Mossbauer spectra interpretation

Contents Basic Physical Concepts. - Hyperfine Interactions. - Experimental. - Mathematical Evaluation of Mossbauer Spectra. - Interpretation of Mossbauer Parameters of Iron Compounds. - Mossbauer-Active Transition Metals Other than Iron. - Some Special Applications. 

Although the EFG of a given system can be easily determined from a Mossbauer spectrum, it may be rather difficult to relate it to the electronic structure of the Mossbauer atom. In order to visualize a few typical cases, the computation of the EFG is described in the following for some selected charge distributions. A comprehensive quantum chemical interpretation of the quadrupole sphtting will be given in Chap. 5. 

Figure 7.12 Analysis of the zero-field Mossbauer spectrum at SOK of H2-reduced A. vinosum [NiFe] hydrogenase. (A) Dotted line experimental spectrum solid line theoretical spectrum for a [3Fe-4S] cluster. (B) Dotted spectrum experimental spectrum in A minus the theoretical spectrum or the reduced 3Fe cluster this was expected to represent the spectrum of the two reduced [4Fe-4S] clusters. Solid line theoretical spectrum for two reduced 4Fe clusters. The additional absorption in the experimental spectrum (arrows) comprised a doublet with a surprisingly small isomer shift (0.05-0.15 mm/s) and accounted for about 8 per cent of the total absorption, i.e. one Fe out of twelve (adapted from Surerus et al. 1994).The present interpretation is that this doublet represents the Fe atom in the Ni-Fe site.

If some iron(III) complexes undergo rapid spin interconversion on the Mossbauer time scale, and some undergo slow interconversion, then it is inevitable that a few will interconvert, at some accessible temperature, at a rate which produces dynamic effects on the Mossbauer spectrum. Such examples have now been found (109, 111). Rate constants have been extracted from these spectra and are necessarily of the order of 106 -107 sec"1. The interpretation of the spectral lineshapes is complex (153, 154), however, and further work will be needed to establish the reliability of the rate data obtained from such spectra. 

The large and positive value of e2qQ (26.4 0.4) and small value of asymmetry parameter rj(0) in Mossbauer spectrum of Ph2Sb(acac)Cl2 have also been interpreted in terms of trans-Ph configuration3635. 

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

Firsova et al. (122) reported that the room temperature Mossbauer spectrum of supported tin molybdate, which had been aged in vacuo at 723 K, showed the presence of tetravalent tin. Only after exposure to oxygen at 473 K did the sample act as an adsorbent for propylene. It then gave a Mossbauer spectrum that showed the reduction of the tetravalent tin to the divalent state. Reduction without exposure to oxygen was achieved at 673 K but supported tin in the absence of molybdenum was not reduced. The results were interpreted in terms of the proposals (123) for the synergistic oxidation-reduction during catalysis. 

More information on this system came from non-destructive Mossbauer measure-ments In cis- and trans-(Co(en)2Cl2)N03 about 10 Co cm were implanted and the targets used as Mossbauer sources. The spectra were compared with emission spectra of the same compounds labelled with Co via chemical synthesis. One must keep in mind that the Mossbauer spectrum contains no direct information on the Co in the matrix but only on the actual Mossbauer nuclide pe arising from the Co -> Fe 3-decay. Because the corresponding compounds cis- and trans--(Fe(en)2Cl2)N03 were not accessible by synthesis, a comparison with the absorption spectra was not possible. The interpretation followed the line ... 

The two principal parameters of a Mossbauer spectrum depend on the interaction of the nucleus with its electronic environment. They can therefore be interpreted in terms of chemical factors. 

The interpretation of a complex Mossbauer spectrum will obviously be simplified if the relative intensities of the various components are known. Once the energy levels of the Zeeman/quadrupole Hamiltonian have been calculated, and the spin quantum numbers for each state assigned (or appropriate linear combinations if the states are mixed), it is possible to calculate the intensities from the theory of the coupling of two angular momentum states [32, 33]. 

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