Iron oxidation state

Horne and Axelrod have suggested that this exchange may not be first order with respect to each iron oxidation state at high thiocyanate concentrations. A lower value for the rate coefficient kg and a slightly higher activation energy than that quoted previously have been reported ... 

A calibration of the popular B3LYP and BP86 density functionals for the prediction of Fe isomer shifts from DFT calculations [16], using a large number of complexes with a wide range of iron oxidation states and a span of about 2 mm s for the isomer shifts, yielded a value for the calibration constant a = —0.3666 mm s a.u. (see Chap. 5). Note the negative sign, which indicates that a positive isomer shift of a certain compound relative to a reference material reveals a lower electron density at the nuclei in that compound as compared to nuclei in the reference material. 

Name of State Iron Oxidation States [Fe4S4] Core Oxidation State Total Spin... 

This behavior, as well as complementary observations, can be explained on the basis of the reaction mechanism depicted in Scheme 5.3. The main catalytic cycle involves three successive forms of the enzyme in which the iron porphyrin prosthetic group undergoes changes in the iron oxidation state and the coordination sphere. E is a simple iron(III) complex. Upon reaction with hydrogen peroxide, it is converted into a cation radical oxo complex in which iron has a formal oxidation number of 5. This is then reduced by the reduced form of the cosubstrate, here an osmium(II) complex, to give an oxo complex in which iron has a formal oxidation number of 4. 

Table 5 Compositions and iron oxidation states in cobalt-iron Prussian Blue analogues Na cCo Fe(CN)6-zH20. ...

The Mars Pathfinder rover carried an Alpha Proton X-ray Spectrometer (APXS), and the two Mars Exploration Rovers (MER - Spirit and Opportunity) carried Alpha Particle X-ray Spectrometers (also called APXS, but in this case more precise versions of the Pathfinder instrument, though without the ability to monitor protons for light element analyses). These instruments contained radioactive curium sources (Fig. 13.16) whose decay produced a-particles, which irradiated target rocks and soils. The resulting characteristic X-rays provided measurements of major and minor element abundances. The MER rovers also carried Mossbauer spectrometers, which yielded information on iron oxidation state. 

Even though little or no material is taken from or added to the rock, it can change its bulk composition in the chemical coordinates which have been chosen through oxidation or reduction of iron. As a result of this change a new mineral assemblage can be produced. One must conclude that the problem of iron oxidation state will be omnipresent. 

The isomer shift is generally an excellent marker for the iron oxidation state. The graph of Figure 2.17 shows the correlation between DFT calculated and experimental isomer shifts of the TAML complexes.16 From this graph, we can see that Ferv- and Fem-TAML complexes are nicely grouped. It is also pleasing to see that theoretical isomer shifts correlate well with experimentally determined 8 values, but this shall not concern us here. 

Van Aken, PA., Liebscher, B. and Styrsa, VJ. (1998) Quantitative determination of iron oxidation states in minerals using Fe L -edge electron energy-loss near-edge structure spectroscopy. Phys. Chem. Miner., 25, 323. 

The Fe304/Fe0 System The basic ferrite cycle operating with iron oxidation states between that of magnetite (Fe304) and wustite (FeO) is another promising option for CO2 activation ... 

Four quadruply bonded Cr(II) complexes, [Cr2(formamidinate) (acetate)4 ] (n = 2 4), have been structurally characterised and exhibit Cr-Cr bond lengths in the range 1.8897(5) to 2.012(1) A, with distances dependent on the presence (long) or absence (short) of axial interactions.7 The heterotrinuclear complex [Cr2Fe(dpa)4Cl2] (dpa = 2,2 -dipyridylamide) incorporates a quadruply bonded Cr2 unit into a linear Cr-Cr Fe framework, with electrochemical studies showing a dramatic stabilisation of the higher iron oxidation states.8... 

Stucki, J. W., K. Lee, L. Zhang, and R. A. Larson. 2002. Effects of iron oxidation state on the surface and structural properties of smectites. Pure Appl. Chem. 74 2145-2158. 

Figure 2 Electron transfer series in native and synthetic model clusters of the Fe4S4 center in the Fe-protein. Redox potential versus NHE, Fe Mossbauer isomer shift (S at 4.2 K vs. iron), mean iron oxidation state, and spin state are indicated for each redox state. Syntheses and ligand-substitution reactions are indicated for synthetic clusters...

In general, minerals in sedimentary and meta-morphic rocks contain ferrous iron (Velde, 1985) which is destined to become iron oxide under conditions of weathering. Oxidation under surface conditions has a tendency to produce iron in the ferric state. Most often the process takes iron out of the silicates and puts it into an oxide phase. In the uppermost layers of mature soils, iron oxide and various silicates, usually non-iron-bearing, are produced. In silicates containing iron, the majority is in the ferric state. The extent of the transformation of iron oxidation state is a rough measure of the maturity of the soil. In the extremely weathered soils one finds only ferric iron and aluminum oxides and hydroxides. These soils are typically red. 

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