Isomer shift correlations

Similar dependencies and trends are observed for other Mossbauer isotopes, for which more information is found in Chap. 7. It should be pointed out again that the nuclear parameter l RIR is negative for Fe in contrast to many other nuclei. The sign of the isomer shift correlations is inverted for nuclei with A/ // > 0. 

Au in 19 metals and semiconductors versus gold metal absorber Electron transfer from isomer shifts, correlation between isomer shift and host electronegativities... 

Fig. 8.26 (Left) Isomer-shift correlation diagram for a systematic series of [(Me3cyclam-acetate) FcOO]"" complexes, whereX is an azide (Na ) or a nitrido (=N) group. (Right) Orbital scheme of [(Me3cyclam-acetate)Fe =N] ...

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. 

Mossbauer measurements on Fe-centered zirconium clusters dramatically illustrate the limitations of the oxidation state/isomer shift correlation just discussed. Indeed, the Fe isomer shifts... 

The equivalence of the oxidation state in both protein active sites and tetranuclear synthetic analogues is also clearly established by comparing the isomer shifts Fe observed in the corresponding Mossbauer spectra. Isomer shifts may be indeed considered to be a measure of the electron density in vicinity of the iron atom in a given species relative to that in a reference compound. For iron compounds with equivalent coordination spheres, the isomer shifts correlate with iron formal oxidation states. Isomers shifts observed for both proteins and model compounds are consistent with the hypothesis that synthetic species [Fe4S4(SR)4] are isoelectronic with the active sites in the 4Fe and 8-Fe-Ferredoxins and HP protein. Mossbauer spectra confirm moreover that all Fe-atoms in Fe4S4 units in synthetic and natural species are strictly equivalent in the time-scale of this technique (ca. 10 s) so they are, as mentioned above, electronically delocalized systems. 

FIGURE 3. Oxidation state/spin quantum state-isomer shift correlation diagram for iron in a variety of compounds. The values of S are referred to the a-Fe standard. (Adapted from Figure 3.2 in Ref. 2b.)... 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

A typical example of a correlation diagram for Fe is given in Fig. 4.3. It summarizes the isomer shifts for a great variety of iron complexes with oxidation states (1) to (VI) in the order of the respective high-spin, intermediate-spin, and low-spin configurations. The plot of the corresponding values marked by grey, hatched and open bars demonstrates three major trends ... 

Another example of the correlation between the isomer shift and covalent bonding properties is n-backbonding. The observed isomer shift of ferrous cyanides [Fe(ll)(CN)5X"] " [24] becomes more negative with increasing... 

Given the success of DFT in the calculation of the isomer shift, it seems appropriate to return to the issue of interpretation which factors are controlling the qualitative behavior of the isomer shift in iron compounds Traditionally, one assumes that there is a correlation of the isomer shift and the charge at the iron center as is suggested from the well-known sensitivity of the isomer shift with respect to the oxidation state. However, things turn out to be more subtle than what is perhaps commonly perceived. 

Fig. 5.5 Lack of correlation between the As orbital population and the valence contribution to the isomer shift (taken from [19])...

Fig. 7.31 Partial ligand-field strength for the ligand L in correlation with the isomer shift 5 (relative to Ru metal at 4.2 K) of the nitrosylruthenium (II) compounds [RuL5(NO)]" (L = Br, Cr, NCS, NH3, CN ) (from [124])...

A correlation of isomer shift, electronic configuration, and calculated -electron densities for a number of ruthenium complexes in analogy to the Walker-Wertheim-Jaccarino diagram for iron compounds has been reported by Clausen et al. [ 127]. Also useful is the correlation between isomer shift and electronegativity as communicated by Clausen et al. [128] for ruthenium trihalides where the isomer shift appears to increase with increasing Mulliken electronegativity. 

Van der Woude and Miedema [335] have proposed a model for the interpretation of the isomer shift of Ru, lr, Pt, and Au in transition metal alloys. The proposed isomer shift is that derived from a change in boundary conditions for the atomic (Wigner-Seitz) cell and is correlated with the cell boundary electron density and with the electronegativity of the alloying partner element. It was also suggested that the electron density mismatch at the cell boundaries shared by dissimilar atoms is primarily compensated by s —> electron conversion, in agreement with results of self-consistent band structure calculations. 

Fig. 7.78 Linear relation of the quadmpole splitting A q = ( jl)eqQ (1 + j /3)l/2 and the isomer shift b for aurous (a) and auric (b) compounds. Also included is a correlation with the relative change in electron density at the gold nucleus, Ali/r(o)P, as derived from Dirac-Fock atomic structure calculations for several electron configurations of gold. An approximate scale of the EFG (in the principal axes system) is given on the right-hand ordinate (from [341])...

Dilute gold alloys with Cu, Ag, Ni, Pd, and Pt as absorbers Correlation of isomer shift with residual electrical resistivity, wave function at Fermi level, s--band population of gold... 

Au microcrystals supported in gelatin Correlation of isomer shift with lattice contraction... 

Alloys of Au with Ag, Sn, Ni, Pd, Pt, A1 Qiarge between host and Au atoms from trends of isomer shift and work function data (ESCA) correlated with electronegativity of host elements... 

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