Chemical Information from Isomer Shifts

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 

7t-acceptor strengths of the ligands, and other parameters. Therefore, the study of a single compound may not be very informative unless data from similar compounds are available for comparison. However, systematic series of related compounds may yield close and revealing correlations of the isomer shift with one or more features of the electronic structure. 

A typical example of a correlation diagram for Fe is given in Fig. 4.3. It summarizes the isomer shifts for a great variety of iron complexes with oxidation states (1) to (VI) in the order of the respective high-spin, intermediate-spin, and low-spin configurations. The plot of the corresponding values marked by grey, hatched and open bars demonstrates three major trends  

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. 

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Most valuable chemical information can be extracted from Mbssbauer parameters such as the isomer shift (5), the quadrupole splitting (AEq), the magnetic splitting (AEm), and the asymmetry parameter (n). 

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. 

Apart from the determination of the structures of stannylenes by diffraction methods (X-ray or electron diffraction) many other physico-chemical techniques can be exployed to characterize these compounds more completely. Besides the classical methods such as IR-, Raman-, PE-, UV- and NMR-spectroscopy, MoBbauer-119 m-tin spectroscopy is widely used for the determination of the oxidation states of tin atoms and of their coordination 1n8-12°-123>. jt is not in the scope of this report to study the dependence of MoBbauer constants such as isomer shift and quadrupole splitting on structural parameters. Instead, we want to concentrate on one question Which information can we deduce from the structure of stannylenes to evaluate their reactivity ... 

The Mossbauer effect involves the resonance fluorescence of nuclear gamma radiation and can be observed during recoilless emission and absorption of radiation in solids. It can be exploited as a spectroscopic method by observing chemically dependent hyperfine interactions. The recent determination of the nuclear radius term in the isomer shift equation for shows that the isomer shift becomes more positive with increasing s electron density at the nucleus. Detailed studies of the temperature dependence of the recoil-free fraction in and labeled Sn/ show that the characteristic Mossbauer temperatures Om, are different for the two atoms. These results are typical of the kind of chemical information which can be obtained from Mossbauer spectra. 

The Mossbauer effect in Te can provide information on both the nuclear properties of the 35.6-k,e,v, first excited state of Te and the chemical properties of pure Te and Te compounds. Nuclear properties which have already been determined include the quadrupole moment, Qj = 0.20 0.03 barn, and the magnetic moment, = - -0.60 0.02 nm. Information on chemical bonding of Te can be obtained from the Te Mossbauer spectra of various Te compounds. The isomer shift which is related to the valence s electrons gives a measure of the ionic character while the quadrupole splitting can provide information on ionic character and hybridized covalent bonds. 

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. 

Frozen solutions of molecular iodine can give information about the solvent-solute interaction [81]. The spectra of iodine in hexane, CCI4, and solid argon are very similar and differ from solid iodine in showing no asymmetry parameter. It therefore seems likely that the species observed is a free iodine molecule. These values were used to derive the currently accepted calibration of the 1 chemical isomer shift. The spectrum in benzene is considerably different because of charge transfer from the benzene to the iodine. 

Similar to the isomer shifts in Mdssbauer spectroscopy, the results from photo-electron spectroscopy are sensitive to the local chemical environment of an atom. [88] However, whereas Mdssbauer spectroscopy reflects the initial electronic structure of the system under investigation, probes the state after elimination of an electron. [84] Furthermore, whereas the Mdssbauer isomer shift gives informations on the s electron density within the nuclear volume, XPS results describe the overall electronic charge denaty of an atom. As such, Moss-bauer spectroscopy and XPS compliment each other in an ideal manner. 

Fifolt [ 130] reported this chemical shift additivity method for fluorobenzenes in two deuterated solvents d6 acetone and d6 dimethyl sulfoxide (DMSO) Close correlations between experimental and calculated fluorine chemical shifts were seen for 50 compounds Data presented in Table 18 result from measurements in deuterochloroform as (he solvent [56] Fluorine chemical shifts calculated by this additivity method can be used to predict approximate values for any substituted benzene with one or more fluorines and any combination of the substituents, to differentiate structural isomers of multisubstituted fluorobenzenes [fluoromtrotoluenes (6, 7, and 8) in example 1, Table 19], and to assign chemical shifts of multiple fluorines in the same compound [2,5 difluoroamline (9) in example 2, Table 19] Calculated chemical shifts can be in error by more than 5 ppm (upfield) in some highly fluonnated systems, especially when one fluonne is ortho to two other fluorines Still, the calculated values can be informative even in these cases [2,3,4,6-tetrafluorobromobenzene (10) in example 3, Table 19]... 

The information concerning the character and degree of the substituent electronic effect transmission from C-2 to C-5(6) and in the opposite direction can readily be obtained from the correlation equation (3.2) of C-5(6) chemical shifts of 2-substituted benzimidazoles and C-2 chemical shifts of the 5(6)- isomers (Scheme 3.12) [220, 689, 691],... 

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