Mossbauer parameters, isomer

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 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. 

A similar situation exists for the isomer shift, and to some extent for the quadrupole interaction. For both interactions, the problem is theoretical in nature that is, for a particular catalytic system, it is not a priori known whether the particle size-dependent isomer shift or quadrupole interaction should be interpreted in terms of a shell model or an internal pressure effect. In the shell model, the Mossbauer parameters of the surface atoms are considered to be different from the corresponding parameters of the bulk atoms, and thus the total spectrum is a sum of these two contributions... 

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. 

The results of this analysis are shown in Table II, after it was found that four peaks provided the most meaningful fit of the data (low y-squared with physically realistic Mossbauer parameters). The outer two peaks form a doublet whose isomer shift is that expected for high-spin Fe2+, and whose quadrupole splitting is temperature dependent, reflecting the change with... 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

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