57Fe isotope

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. 

Mossbauer Measurements. Co-Mo catalysts cannot be studied directly in absorption experiments since neither cobalt nor molybdenum has suitable Mossbauer isotopes. However, by doping with 57Co the catalysts can be studied by carrying out Mossbauer emission spectroscopy (MES) experiments. In this case information about the cobalt atoms is obtained by studying the 57Fe atoms produced by the decay of 57Co. The possibilities and limitations on the use of the MES technique for the study of Co-Mo catalysts have recently been discussed (8., 25.). 

Fig. 14. EPR spectra of carbon monoxide oxidoreductase from C. thermoaceticum, treated with CO plus coenzyme A. Solid lines are experimental spectra, dashed lines are computer simulations, with gx= gf = 2.074, g2 = 2.028. Substitutions with 61Ni and 57Fe were made by growth of the organism on the appropriate isotopes, (a) Effect of substitution with 6lNi. Simulation assumes AM = 3 MHz, A, = 20 MHz. (b, p. 328) Effects of substitution with 5,Fe and l3C. The simulation of the 57Fe spectrum assumes one iron atom with Ah = 40 MHz, A = 60 MHz, and two iron atoms with A, = 20 MHz, A = 30 MHz. The simulation of the l3C spectrum assumes An = 26 MHz, A = 13 MHz. Spectra provided by courtesy of Dr. S. G, Ragsdale.

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. 

In Appendix I are collected values of R/ for those Mossbauer isotopes where estimates of A could be found (55). For 57Fe (14.41 keV), R/ is equal to 20 experimentally the range of isomer shifts for this isotope is approximately 2 mm sec"1 and the observed width of the resonance is equal to 0.19 mm sec" h Thus, the experimental value of Rt is of the order of 10, consistent with the above estimate of R,. For the heavier isotopes the previous estimate of i (0) 2 [ is about an order of magnitude too small (85) and, therefore, the value of R/ should be increased by a factor of ten for these isotopes. 

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. 

Impulseless resonance absorption of y-quants (gamma radiation) from a radioactive isotope, here Cobalt 57Co 57Fe + y (main quant 122 keV quant used for spectroscopy has a different energy)... 

As mentioned above, 57Fe is the most important isotope that exhibits the Mossbauer effect and Mossbauer spectra provide valuable information about the chemical environments of iron nuclei. At the trivial level it is able to provide quantitative discrimination between Fe11 and Fem non-invasively, a valuable technique particularly for unstable samples such as, for example, air-sensitive sediments. Also, because the technique is specific for individual isotopes, it is able to detect and identify small amounts of iron-rich phases in the presence of large quantities of other compounds. A good example here is the case of soil and mineral specimens, where the various oxide and oxyhydroxide species can all be distinguished from one another on the basis of their Mossbauer spectra at different temperatures (see e.g. Goodman, 1994). 

Table 1. Fe-N(ligand) stretching vibrations (Pin cm in [Fe(phen)2(NCS)2l, (Fe(phen)2(NCSe)2 J and (Fe(bipy)2(NCS>2 J at 298 and 100 K from FIR measurements using the 54Fe/57Fe metal isotope technique...

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