Mossbauer spectroscopy other cases

Apart from fluorescence, several other methods may be used to obtain time-resolved information. In the case of proteins containing an iron atom, Mossbauer spectroscopy allows the determination, in the iron binding site, of not only root-mean-square shifts of atoms but also the times over which such shifts occur. Detailed investigations of myoglobin have yielded relaxation times on the order of 10 8 Proton NMR spectroscopy can be used to... 

Table I shows the various Mossbauer nuclides—i.e., the nuclides where the Mossbauer eflFect has actually been seen. Not all of these are as easy to exploit as the Fe and 9Sn cases referred to above. However, with improved techniques a number of these should prove accessible to the chemist. Representative elements of almost all parts of the periodic table are tractable by these techniques. It seems clear, however, that the methods of Mossbauer spectroscopy are no longer technique-oriented but that this field is becoming a problem-oriented discipline. In other words, the Mossbauer effect is now used successfully in many cases not only to demonstrate the effect or to corroborate physical evidence obtained by other means—NMR, or infrared, or kinetic studies— but also to solve new chemical problems.

The other mechanism is called the Fermi contact interaction and it produces the isotropic splittings observed in solution-phase EPR spectra. Electrons in spherically symmetric atomic orbitals (s orbitals) have finite probability in the nucleus. (Mossbauer spectroscopy is another technique that depends on this fact.) Of course, the strength of interaction will depend on the particular s orbital involved. Orbitals of lower-than-spherical symmetry, such as p or d orbitals, have zero probability at the nucleus. But an unpaired electron in such an orbital can acquire a fractional quantity of s character through hybridization or by polarization of adjacent orbitals (configuration interaction). Some simple cases are described later. 

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

Mesoporous tin-containing analogues of MCM-41 and tin oxide-modified me-soporous SBA-15 have also been characterized by Mossbauer spectroscopy (162,163). In the first case, the results suggest that tin was incorporated in the structure of the silicate and, in the second, they indicated that two types of supported tin species were formed, depending on the tin content. One would correspond to atomically isolated species stabilized in the wall of the pore and susceptible to reduction to Sn under reductive treatment conditions and the other to large oxide clusters distributed in the external pore structure. 

The preparation of polypyrrole doped with sulpho-nated metallocyanine anions and other complex anions gave rise to detailed Mossbauer effect studies in these systems. It should be stressed that Mossbauer spectroscopy of the above mentioned compounds is more complex than in the case of polypyrrole doped with metal halides or metal cyanides. 

It is useful at this juncture to point out that some forms of Fe, as well as other transition elements, can exist in diamagnetic forms as well as in forms containing an even number of electrons. For the first there can be no EPR since there is no magnetic moment. For the latter situation, exemplified by high-spin Fe(II) and Mn(III), EPR may be observable using special methods, or may not be observable at the conventional frequencies. (For details, see [22].) In the case of Fe, however, one has the option of employing Mossbauer spectroscopy. It is useful in all oxidation states and spin-coupling schemes. 

In Mossbauer spectroscopy, the peak-shape of the transmission spectrum (measured with a thin absorber, see O Fig. 9.22) is described as the convolution of the Lorentzians characteristic of the source and the absorber. According to the addition theorem, the result of such a convolution will be another Lorentzian with a halfWidth equal to the sum of both halfwidths. In other words, in this case, the FWHM is twice of the natural linewidth F. 

For the determination of concentration Ma all the parameters in the above equation have to be known. However, in most cases, the exact values of the Mossbauer-Lamb factor /a and the constant k are unknown. In such cases, calibration graphs obtained with etalons are needed. However, the relative /-factors belonging to different microenvironments are usually known. Consequently, Mossbauer spectroscopy has a unique ability for the determination of the relative concentration of individual Mossbauer species in the same sample. The absolute concentration can be determined more accurately using other techniques. 

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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