Ferric iron pairs

The small quadrupole splitting in the oxidized protein spectra imply that the electron density around the iron atoms is nearly spherical. A spherical charge density indicates that the iron is an S-state ion, although low-spin ferric atoms can have small quadrupole splittings 158). In addition, the oxidized protein spectra show a single quadrupole pair, which indicates that the environments for the two iron atoms are nearly identical. The isomer shift for this quadrupole pair is most consistent with that of ferric iron, although low-spin ferrous iron cannot be ruled out as possibility by the isomer shift value alone. 

Figure 3.16 Energy level diagram for ferric iron matched to spin-forbidden crystal field transitions within Fe3+ ions, which are portrayed by the polarized absorption spectra of yellow sapphire (adapted from Ferguson Fielding, 1972 Sherman, 1985a). Note that the unassigned band at -17,600 cm-1 represents a paired transition within magnetically coupled Fe3+ ions located in adjacent face-shared octahedra in the corundum structure.

Aqueous speciation. The distribution of dissolved components among free ions, ion pairs, and complexes. For example, dissolved iron in acid mine drainage (AMD) can be present as Fe( q) (free ferrous iron), FeS04(aq> (ion pair), Fef q) (free ferric iron), Fe(OH)(j q), and FeS04(aq) species. These species are present in a single phase, aqueous solution. Aqueous speciation is not uniquely defined but depends on the theoretical formulation of mass action equilibria and activity coefficients, i.e. it is model dependent. Some aqueous speciation can be determined analytically but operational definitions and assumptions are still unavoidable. 

The one-electron reduced cluster, [3Fe-4S]° has a system spin 5 = 2, which is envisioned to be the result of parallel coupling (i.e., through double exchange) a ferric 5 = 5/2 and a ferrous 5 = 2 ion into a delocalized pair with 5 = 9/2, and subsequently coupling this structure antiparallel to the remaining 5 =5/2 iron into an overall system spin 5 = 2 (Papaefthymiou et al. 1987). 

Nessler cylinder 1 pair Ferric ammonium sulphate 1.726 g Sulphuric acid (0. 1 N) 10.0 ml Iron-free citric acid (20% w/v) 2.0 ml Thioglycollic acid 0.1 ml Iron-free ammonia solution 20 ml. 

For iron and tin the magnitude of 8 has been found to be related to the oxidation state of the metal (15, 28) (Table I). In iron complexes, the spin of the 3d electrons of the iron atoms can be paired (low spin) or unpaired (high spin). In low spin ferrous and ferric complexes, 8 and AEq values are similar, but the value of AEq differs greatly for the high spin complexes (Figure 1). Mossbauer spectroscopy has been used to... 

The active site in the aerobic, inactive D. vulgaris enzyme shows no EPR signals. The FTIR spectrum indicates the presence of a bridging CO, in addition to the end-on bound CN/CO pair to each iron atom (Fig. 7.2, trace A). We presently assume that both Fe atoms in the bimetallic site are low-spin ferric and that their S = 1/2 systems are anti-ferromagnetically coupled to a diamagnetic state. 

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