Quadrupole Splitting and the Isomer Shift

Spherical symmetry and asymmetry of electric field gradients affecting 

TABLE 3.2 Mossbauer Parameters for Some Biological Species  

Reference 36 provides the Table 3.2 data on iron sulfur clusters found in many bioinorganic species.  

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The data from all of these spectra are compared in Table II, which lists the quadrupole splitting and the isomer shift (relative to iron in palladium) in millimeters per second and the ferric-ferrous ratio (as obtained from the Mossbauer data, by comparing the areas of the two sets of lines). The variations in the quadrupole splitting, ranging from 1.84 to 2.08 mm./sec., are certainly outside the range of experimental error and must be attributed to actual variations in the different tektites studied. The only type of tektite for which we measured many diflFerent samples was the indochinite, and the quadrupole splitting and isomer shift of these samples were the same for all samples within experimental error. Hence, these two parameters would be useful in verifying the classification of different types of tektites. The difficulties involved in... 

The nuclear electric quadrupole splitting and the isomer shift effect have their origins in the interaction of the nucleus with its electrostatic environment. Assuming that the electric potential V(r) due to all charges other than the nuclear charges varies slowly over the nuclear volume, it may be expanded about the origin (at the center of charge of the nucleus) as ... 

The theory of the quadrupole splitting and chemical isomer shift of (see preceding section) may be applied to xenon, and values of Up, the imbalance in the 5p-orbitals, are calculated [120] directly from using... 

Figure 5.7 Relationship between the isomer shift, the quadrupole splitting and the oxidation state of iron.

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

This internal pressure effect may actually be quite general in Mbssbauer effect studies of small particles, as discussed by Schroeer et al. for the recoil-free fraction (156) and the isomer shift (157). In addition, Schroeer (152) has summarized a number of origins for Mossbauer parameters being particle size dependent. Thus, from the above discussion, it seems apparent that a priori particle size determination using the recoil-free fraction, quadrupole splitting, or isomer shift is not possible for an arbitrary catalytic system. However, the "experimental calibration of these parameters, which not only facilitates particle size measurement, may also provide valuable information about the chemical state (e.g., electronic, defect, stress) of the small particles. This point will be illustrated later. 

Three types of correlation between MtSssbauer amd NMR parameters for tin(IV) systems have been proposed namely, between the Isomer shift and the chemical shift of a-protons in organic substituents (70), between the isomer shift and the coupling constants J( Sn-CH3) euxd J( Sn-H) (105), and between the quadrupole splitting and the coupling constant (27). None appezurs to be particuleurly successful. 

It is often the case that it is only through a combination of the isomer shift, the quadrupole splitting and the magnetic splitting that enough information can be obtained to allow us to understand a chemical process or to... 

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