Mossbauer spectroscopy suitable nuclei

The study of metals in biological systems requires techniques, some of them highly specific, some limited to specific characteristics 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 with a Mossbauer nucleus available is Fe. The EPR spectroscopic techniques will only be of application if the metal centre has an unpaired electron. In contrast, provided that suitable crystals can be obtained. X-ray diffraction allows the determination of the three-dimensional structure of metalloproteins and their metal centres. 

The " Sn nucleus is suitable for Mossbauer spectroscopy (see Section 2.12) and isomer shift values can be used to distinguish between Sn(II) and Sn(IV) environments. The spectroscopic data may also provide information about the coordination number of the Sn centre. 

The last contribution (Chapter 7) deaHng with the role of Mossbauer spectroscopy in the science of molecular sieves was provided by Lovat V.C. Rees, one of the pioneers in this field. Although Mossbauer spectroscopy is appHcable in zeolite research only to a small extent because of the limited number of suitable Mossbauer nuclei, we are indebted to this technique for valuable knowledge of and a deeper insight into some special groups of zeoHtes and zeolite/guest systems. This is particularly true of molecular sieves, which contain the most important Mossbauer nucleus Fe in their framework and/or extra-framework guests (cations, adsorbates, encapsulated complexes, and so on). 

Mossbauer spectroscopy therefore appears in principle to be suitable for following changes brought about by solvation. However, the practical application is considerably limited by the fact that the Mossbauer effect (recoilless y-ray resonance absorption) appears only in solid substances, where the Mossbauer nucleus is so firmly bound in the crystal lattice that recoilless y-ray emission and absorption are ensured. 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

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