Mossbauer parameters, isomer shift

Figure 2A. Schematic diagram of Mossbauer parameters isomer shift (6), quadrupole splitting (AEq) and magnetic dipole splitting of the nuclear energy states of 57pe leading to various hyperfine splitting in Mossbauer spectra.

Mossbauer parameters isomer shift, 6, and quadrupole splitting, AEq, in units of mm/s. 

Iron-supported-on-MgO catalysts behave in some ways differently from the above catalyst systems. That is, while the catalytic activity of these metallic-iron particles for the atmospheric-pressure ammonia synthesis depends markedly on particle size in the range 1.5-10 nm (206), the Mossbauer parameters (isomer shift, quadrupole splitting, and magnetic hyperfine splitting) are independent of iron particle size in this range (97). This thus rules out an electronic effect in the interpretation of the effect of particle... 

INTRODUCTION MoSSBAUER PARAMETERS Isomer Shift Quadrupole splitting Hyperfine nlagnetic splitting Relative Area INSTRUMENTATION APPLICATIONS... 

The experimental MOssbauer parameters, isomer shift, 5, and nuclear quadrupole splitting, A p, mm s" of organotin-antibiotic derivatives. Tables 12.3-12.5, are characteristic of organotin(lV) derivatives [2,5,72]. The isomer shift, 8, increased within the organotin(IV) antibiotics series from diphenyl to dibutyltin(IV) compounds, and from triphenyl to tributyltin(lV) for the triorganotin(rV) derivatives [2,5,72-74], The small differences could result from small differences in the covalency of the tin-carbon bonds in the complexes. 

Table 1Z3 Experimental Mossbauer parameters, isomer shift, 8, mm s", nuclear quadmpole spUttings mm s , measured at Uquid temperature, calculated nuclear quadmpole splittings, caicd C n-C angles for RjSn(IV) Q antibiotic and RjSnflV) Cl emtibiotic derivatives [emtibiotic amoxidUin, ampiciUin, methicillin and peniciUin] [78-80]...

Table 12.4 Experimental Mossbauer parameters, isomer shift, 6, mm s, nuclear quadrupole splittings A, mms" , measured at liquid temperature, calculated nuclear quadrupole splittings, and C-Sn-C angles for and RjSn(lV)antibiotic derivatives (antibiotic = amoxicUlin, ampicillin and cephalexin) [78,79,81,82]...

The analysis of the Mbssbauer spectra of the three diorganotin(IV) ciprofloxacinate complexes allowed the calculation of the Mossbauer parameters, isomer shifts, 8, and quadrupole splittings, lA I, reported in Table 12.5. Each complex showed the occurrence of a characteristic two doublets spectrum. This indicates that two different tin(fV) environments exist in the organotin polymers. The values of Mossbauer... 

Mossbauer parameters—isomer shift (5, mm s relative to a-Fe) and quadrupole splitting (AEq, mms )—plotted for different forms of Co" in aqueous suspension of live cells of Azospirillum brasilense (strain Sp245) rapidly frozen after (1)2 min and... 

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. 

In Mossbauer spectroscopy, isomer shift, which is one of the important parameters to discuss the hybridization between 5f eiectrons at uranium atoms and electrons at the other atoms, is difficult to observe as mentioned above. However, hyperfine coupling constant at nuclei is a complementary parameter to discuss the hybridization at the uranium site. Typical coupling constants in uranium-based intermetallics are about I50T//xb, to our best knowledge. When coupling constants smaller than 150 TZ/ b te obtained in some compounds, it can be concluded that the nature of the 5f electrons in them is more delocalized than that in typical uranium-based intermetallics because of the expansion of the wave functions of 5f electrons. 

TABLE 26.1 Mossbauer spectral parameters - Isomer Shift (IS, mm/s), Quadrupole Splitting (QS, mm/s), Hyperfine field (HI, kOe) for polycrystalline samples prepared at FSU. Observed linewidth 0.29-0.35 mm/s... 

The spin state of the compounds XFe(R2dtc)2 is 3/2 (64). Mossbauer spectra of ClFe(Et2 tc)2 in solution are almost identical with the spectrum of the six-coordinated Fe(Et2magnetic susceptibility and in the isomer shift and quadrupole splitting parameters suggests a geometrical correspondency in solution, which can be attained by the binding of a solvent molecule to the sixth coordination site of the ClFe(Et2[Pg.98]

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

Similar dependencies and trends are observed for other Mossbauer isotopes, for which more information is found in Chap. 7. It should be pointed out again that the nuclear parameter l RIR is negative for Fe in contrast to many other nuclei. The sign of the isomer shift correlations is inverted for nuclei with A/ // > 0. 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Griesinger et al. [56] recorded Zn Mossbauer spectra with sources of Zn diffused into ZnO, ZnS (both wurtzite and sphalerite), ZnSe, ZnTe, and Cu, and an enriched ZnO absorber. The isomer shifts extracted from their spectra cover a velocity range of 112 pm s and were found to follow linearly the lattice spacing parameter where p and Mav are the host density and average... 

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