Isomer shift europium

The relative amounts of sodium alkenesulfonate isomers and sodium hydroxyalkanesulfonate isomers have been determined with 1.19-2.07% precision by NMR measurements of the methyl signals of methyl esters in carbon tetrachloromethane with 1.3 mol equivalent tris(heptafluorobutanoylvivaloyl-methanato)europium(I) and tri(dipaloylmethanato)europium as shift reagent. The... 

Table I. Differences in Energy in Terms of Isomer Shift of the 21.7-k.e.v. Line of in Different Europium Compounds...

Europium has been characterized by several workers (3, 13) as divalent in the metallic state. This is again borne out by the isomer shift of the metal, which is clearly much closer to that of the covalent divalent compounds than that of the covalent trivalent compounds. (The comparison should be made with covalent compounds because in the metallic state the atoms are bonded together by sharing the valence electrons.)... 

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. 

K. The stable antiferromagnetic ground state was also evident from the negative Weiss constant of —14 K and the monotonically increasing magnetization curve with a value of 2.8 p.b/Eu at 2 K and 5.5 T.151 Eu Mossbauer spectroscopic studies confirmed the divalent nature of europium in EuAuPb with an isomer shift value of —10.9 mm/s at 78 K. Line broadening in the Mossbauer spectra occurred around... 

Fig. 17.8 Chemical isomer shifts of divalent europium selenides correlated with the mean Eu -Se " distance. [Ref. 31, Fig. 3]...

A systematic study of the Eu/Yb and Eu/Ba alloys has been made [52, 53]. In the ytterbium system, the Curie temperature falls from 90 to 5 K and the saturation field also falls from 265 to 160 kG as the ytterbium content increases from 0 to 92 at. %. The relationships are linear apart from a discontinuity at 50 at. % where there is a phase change. Similarly for barium the Curie temperature falls from 90 to 40 K and the field from 265 to 206 kG as the barium content rises to 50 at. %. However, the chemical isomer shift is not significantly altered. The sign of the magnetic field is known to be negative from neutron diffraction data. Calculations suggest that a contribution of —340 kG to the field in europium metal arises from core polarisation, that +190 kG comes from conduction-electron polarisation by the atoms own 4/-electrons, and that —115 kG comes from conduction-electron polarisation, overlap, and covalency effects from neighbouring atoms. 

Chemical isomer shifts for all four europium resonances have been measured in a number of compounds. The Eu values are given in Table 17 5. Comparison of pairs of values for two transitions should show a linear relationship. An early attempt to verify this for the Eu and Eu (97 and 103 keV) transitions in EU2O3, EUSO4, and Eu metal disclosed a large deviation [59], but this was later shown to be a result of impurity in the metal [62]. 

In addition to using the lattice parameter(s) of the RM compound to estimate the valence state of europium and ytterbium one can use magnetic susceptibility data (Gschneidner 1969b, Wohlleben 1981) and Lm absorption edges, XPS and UPS (Wohlleben 1981) to distinguish between the 4f" and 4f" configurations and intermediate valences. For europium one can also use Mossbauer isomer shift data (Brix et al. 1964, Clifford 1967, van Steenwijk and Buschow 1977, de Vries et aL 1984). 

Wortmann, G., U.F. Klein, G.M. Klavius and W.B. Holzapfel, 1972, Pressure Dependence of the Isomer Shifts and CJuadrupole Splittings of Di- and Trivalent Europium Compounds, Presented at the International Conference on Applications of the Mdssbauer Effect, Ayeleth Hashahar, Israel, 1973. 

The large and well-defined range of isomer shifts observed for the NGR of Eu in the di- and trivalent states of the europium ion make this isotope particularly valuable for the study of intermediate valency states in lanthanide intermetallics. Such intermediate valency states have their origin in interconfiguration fluctuations (ICF s). In the case of europium intermetallics, the unusual isomer shifts observed are interpreted in terms of fast fluctuations, on the order of 10 " sec, of an electron between a localized 4f level and the conduction band, resulting in a fluctuating charge density at the Eu nuclei. 

The pressure dependence of the hyperfine fields and isomer shifts in the europium chalcogenidesihas also been studied [Klein et al. (1973), Kalvius et al. (1974), Klein et al. (197 ]. In EuO and EuS there occurs a pronounced increase in Htn with high pressure (up to 60 kbars), whereas in EuTe there is a very small... 

The equiatomic compounds EuLiBi and YbLiBi, containing divalent rare earths, are known. EuLiBi, which is extremely moisture-sensitive, was prepared from reaction of Eu, Li, and Bi in a 1 2 1 atomic ratio within Mo crucibles placed in steel ampoules jacketed by silica tubes at 1000 °C for 5-7 d (Prill et al., 2002). Powder X-ray diffraction indicates an orthorhombic TiNiSi-type structure (Pearson symbol oPl2, space group Pntm (No. 62), Z = 4), but atomic positions were not refined (section 5.4, fig. 9a). The presence of divalent europium is supported by Eu Mossbauer spectroscopy (isomer shift of -11 mm/s) and magnetic susceptibility measurements (effective magnetic moment of 8.3 mb, close to that of the free-ion value of 7.94 MB for Eu +). EuLiBi orders antiferromagnetically below 7.5 K. 

