57Fe Mossbauer parameters

The main objective in carrying out LIESST experiments on (bt, S) was to elucidate the nature of excitations (metastable pairs) which appear at low temperatures after light irradiation of a ground [LS-LS] state. As is illustrated in Fig. 11a, at 4.2 K before irradiation the Mossbauer spectrum of a sample, which was enriched with 20% of 57Fe, reflects the presence of mainly LS species. The Mossbauer parameters obtained from the fitting of the spectrum are dLs=0.357(l) mms, AEq(ls)=0.452(2) mms. After irradiation of the sample for one hour (2=514 nm) at 4.2 K, the Mossbauer spectrum of (bt, S) shows a decrease in the intensity of the LS species (62.0%) in favour of an increase of the HS species (38.0%) (Fig. lib). Time-dependent measurements revealed the decay of the HS component (Fig. 11c, d), which... 

The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32, 87, 88). In the latter technique, the spectrum of a compound with well-established Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. 

Mossbauer spectrometry is a powerful means for the elucidation of the state of iron in materials [44,138-142,145]. Figure 4.63 [44] shows the 57Fe Mossbauer spectra of the natural zeolite rocks, such as MP, C2, Cl, and C4 (see Table 4.1). In Table 4.12, the Mossbauer parameters calculated with the help of the numerical resolution of the spectra presented in Figure 4.63 are reported. That is, with the help of the recorded spectra, the accurate peak positions, integrated intensities, as well as the FWHM of each peak were calculated. This calculation was carried out by fitting the spectra with three quadrupole doublets one for site 1, another for site 2, and a last one for site 3 [44], The peaks were simulated with Gaussian functions and the fitting process for the numerical resolution of the spectra was carried was carried out with a peak separation and analysis software, developed for this purpose [44,145] based on a least square procedure [48],... 

Combined EPR and 57Fe Mossbauer spectroscopic studies of the MoFe protein in various overall oxidation states39 40,42,44 48,49) have provided strong evidence for the presence of six metal clusters two M centers that are the protein-bound form of the FeMo-cofactor (a novel Mo-Fe-S cluster) and four unusual tetranuclear iron clusters referred to as the P clusters. These will be discussed separately below. In addition, Mossbauer spectra of MoFe proteins from Azotobacter vinelandii, Clostridium pas-teurianum, and Klebsiella pneumoniae all show an additional component termed S39 40 42, which accounts for 6% of the total Fe present (2 Fe per molecule) and which has Mossbauer parameters (AEq = 1.35 mm/s d = 0.60 mm/s) different from those expected for likely impurities such as high-spin Fe2+ or Fe3+ ( adventitious iron). At present, it is difficult to decide whether species S is an unusual and persistent impurity or an integral part of the MoFe protein. 

Mossbauer effect Resonance absorption of gamma radiation by specific nuclei arranged in a crystal lattice in such a way that the recoil momentum is shared by many atoms. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is 57Fe. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine cou-... 

Table 4.2 Summary of Mossbauer Parameters for Reduced Pt, Ir, and Ptlr Catalysts Containing 57Fe (41)... 

Table 1. Parameters of 57Fe Mossbauer spectra recorded form the Fe2O3-In2O3 composites and individual iron oxides annealed at 300 °C.

Recently, the stochastic models for the Mossbauer line shape problem have been discussed by several investigators.20 Such models can be treated in a systematic way as we have described in the above. For example, in a 57Fe nucleus, the spin in the excited state is / = and that in the ground state is / = i, so that the Hamiltonian is a 6 x 6 matrix. If a two-state-jump model is adopted, the dimension of the matrix equation, Eq. (63), is 6 x 2 = 12. If the stochastic operator is of the type (26), then the equation is a set of six differential equations. These equations can be solved, if necessary, by computers to yield the line shape functions for various values of parameters. 

Ri. The value of R, will then be compared to the experimentally determined value of Rt for the 57Fe resonance. The result of this comparison taken together with other information will lead to better estimates of R, which will be called R . Finally, the values of R, (our best estimate of Rf) for a particular isotope will be used to decide if that isotope may be of interest in the studies of catalytic and chemical phenomena. It should be noted here that the analysis of the Mossbauer isotopes in terms of the ratio R, provides a simple physical feeling" for the associated nuclear parameters. The treatment in this section is based on nuclear parameters available from a variety of sources (/- 7, 30,85). However, these parameters are not available in a form readily usable for chemists. 

Fig. 9. Mossbauer data for a methanol solution of 57Fe bound to enterobactin in an applied field of 1.3 kOe at 4.2, 10.2, 18.9 K. The solid curves are calculated from the electronic model with rhombic symmetry and the parameters given in Table 4. The contributions from the three Kramers doublets are given at the top in descending order of energy...

A rather complete Mossbauer study of the 57Fe complexed by deferoxamine has been done by Bock and Lang (38) and their results are in excellent agreement with a model spin Hamiltonian in which the coupling constants have the values D=0.5 cm-1, A = 77/3 =0.46, Aex = 1.474 mm/sec, P =—0.30 mm/sec (or AEq =0.77 mm/sec in absence of magnetic interactions). The asymmetry parameter A = /3> 1/3 merely means that the principal distortion axis is in the y-direction instead of the z-direction for the axis system assumed for the spin Hamiltonian. We will use an axis system in which 0 < X < 1 and D >0 so that we can compare with other molecules. Some of the data and the fitted curves show the typical features associated with these iron transport compounds. 

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