Mossbauer basics

This contribution Is concerned with the magnetic and Mossbauer characterization of (a) Fe/zeollte (mordenlte) systems, and that of (b) Fe and/or Ru on boron-doped carbon substrates. Some correlations between the characterization and CO hydrogenation parameters will be pointed out. Because of limitations of space, we shall present salient features of these Investigations. At the outset. It would be befitting to present a succinct background on the basic principles of magnetic and Mossbauer characterization. 

Contents Basic Physical Concepts. - Hyperfine Interactions. - Experimental. - Mathematical Evaluation of Mossbauer Spectra. - Interpretation of Mossbauer Parameters of Iron Compounds. - Mossbauer-Active Transition Metals Other than Iron. - Some Special Applications. 

This review will introduce basic techniques for calculating equilibrium and kinetic stable isotope fractionations in molecules, aqueous complexes, and solid phases, with a particular focus on the thermodynamic approach that has been most commonly applied to studies of equilibrium fractionations of well-studied elements (H, C, N, O, and S) (Urey 1947). Less direct methods for calculating equilibrium fractionations will be discussed briefly, including techniques based on Mossbauer spectroscopy (Polyakov 1997 Polyakov and Mineev 2000). 

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 results are misleading simply because the basic set of functions used is inappropriate. The great value of the Mossbauer technique is that once SR/R is established, one then has a rather severe test to apply to theoretical calculations concerning the structure of compounds. If... 

Despite all the information that might be obtained using Mossbauer spectroscopy, some of its limitations naturally discouraged many chemists from using this new technique. Unfamiliarity with the basic principles, the fact that most of the early work was done only on iron and tin, and the lack of commercially available research quality equipment until 1965 were other reasons for the lack of interest. This symposium. The Mossbauer Effect and Its Application in Chemistry, was sponsored by Nuclear Science (formerly Nuclear Science Engineering Corp.), a division of International Chemical Nuclear Corp., with the hope that more chemists would learn how Mossbauer spectroscopy has been and can be used. 

In a recent study, Fernandez-Bertran et al. used mechanochemical reactions to prepare a number of hemin complexes with amino acids such as arginine, histidine, lysine, methionine and tryptophan. The basic amino acids react with the hemin peripheral propionic acid groups, while arginine is also able to form a pentacoordinated complex at the Fe(III) centre. The reactions were followed by IR and Mossbauer spectroscopies [77a]. The solid-state reaction of hemin with KCN, Na2S and various substituted imidazoles has also been investigated [77b]. 

There have been two additional experiments which verified this basic picture of the nuclear hyperfine interaction in hemins. Johnson (78) increased the spin-lattice relaxation time by performing the Mossbauer experiment under field and temperature conditions which provide a large value of H/T. At 1.6 °K and in an applied field of 30 kG, a magnetic hyperfine interaction corresponding to that expected for high spin Fe(III) and for the g-values is measured experimentally. Recently, Lang et al. have found that a portion of hemin chloride dissolved in tetrahydro-furan at 1 mM concentration displays a hyperfine interaction at 4 °K in zero applied magnetic field. Their conclusion is that a portion of the hemin is present in a monomeric form in this solvent, a situation which is not apparent to any extent in water, acetic acid, chloroform, or dimethyl sulfoxide (77) at any concentrations used. 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

We discuss briefly some basic topics in materials physics such as crystallography, lattice vibrations, band structure, x-ray diffraction, dielectric relaxation, nuclear magnetic resonance and Mossbauer effects in this chapter. These topics are an important part of the core of this book. Therefore, an initial analysis of these topics is useful, especially for those readers who do not have a solid background in materials physics, to understand some of the different problems that are examined later in the rest of the book. 

In a basic Mossbauer experiment, the reduction in transmission (9) (Figure 2) or the increase in scattered intensity of radiation (2) (Figure 3) is observed as a function of the relative velocity between a source and an absorber. The full width at half maximum of the resonance curve r is related to the mean life of the radiating state by the uncertainty relation r 2h/r. The depth of the curve, c, is related to /, the magnitude of the recoilless fraction of gamma rays emitted, and hence to the crystalline properties of the solid. Finally, the displacement of the curve from zero relative velocity indicates the energy difference between emitted and absorbed radiation and is proportional to the s-electron... 

Mossbauer spectroscopy may be important and useful when applied to electrodes which contain ferromagnetic components. It is basically an in situ tool which provides valuable information on possible orientation and oxidation states of ferromagnetic species in the electrodes as a function of the electrochemical process and the potential applied. For example, electrodes for oxygen reduction may be highly catalytic when containing macrocycles with transition metal cations such as Fez+, Niz+, Coz+ [89,90], A typical apparatus for this technique is described in Ref. 91. 

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