Mossbauer spectroscopy symmetry

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

From the applications of Ni Mossbauer spectroscopy in solid-state research, it is clear that (1) information from isomer shift studies is generally not very reliable because of the smallness of the observed isomer shifts and the necessity of SOD shift corrections which turn out to be difficult, and (2) useful information about magnetic properties and site symmetry is obtained from spectra reflecting magnetic and/or quadrupolar interactions. 

Ga( Zn), Sn, Te( I) Mossbauer spectroscopy, no modifications of the local symmetry of lattice sites, electronic structure of atoms and intensity of electron-phonon interaction are revealed for Pbi Sn Te solid solutions in the gapless state at 80 and 295 K... 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

Although in most of the above reports structural information is based on X-ray data, Au Mossbauer spectroscopy has also been successfully employed for the investigation of two-, three-, and four-coordination in gold(I) complexes.2551 An increase in coordination numbers leads to a decrease in the isomer shift (IS) by 1-2mm s 1 (three-coordination) or 2-4mm s 1 (four-coordination) relative to two-coordination. For the same ligands, the quadrupole splitting (QS) for three-coordinated complexes is expected to be very similar to that for the two-coordinate derivatives, while that for complexes with Td symmetry should be zero. 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

The variety of symmetries in the garnet structure (coordinations 4, 6, and 8) allows considerable compositional range. Table 5.16 lists the elements commonly present in positions X, X and Z. The diadochy of Al, Ti" ", and Fe in the tetrahedral site has been confirmed by Mossbauer spectroscopy on natural Fe-Ti-bearing garnets (Schwartz and Burns, 1978), and the presence of phosphorus in these sites, observed in upper mantle garnet, is attributable, according to Bishop et al. (1976), to coupled substitutions of the type... 

A novel cytochrome b model compound containing two heme binding sites was tested by EPR and Mossbauer spectroscopy. Both sites were found to be occupied in a 1 1 stoichiometry. For one of the sites a normal b-hemichrome EPR signal with rhombic -tensor symmetry was found as expected for a bis-histidine ligation with parallel histidine planes. For the other site unusual EPR features (e.g. axial y-tensor) were obtained which were ascribed to a configuration with perpendicular histidine planes.267 A new type of cytochrome b was described for a preparation from the cytoplasmic fraction of an archaeon, Acidianus ambivalens. This is thus the first soluble cytochrome found in this... 

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. 

Table 1 lists some of the properties of the plant-type iron sulfur-proteins for which extensive study by EPR and Mossbauer spectroscopy has been reported. The physical properties summarized show that the plant-type iron sulfur proteins have molecular weights in the range from 12,000 to 24,000 and have EPR g-values (gx, gy, gz) all of the g = 1.94" type shown in Fig. 6 but with minor variations reflecting axial or nonaxial symmetry of the paramagnetic center. The amino-acid sequences of four plant-type iron-sulfur-proteins are known alfalfa (136), L. glauca (137), Scenedesmus (138), and spinach (139). Each protein has about 97 residues, all in a single peptide chain these are shown in Table 2. 

Magnetic nanoparticles in the sol-gel silica glass were obtained by heat treatment at 1000°C in air during 6 h and identified by X-ray and Mossbauer spectroscopy as y-Fe203 (maghemite) [11], The magnetic parameters of maghemite used in computer simulations of the SPR spectra, are Ms = 370 kA m 1 and = —4.64 kJ m"3 (cubic symmetry) [35],... 

The all-iron(III) nature of 22 and 23 was determined by Mossbauer spectroscopy. In [Fe4(L7)4] (22), four octahedrally coordinated iron centers constitute the apices of a tetrahedron, and the four tripodal, tris(bidentate) ligands (L7)3 are centered above the triangular faces of the tetrahedron [86]. Hence, 22 has nearly / -molecular symmetry, and the crystals are composed as racemic mixtures of homoconfigurational (A,A,A,A)/(A,A,A,A)-/ac stereoisomers. There is no evidence that the cavity of the tetrahedron hosts a guest (Fig. 8, top) [49, 62, 91-99]. 

Application of the Mossbauer effect, which is essentially a bulk phenomenon, to the study of surfaces has received significant attention in recent years. The usefulness of this technique lies in its ability to determine the electronic environment and symmetry of the surface nucleus, and it offers a method of investigation that is clearly complementary to other physical methods for the characterization of solid surfaces. Mossbauer spectroscopy has the attractive advantage that it may be used at a variety of pressures and can be applied to the study of heterogeneous catalysis and adsorption processes to probe the nature of the solid surface and its electronic modification when holding adsorbed species. 

Experimentally, the structures are determined by the use of various physical methods such as vibrational spectroscopy, both in the Raman and IR, NMR, or diffraction methods. Other accepted physical measurements, such as conductivity, cryoscopy, magnetic properties, electronic spectra in the UV and visible ranges, nuclear quadmpole resonance (NQR), and Mossbauer spectroscopy have also been applied. Table 3 summarizes various geometric structures found in halogen compounds, coordination numbers, bonding and nonbonding electronic arrangements, and symmetries. 

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