Mossbauer effect isomer shift, correlation with

The Mossbauer parameters of the tin halides show that in these systems the correlation between the isomer shifts and the donicity values is by no means as clear as for antimony pentachloride. The isomer shifts of tin tetrachloride agree, within experimental error, in frozen dimethyl sulphoxide, dimethylformamide and tributyl phosphate solutions, and those of tin tetraiodide agree in dimethyl sulphoxide, dimethylformamide and ethanol solutions. The Gutmann donicities of these solvents lie in the range 20-30. For both tin compounds, a further decrease in the donicity of the solvent causes an increase in the isomer shift, which indicates an increase in the electron density at the tin nucleus. This effect appeared in acetonitrile solution with tin tetrachloride, and in tributyl phosphate and carbon tetrachloride solutions with tin tetraiodide. (It must be emphasized that the last two solvents have low relative permittivities, whereas that of acetonitrile is high.)... 

The charge density at the nucleus is mainly determined by s electrons and only partially by p-elec-trons. The main effect of the p electrons and d electrons and any other electron shells that do not contribute directly to the electron density ip(0) p, is to shield the s electrons. The determination of the scale factor (r -rl) in Equation [9] is called the isomer shift calibration. The interpretation of isomer shifts in Mossbauer spectra involves the correlation of a given A/ (0)=l /(0)l -l /(0)lg value (the electron density difference) with the known electronic structure of the Mossbauer atom or the change of the... 

The electronic density at the nucleus may be separated into contributions due to inner-core filled orbitals and to partially filled valence orbitals, and thus the isomer shift may be expected to be sensitive to the specific nature of the chemical bonds involving the Mossbauer-active species and adjacent atoms in the molecule or lattice. Furthermore, the value of 8 may be modified not only by varying the 5-orbital occupation but also by a shielding effect associated with orbitals of other types. The removal of a d-electron from an iron ion, for instance, will lead to a contraction of the occupied s-orbitals and thus to a shift of 8 in the negative direction. A correlation between the isomer shift and oxidation states, which may be useful in the analysis of certain spectral features associated with iron in molecules and other materials, is shown in Figure 3. 

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