Isomer shift references

Fig. 7.68 Ir Mossbauer spectra of Ba2CaIr06 at 4.2 and 65 K (notice the different velocity scale) and of Ir metal (the isomer shift reference) at 4.2 K (from [310])...

Sample Fe state Isomer shift (mm/s) Quadrupol splitting AE(mm/s) Reference... 

A calibration of the popular B3LYP and BP86 density functionals for the prediction of Fe isomer shifts from DFT calculations [16], using a large number of complexes with a wide range of iron oxidation states and a span of about 2 mm s for the isomer shifts, yielded a value for the calibration constant a = —0.3666 mm s a.u. (see Chap. 5). Note the negative sign, which indicates that a positive isomer shift of a certain compound relative to a reference material reveals a lower electron density at the nuclei in that compound as compared to nuclei in the reference material. 

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

Fig. 4.9 Magnetic dipole splitting (nuclear Zeeman effect) in pe and resultant Mossbauer spectrum (schematic). The mean energy of the nuclear states is shifted by the electric monopole interaction which gives rise to the isomer shift 5. Afi. g = Sg/tN and A M,e = refer to the...

Fig. 7.57 Plot of the isomer shift 5 of the 36.2 keV Mdssbauer transition of Os versus Dirac-Fock values for the electron density differences at the Os nuclei in free ion 5d configurations. The numbers of the data points refer to the numbering of the compounds in Table 7.9 (from [258])...

Isomer Shift Derived from NFS (Including a Reference Scatterer)... 

Fig. 9.19 Experimental NFS spectrum of Fe02(SCsHF4) (TPpjvP) at 4.2 K together with stainless steel (SS) at 300 K as reference scatterer, the latter placed 30 cm downstream and outside the cryostat. A lit with the CONUSS program [9,10] yields the value 5 = 0.30 mm s as isomer shift of the porphyrin relative to stainless steel [25]...

Figure 3.26 Cobalt-57 source of 14.41-keV y radiation used in Mossbauer experiments. Isomer shift and quadrupole splitting characteristics are shown at right. (Adapted from Figure 2.26 of reference 3 and Figure 1 of reference 24.)...

The isomer shift, S, is the consequence of the Coulomb interaction between the positively charged nucleus and the negatively charged s-electrons. Since the size of the nucleus in the excited state differs from that in the ground state, the Coulomb interaction energies are different as well. The isomer shift is therefore a measure of the s-electron density at the nucleus, and yields useful information on the oxidation state of the iron in the absorber. Isomer shift values are expressed in velocity units, mm/s, and are usually given with respect to the peak position of a reference such as metallic iron. Table 5.2 lists a few isomer shift values of common iron compounds. 

A 10% w/w highly enriched - FePc/XC-72 dispersion, prepared in exactly the same fashion, was used in the Hossbauer measurements. (The isomer shifts, 5, are referred to the a-Fe standard, and 5, the quadrupole splittings A, and widths T, are all given in mm-s"1, throughout the text.)... 

The second problem in relation to Mossbauer spectroscopy, which is trivial but irksome, is agreeing on a reference point with respect to isomer shifts. Each particular nuclide has several possible chemical environments for the source and therefore several possible reference... 

There has been some discussion of what to use as a reference point for the isomer shift in the case of cobalt-57. Two years ago 12) I discussed the possibility of using sodium nitroprusside, which I had origi-... 

In order to examine in which cases the different Mossbauer parameters can be observed, we will in this section make use of parameters R (which will be defined below) where i = 1,2, and 3 refers to the isomer shift, quad-rupole splitting, and magnetic hyperfine splitting, respectively. 

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.)...

Figure 1. Isomer shift versus quadrupole splitting data for Fe2+ ions in a variety of coordination environments in silicate and oxide minerals. Calibration of each 295K Mossbauer spectrum from which the parameters are derived is based on reference zero velocity at the midpoint of the a-Fe spectrum. The legend to mineral symbols is contained in Table I.

Fig. 17. Some representative Mossbauer spectra of (Fe(2-CH30-phen)3](ClC>4)2 H2O at various temperatures the zero point of the velocity scale refers to the isomer shift of Na2[Fe(CN)sNO] 2 H2O relative to the 57Co/Cu source at 298 K. The doublets A, B and C are assigned to the 1 Aj, 5Aj and 5E states, serially105)...

By combining data for a range of almost completely ionic iodides, it has been possible to calibrate the isomer shift scales in terms of the electron configurations. The closed-shell configuration of the iodide ion is again taken as the reference point, and the following relationships have been derived1,5 ... 

Compound Isomer shift Quadrupole splitting Reference... 

The Mossbauer data for a number of sigma bonded divalent tin compounds are collected in Table V (64, 72, 74, 77, 88, 89). The reader is referred to reference 74 for a more complete list of divalent amides we have listed here only Sn[N(SiMe3 )2]2 for comparison to the isoelectronic alkyl compound. The exceptionally low value for the divalent compound Sn[CH(SiMe3)2]2 is no doubt due to its unusual dimeric structure in the solid state. All the other compounds listed in Table V as divalent structures have isomer shifts above the often-used dividing line of 2.56 mm/sec for tin(II). 

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