes at 77 K lower table lists isomer-shift values reported in the literature for other [4Fe 4S] and [2Fe 2S] clusters. Data source Petrouleas, Brand, Parrett and Golbeck (1989) A Mossbauer analysis of the low-potential iron-sulfur center in photosystem I Spectroscopic evidence that Fx is a [4Fe-4S] cluster. Biochemistry 28 8982.

The 4-Fe proteins show a cubane-like iron-sulfur center with an iron atom at each of the four alternate corners of a distorted cube. The oxidized high-potential 4-Fe cluster is paramagnetic, with Seff = 1/2, and displays a nearly axial EPR spectrum. AK -band ENDOR study of such an iron protein isolated from C. vinosum ( Fe enriched) has been reported by Reid In this microwave frequency region the proton ENDOR signals do not overlap with the Fe resonances. The hfs of Fe were found to be nearly isotropic with coupling values of 22.6 and 32.5 MHz. Similar values have been found by Anderson et al. / from Fe- Fe difference spectra. Since only two different A " tensors were observed, the iron atoms were classified in two pairs of equivalent nuclei. Mdssbauer data show that the two hf couplings are of opposite sign ... 

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