57Fe Mossbauer spectroscopy technique

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

The oxidation state of the central iron ions in the neutral species [FeL], as well as their monocation and dicationic oxidation products, [Fe(L )]+ and [Fe(L )]2+, are amenable to direct measurement by Mossbauer spectroscopy where the isomer shift gives direct information for the dn electron configuration, provided that the coordination number and the nature of the donor atoms are invariant. Frozen acetonitrile solutions of the above 57Fe enriched species have been investigated by this technique (147). 

The study of metals in biological systems requires techniques, some of them highly specific, some limited to certain aspects of the metal ion in question, some of more general applicability. Thus, Mossbauer spectroscopy in biological systems is restricted to iron-containing systems because the only element available with a Mossbauer nucleus is 57Fe. The EPR spectroscopic techniques will be of application only if the metal centre has an unpaired electron. In contrast, provided that crystals can be obtained, X-ray diffraction allows the determination of the 3-D structure of metalloproteins and their metal centres. 

Freeze-quenching technique in combination with ESR and Mossbauer spectroscopy was used for monitoring intermediates in the reaction of substrate free 57Fe-P450C8Itl with peroxy acetic acid (Schunemann et al., 2000). In such a condition, the oxidant oxidized the enzyme active site iron (III) to iron (VI) and Tyr 96 into tyrosine radical, 90% and 10% from the starting material, respectively. Thus the tyrosine residue may be involved in the catalytic process. 

Mossbauer spectroscopy is of special significance for species containing paramagnetic 57Fe nuclei, which render the NMR technique not as useful. This technique could also be utilized for homo- and heterometallic alkoxides of Mossbauer active nuclei such as ll9Sn and mSb. 

Since its discovery, Mossbauer spectroscopy has grown into a highly sophisticated technology in which precise measurements (10-8 ev) are made and detailed information about the environment of the Mossbauer nucleus can be found. In the case of 57Fe, the Mossbauer technique has seen its greatest development so that an accurate description of the chemical state of an iron ion can be determined which includes the details of the electronic states and their energies, the symmetries of the crystalline or molecular environment and the covalent sharing of the valence electrons. 

Spectroscopic techniques are valuable in studies of the structure of polymers because they give information about the environment of the probe atom on a microscopic scale. Mossbauer spectroscopy is particularly useful because the absorption spectrum is entirely due to one isotope of a single chemical element, most commonly 57Fe. By introducing iron into polymers containing acid groups, it is possible therefore to examine the ionic phase specifically. Its structure can be defined to some extent, and interactions of the cations with their surroundings can be determined. 

Mossbauer Measurements. Co-Mo catalysts cannot be studied directly in absorption experiments since neither cobalt nor molybdenum has suitable Mossbauer isotopes. However, by doping with 57Co the catalysts can be studied by carrying out Mossbauer emission spectroscopy (MES) experiments. In this case information about the cobalt atoms is obtained by studying the 57Fe atoms produced by the decay of 57Co. The possibilities and limitations on the use of the MES technique for the study of Co-Mo catalysts have recently been discussed (8., 25.). 

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