Mossbauer spectroscopy structural information from

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

In the expression for the isomer shift, the term [c — nuclear constant, which has been determined either by direct nuclear measurement or by the measurement of the isomer shift for compounds with known electronic structures (57). Ideally then, a measure of 8 provides a determination of 0S(O) 2, the latter related to the electronic structure as expressed, for example, by the occupation numbers of the various orbitals, e.g., 3d74s1 for metallic iron (Fig. 7) (52). In this manner, it is often possible to identify the oxidation state of the Mossbauer atom and to deduce information concerning the bonding of this atom to its surroundings. In some cases, different electronic structures may have similar values of 0S(O) 2 (the low-spin Fe2+ and Fe3+ pair is an example), and electronic structure information from Mossbauer spectroscopy is most... 

Four different material probes were used to characterize the shock-treated and shock-synthesized products. Of these, magnetization provided the most sensitive measure of yield, while x-ray diffraction provided the most explicit structural data. Mossbauer spectroscopy provided direct critical atomic level data, whereas transmission electron microscopy provided key information on shock-modified, but unreacted reactant mixtures. The results of determinations of product yield and identification of product are summarized in Fig. 8.2. What is shown in the figure is the location of pressure, mean-bulk temperature locations at which synthesis experiments were carried out. Beside each point are the measures of product yield as determined from the three probes. The yields vary from 1% to 75 % depending on the shock conditions. From a structural point of view a surprising result is that the product composition is apparently not changed with various shock conditions. The same product is apparently obtained under all conditions only the yield is changed. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The discussion of activity from X-ray data in conjunction with kinetic data is also difficult because, apart from considerations of dynamics, these techniques do not provide the essential knowledge about the energy states of given atoms or groups. It is necessary to inspect the electronic structure of at least certain regions of the protein. Methods exist for this inspection, and these include electron paramagnetic resonance, ultraviolet, circular dichroism, Raman and Mossbauer spectroscopies. The full understanding of activity can only come when the information derived from all available methods is assimilated and rationalized. 

Mossbauer spectroscopy also provides structural information.97-100 In particular it is possible to deduce from peak asymmetry the separate appearance of surface atoms as size is decreased they cause an increase in intensity of quadrupole splitting, and two quadropole components indicate two kinds of surface atoms.97 Au1 and Au111 species are also sometimes recognised,99 and sometimes not.98... 

Mossbauer spectroscopy can also be used to obtain information about fine structure parameters like zero-field splitting D and rhombicity parameter E D. The zero-field splitting is a second-order effect, which can be classically visualized as resulting from the circular currents generated in the atomic shell by the electron spin. In Section 3.10.5, we describe how D and E D can be calculated. 

The spectroscopic techniques, on the other hand, probe individual species which make up the various regions. Infrared (chapter 8) and nuclear magnetic resonance (chapter 7) address themselves to water and the interactions of water with the various species with which it is in contact. Mossbauer spectroscopy (chapter 9), in addition, provides valuable information on the proximity of the cations and their environment. Mechanical (chapter 6) and transport (chapter 4) properties provide more indirect insight into the structural aspects, which is supplemented by thermodynamic studies (chapters 2 and 5) of the interaction between the polymer and water or other liquids. All these techniques are discussed in the present volume, and from these studies several structural models have emerged (chapter 13). 

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