Iron compounds spectra

The pentanuclear carbido species Ms(CO)lsC (M = Fe, Ru, Os) have been prepared. The iron compound has been known for some considerable time (209), but the ruthenium and osmium complexes were prepared recently by pyrolysis reactions (210). The ruthenium adduct was only isolated in low yield (—1%), while the osmium complex was obtained in higher yield (—40%). The infrared spectrum and mass spectral breakdown pattern indicate a common structure to these compounds. The molecular structure of the iron complex is shown in Fig. 46. 

JJor chemists interested in modem theories of chemical bonding, the most useful data obtainable by the Mossbauer technique are the magnitude and sign of the electric quadrupole field gradient tensor and the magnitude of the shift, 8, (which we prefer to call the chemical isomeric. Cl, shift), of the center of the Mossbauer spectrum relative to some standard absorber. Although a considerable amount of chemical and structural information is potentially available from quadrupole data on iron compounds, relatively little use has been made of such data in the literature, and we will not discuss this parameter here. We will instead restrict ourselves to two main points review of the explanations put forth to explain Cl shift data in iron compounds, and a survey of some of the correlations and generalizations which have been found. 

The Mbssbauer effect involves resonant absorption of y-radiation by nuclei in solid iron oxides. Transitions between the I = Y2 the I = 72 nuclear energy levels induce resonant absorption (Fig. 7.4). A Mbssbauer spectrum is a plot of the transmission of the rays versus the velocity of their source movement of the source ( Co for iron compounds) ensures that the nuclear environments of the absorber and the source will match at certain velocities (i.e. energies) and hence absorption takes place. In the absence of a magnetite field the Mbssbauer spectrum consists of one (if the absorbing atoms are at a site of cubic symmetry) or two (symmetry distorted from cubic) absorption maxima. When a static magnetic field acts on the resonant nuclei, this splits the nuclear spin of the ground state into two and those of the ex-... 

In a recent discussion of the spectra of haem-compounds, one of us (10) suggested that d—d transitions, as well as charge transfer bands, could be responsible for part of the near infra-red absorption, and that n—ji triplets could also be involved. In particular this could be the case for the weaker bands in the mesoporphyrin iron methoxide spectrum. The strongest band in the near infra-red, which is the only one seen clearly in haemoproteins, is very probably largely charge transfer. The... 

For liquid solutions containing iron compounds or proteins, the Mossbauer-Lamb factor goes to zero, and therefore Fe Mossbauer spectroscopy on solutions is only possible in the frozen state. Since the natural abundance of Fe is only 2%, it is advisable to perform Mossbauer studies on solutions with Fe-enriched samples. For protein studies, Fe enrichment is essential. Typical sample volumes can range from 0.25-1.0mL. In some cases, frozen solution studies can be performed down to an Fe concentration of 0.1 mM, but taking a Mossbauer spectrum of such a sample can take weeks depending on the shape of the spectrum and the outcome is uncertain. Therefore, it is strongly recommended that the concentration of Fe be at least 1 mM. Samples with Fe concentrations of, for example, 10 mM can very conveniently be measured in a couple of hours—such samples the Mossbauer spectroscopists like the most. In any case, for... 

One remaining electron-nuclear interaction which is measureable is the isomer shift which is the centroid of the absorption spectrum. This is a measure of the electronic charge density at the 57Fe nucleus, and can be used to group iron compounds according to their oxidation state. The isomer shift varies because of the shielding effects of the valence electrons on the s-electrons at the nucleus. 

Both catalysts exhibit a broader iron 3d band than that of elemental iron. A large fraction of the total valence band intensity arises from these iron 3d states (cross section of Fe3d 4.5 x 10" compared to O 2p with 5 x lO " ). This reflects the presence of covalently bonded iron compounds in the mixed surface of the catalysts. The broad feature at 5.5 eV in the top spectrum arises largely from oxygen, since a similar structure with low intensity at the Fermi edge was found for iron foil exposed to 8 L oxygen at room temperature. The shoulder around 10 eV, indicative of iron oxides, was not observed in the chemisorption experiment. 

A compound C3F5Fe(CO)2(7r-C5H5) is obtained by treating the salt 7T-C5H5Fe(CO)2Na with perfluoroallyl chloride (Section VI, B). The pattern of the F spectrum of the iron compound consists of three multiplets... 

A preliminaiy characterization of a new iron—sulfur protein isolated from Desulfovibrio vulgaris Hildenborough was reported in 1989 124). The protein contained approximately 6 iron and 6 inorganic sulfur atoms per molecule. The FPR spectrum of the dithionite reduced protein exhibited an S = signal similar to what was found for synthetic compounds with a [6Fe-6S] core (prismane core). No other FPR signals were reported at this time, and based on the observed similarity it was suggested that this peculiar iron-sulfur protein contained a [6Fe-6S] cluster. Because it had no known function, the pro-... 

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