The principal limitation of the method is the lack of suitable isotopes. It is unfortunate that elements such as carbon, oxygen, and silicon do not show a Mossbauer effect. In addition to iron, Mossbauer effects are readily observed in isotopes of tin, xenon, iodine, europium, gold, and a few others. Isomer shifts make it possible to distinguish the valence states of iron and tin, but most of the other nuclei have greater resonance linewidths, making it difficult to observe chemical effects. 

A 100 MHz. proton magnetic resonance spectrum (chloroform d) of the amine in the presence of an equal amount of the chiral shift reagent, tris[3-(trifluoromethylhydroxymethylene)-d-camphorato]euro-pium(III)4 (submitters), or in the presence of an equal amount of tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato]europium-(III) (checkers), revealed that the product contained no detectable enantiomeric isomer. 

Z-isomer.56 The isomers of the cyclic phosphonate (36) were assigned from the effect of Eu(dpm)3 on the methyl signal.67 Shifts of the butyl signals of tributyl phosphate by europium chloride have also been recorded.58... 

The proton NMR spectrum of 1,10-phenanthroline has been obtained and analyzed by several authors in nonaqueous solvents 04-109 and in water at various pH values.28,110 Examples of studies of the NMR spectra of substituted 1,10-phenanthrolines that have been investigated in some detail are also worthy of mention.47,104,106,11° The NMR spectra of all ten phenanthrolines have been determined in deuterochloroform, and the spectra were interpreted12 (Table IV). The spectra of the 1,7-, 1,10-, and 4,7-isomers have also been compared with that of phenanthrene.111 Shifts in the NMR spectrum of 1,10-phenanthroline induced by a europium shift reagent have been discussed,112 and 13C chemical shifts of free and protonated 1,10-phenanthroline were measured.113... 

II, A,3,a) its dipole moment, IR, UV, and NMR spectra were distinguishable from (Z)-isomer 40 (R = H) (72TL4565). NMR studies were particularly relevant. Shift reagent experiments showed the europium complex coordinated more strongly with (Z)-compound 40 (R = H) than with ( )-isomer 12 (R1 - R4 = H) (72TL4565). Also, the (E)-isomer and its 3,8-disubstituted derivatives (12) showed nonequivalent methylene and meth-ine protons, in contrast to (Z)-isomers 40 (81JOC303). 

Commercially available Eu(hfc)3, tris [3-(peptafluoropropylhydroxy-methylene)-d-camphorato]europium III, NMR shift reagent, was used as received from Aldrich Chemical Company. The proton on the chiral carblnol carbon was shifted downfield to - 11 ppm in CDClj. The R isomer was shifted 0.5 ppm further downfield than the S isomer. 

Achiral lanthanide chelates can also be added to CSAs such as arylperfluoroalkyl-carbinols, the ethyl ester of 3,5-dinitrobenzoyl-L-leucine (25) , the 3,5-dinitrobenzoyl derivative of 1-phenylethylamine, Af-(l-(l-naphthyl)ethyl)trifluoroacetamide (26) and a series of l-(l-naphthyl)ethyl urea derivatives of amino acids (27) to enhance the enantiomeric discrimination. With sulfoxide or lactone substrates , the europium ion preferentially associates with the substrate in the bulk solution. Provided the enantiomers have different association constants with the CSA, the isomer that shows the weaker association with the CSA shows the larger lanthanide-induced shifts. Low concentrations of lanthanide relative to the substrate and CSA lead to enhancements of enantiomeric discrimination in the NMR spectrum. If the concentration of lanthanide is too high, binding of the substrate to the lanthanide strips the substrate from the chiral solvating agent and diminishes the chiral discrimination in the NMR spectrum. 

Then we changed over to the isomer allylic alcohol, to 3-methyl-2-buten-l-ol (prenol). Being a primary alcohol, it was smoothly epoxidized under both stoichiometric and catalytic Sharpless conditions. While the stoichiometric method provides only moderate yields as the dimethyl glycidol is fairly watersoluble, the catalytic method affords the double yield. The e.e. amounts to 90% in both cases. Optical purity and e.e. of the 3,3-dimethyl glycidol were determined by polarimetry and -NMR in the presence of chiral europium shift-reagent [22]. 

The earlier structural assignments of the products of oxidation of tropine (4) with hydrogen peroxide have been corrected. Analysis by n.m.r. at 220 MHz establishes the major isomer to be (5) and the minor isomer to be (6). The shift reagent europium tris-(l, 1,1,2,2,3,3-heptafluoro-7,7-dimethyl-octane-4,5-dione) has been used with (7) but fails to establish conclusively whether contact shifts are important close to the co-ordination site. 

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