Isomer shift interpretation

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. 

The traditional interpretation of the isomer shift in Fe Mossbauer spectra is based on the following assumptions (a) the influence of the 3d electron configuration on the IS occurs via the shielding effect of the 3d electrons on the 3s and 4s electrons, (b) the variations in the 3s shell are dominant, and (c) the influence of the 4s shell occurs via the 4s population. 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

Ir/transition metals Description of a new model (Atomic cell model) for the interpretation of isomer shift values, with electronegativity and cell boundary electron density as parameters... 

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. 

Au and " Sn in various metallic matrices as absorber Interpretation of isomer shift... 

Figure 7.12 Analysis of the zero-field Mossbauer spectrum at SOK of H2-reduced A. vinosum [NiFe] hydrogenase. (A) Dotted line experimental spectrum solid line theoretical spectrum for a [3Fe-4S] cluster. (B) Dotted spectrum experimental spectrum in A minus the theoretical spectrum or the reduced 3Fe cluster this was expected to represent the spectrum of the two reduced [4Fe-4S] clusters. Solid line theoretical spectrum for two reduced 4Fe clusters. The additional absorption in the experimental spectrum (arrows) comprised a doublet with a surprisingly small isomer shift (0.05-0.15 mm/s) and accounted for about 8 per cent of the total absorption, i.e. one Fe out of twelve (adapted from Surerus et al. 1994).The present interpretation is that this doublet represents the Fe atom in the Ni-Fe site.

Mossbauer spectra of a series of Sn(Pc)X2 have been studied. The linear relationship between the isomer shift and the axial ligand electronegativity is interpreted in terms of an electron density increase on the Sn atom and a decrease in n donation from Pc to Sn with an increase in axial tr donation.198 This is supported by X-ray photoelectron spectroscopy. 

A similar situation exists for the isomer shift, and to some extent for the quadrupole interaction. For both interactions, the problem is theoretical in nature that is, for a particular catalytic system, it is not a priori known whether the particle size-dependent isomer shift or quadrupole interaction should be interpreted in terms of a shell model or an internal pressure effect. In the shell model, the Mossbauer parameters of the surface atoms are considered to be different from the corresponding parameters of the bulk atoms, and thus the total spectrum is a sum of these two contributions... 

Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206), the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an electronic effect in the interpretation of the effect of particle... 

Figure 44 IS/QS relationship (IS = isomer shift and QS = quadrupole splitting) for complexes 115a and 115b (red triangles), representative phosphine Au(l) complexes (green squares) and Au(lll) (blue rhombuses) complexes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this chapter.)...

The dependence of the isomer shift (41) and recoil-free fraction (9, 42-48) on particle size has also been suggested, but such relationships may be somewhat tenuous. It is clear that caution must be exercised in the use of methods hitherto described in the interpretation and correlation of microcrystallite size. Recent work has suggested (12) that ferric oxide may react with the support when calcined at high temperatures, e.g. for 2 hr at 500°C. The presence and contribution... 

Tin-119 Mossbauer data have been reported for a number of stannylene complexes and are collected in Table VI. The IS (isomer shift) of the stan-nocene complexes is much lower than that of stannocene, while the QS (quadrupole splitting) is much higher. This has been interpreted as indicating synergistic cr- plus 7r-bonding of the stannylene to the transition metal (52). 

---