Ytterbium systems

Watts and Richter (36) in their study of energy transfer between ytterbium and holmium in YF3 has also found that diffusion in the ytterbium system may be important when the ytterbium concentration is increased. By variation of the ytterbium concentration they were able to vary the diffusion coefficient by three orders of magnitude and show consistence of their results with Eq. (28). 

It has also turned out mainly during our studies of mixed-ligand organo-ytterbium systems that many Yb3+-complexes present excellent conditions for the observation of their 1H-NMR spectra in spite of the strong paramagnetism of this f13-system.Previous... 

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. 

At the other end of the rare earth series the results for the ytterbium system show the stable existence of anion-excess superstructure phases completely analogous to those found in the yttrium system, but there is also evidence for a hypostoichiometric fluorite-related phase YbX2- . Much more work in this general area needs to be done in order to clarify the situation. 

NOL-based systems for addition of (substituted) anilines to meso epoxides. Hou found that a ytterbium-BI NO L complex catalyzed desymmetrization of cyclohexene oxide in up to 80% ee [15], Shibasaki demonstrated that a praseodymium-BINOL complex could promote addition of p-anisidine to several epoxides in moderate yields with modest enantioselectivities (Scheme 7.7) [16]. 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

Symmetrical and unsymmetrical benzoins have been rapidly oxidized to benzils in high yields using solid reagent systems, copper(II) sulfate-alumina [105] or Oxone-wet alumina [105, 106] under the influence of microwaves (Scheme 6.32). Conventionally, the oxidative transformation of a-hydroxy ketones to 1,2-diketones is accomplished by reagents such as nitric acid, Fehling s solution, thallium(III) nitrate (TTN), ytterbium(III) nitrate, ammonium chlorochromate-alumina and dayfen. In addition to the extended reaction time, most of these processes suffer from drawbacks such as the use of corrosive acids and toxic metals that generate undesirable waste products. 

Aromatic halides such as chlorobenzene and p-fluorololuene were rapidly hy-drogenolyzed in 100% conversion by NaH of nanometric size in the presence of homogeneous catalysts. One- or two-component (e.g., Ni(OAc)2/TiCl4) systems were effective. The combination of ytterbium chloride and a transition-metal chloride showed a remarkable synergistic effect [37, 38]. 

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