Isomer-shift

The isomer shift, 3, is the consequence of the Coulomb interaction between the positively charged nucleus and the negatively charged s-electrons. Since the size of the nucleus in the excited state differs from that in the ground state, the Coulomb interaction energies are also different. The isomer shift therefore is a measure of the s-electron density at the nucleus, and yields useful information on the oxidation state of the iron in the absorber. Isomer shift values are expressed in velocity units (mm s 1), and are usually given with respect to the peak position of a reference such as metallic iron. Some isomer shift values of common iron compounds are listed in Table 5.2. 

The isomer shift contains a contribution from the thermal motion of the individual atoms in the absorber, the second-order Doppler shift, which makes the isomer shift temperature-dependent  

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Ingalls R, Drickamer H G and De Pasquali G 1967 Isomer shift of iron-57 in transition metals under pressure Phys. Rev. 155 165... 

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

Summary of EPR -Values, Fg-Hyperfine Coupling Constants, Isomer Shifts, and Quadrupole Splittings for Some Representative [Fg3S4] Clusters... 

Mossbauer data also seemed to agree with the proposal of a [6Fe-6S] cluster ill). About two-thirds of the Fe intensity of the reduced protein (i.e., four out of the proposed total of six) had isomer shift... 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

Most valuable chemical information can be extracted from Mbssbauer parameters such as the isomer shift (5), the quadrupole splitting (AEq), the magnetic splitting (AEm), and the asymmetry parameter (n). 

Figure 2A. Schematic diagram of Mossbauer parameters isomer shift (6), quadrupole splitting (AEq) and magnetic dipole splitting of the nuclear energy states of 57pe leading to various hyperfine splitting in Mossbauer spectra.

Figure 8. Saturation magnetization (Gs) and the Mossbauer Isomer shift (6) vs S102/A1203 ratio. The arrows show the ratio =17, at which the acidity is highest.

Sample Fe state Isomer shift (mm/s) Quadrupol splitting AE(mm/s) Reference... 

Table 1. Isomer shifts (IS) and quadmpole splitting (QS) of Au55(PPh3)i2Cl6 and of bulk gold.

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

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