Sodium nitroprusside isomer shift

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. 

Figure 5.6 Isomer shifts (relative to sodium nitroprusside) as a function of temperature in the Debye model, along with measurements on a-FeOOH (adapted from [11]).

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-... 

FIGURE 2.1 Energy levels, isomer shift, and quadrupole splitting for 57Fe. The Mossbauer spectrum shown was recorded at T— 4.2 K with a sodium nitroprusside absorber, a diamagnetic (.S 0) compound often used for velocity calibrations. The isomer shift, indicated by the vertical dashed line, has the value 5 — —0.18 mm/s relative to Fe metal. 1 mm/s Doppler shift... 

In the older literature isomer shifts are often reported relative to Na2[Fe(CN)5NO] (Table 2) sodium nitroprusside. The values are related by 5np = a-Fe + 0.26 mm s . ... 

This gives 29% population of the n (NO) orbitals, in good agreement with other estimates. Since the 3i/-electron population is effectively only 4-5, and there is a 45-population of about 0-5, the electron density at the nucleus is much higher than usual and confers on sodium nitroprusside its unusually low chemical isomer shift. The electric field gradient and mean square displacement tensors have been completely determined in sodium nitroprusside single-crystal absorbers from the polarisation dependence of the absorption cross-section [37, 38]. The principal axes of these tensors do not coincide. 

Because the shielding of the nucleus by the fully occupied t2g subshell of low-spin iron(II) is less effective than in the case ofhigh-spin iron(II) compounds (seeSection 3.4.2), the charge density at the nucleus caused by the s electrons is increased compared to high-spin iron(II). This leads to isomer shifts ranging from —0.258 mm s at 298 K for sodium nitroprusside Na2[Fe(CN)gNO] 2H2O up to 0.31 mms for [Fe(phen)3] (0104)2 or even 0.46 mm s for several ferrous low-spin heme models and cytochromes. ... 

Note Heff = effective magnetic field IS = isomer shift relative to a-Fe, QS = quadrupole splitting FWHM = absorption line full width at half-maxima S = relative area of component So = areas of components reduced with respect to the six-line spectrum of hematite. IS s have been reduced with respect to sodium nitroprusside. Measurement error IS, QS and FWHM + 0.03 mm/s, S <10 %. 

Mossbauer Absorption Spectroscopy. Spectra were acquired at room temperature in a constant acceleration spectrometer using a Co in Rh source. Isomer shifts are relative to the NBS standard sodium nitroprusside. Magnetic hyperfine fields were calibrated with the 515 kOe field of a-FcjO, at RT. Mossbauer parameters were determined by fitting the collected spectra with reference sub-spectra consisting of Lorentzian-shaped lines using a non-linear iterative minimization routine. 

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