Isomer shift correlation with quadrupole splitting

The two major parameters of the MOssbauer spectra of tin(IV) compounds—the isomer shift and the quadrupole splitting—can be correlated with the nature and arrangement of the ligands bound to the tin atom. In systems in which the ligands are either all identical... 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

A correlation between the isomer shift and quadrupole splitting is adumbrated. However, more data would be needed to confirm this. Such a correlation might be consistent with the molecular orbital approach and the linear relationships in Equations 3 and 5. The decreased quadru-... 

This hydrogen bonding also impacts Mossbauer spectroscopic properties as shown in Table 3. It should be noted that the use of the B3LYP method has enabled accurate predictions of the Mossbauer quadrupole splittings and isomer shifts in a wide variety of iron proteins and model systems covering all iron spin states and coordination states [8-14,23], with a theory-versus-experiment correlation coefficient R2=0.98 over an experimental range of 8.80 mm/s in 47 systems... 

A more cursory examination of 24 of these tris-(N,N-disubstituted dithiocarbamato)iron(III) complexes showed no significant variation in the chemical isomer shift which was recorded as 0-38 mm s at room temperature [18]. A significant variation was found in the quadrupole splitting despite the distance of the substituent groups from the central iron atom, and correlation with the structural type of ligand was attempted. In general the experimental parameters were similar to those listed in Table 8.2 from other workers, and are therefore not given. 

It is cleaur, however, that these two parauneters measure very different types of bond polarity. The isomer shift is correlated with electronegativity and places the orgamic groups between iodine and bromine. Quadrupole splitting, on the other hauid, can be correlated with Taft a -values amd places the orgauiic groups and the halogens at opposite extremes of the... 

The values of isomer shifts (S= 1.08 and 1.05-1.07 mm s relative to a-Fe) and quadrupole splittings (A q = 3.0-3.1 and 2.3-2.4mms see Table 17.3) obtained for the two doublets allowed those components to be correlated with the two cation-binding sites in the GS active center (sites n2 and n I, respectively [42-44]). As mentioned above, these sites of bacterial GSs have different coordination environments, as well as a correspondingly lower (for site n2) and higher (for site nl) affinity to the cation. The latter difference is in line with the nonuniform distribution of Co" between the spectral components (as the areas of quadrupole doublets I and 2 in each spectrum corresponding to different Co" forms are significantly different see Fig. 17.6). 

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