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Crystalline field compounds

The cyclodextrins (cycloamyloses) are torus-shaped molecules that can form crystalline inclusion compounds, recently attracting much attention as enzyme-site models. Their history has been seen in three phases. From 1891 to 1935 they were known as natural products, but with no recognition of their exact chemical structure. This recognition emerged in the second period, to about 1970, when most of their characteristics were also elucidated. The period from 1970 to the present has seen considerable research into their industrial use and production.239 Their inclusion compounds or complexes have found employment in such diverse fields as explosives, insecticides, pharmaceutical products, rust-prevention agents, and even baking powder. [Pg.70]

There are however problems with such a blanket assignment of peaks at ca. 30 ppm toAF >, since they generally show only a very small magnetic field dependence (Meinhold et al. 1993a), by contrast with the strong field dependence of the crystalline AT compounds (Jansen et al. 1998). Furthermore, a consequence of such an assignment is that the position of these peaks coincides with the isotropic chemical shift, implying... [Pg.284]

A comparison of the embedded ion method and the surface charge representation of the electrostatic embedding potential method, two methods which can be used to calculate solid-state effects on NMR chemical shifts, has been reported. The results in a selected group of compounds with known singlecrystal solid-state NMR data and neutron diffraction structures, confirmed that these effects are important in both C and chemical shifts. The solid-state effects calculated by both methods were similar and of equal statistical quality when compared with the experimental data. It was founf that iterative schemes to calculate, in a self-consistent fashion, the charges used to simulate the crystalline field are very important for ionic compounds. [Pg.280]

Arrese-Boggianq et al. (1977) have studied DyFes and again find a Dy hyperfine field close to the crystalline values and broadened by only about 1%, while in contrast the Fe field is broadened by 20% and is higher than that in the crystalline counterpart compound a result found also in DyNia, DyCos, and YFea with small amounts of Fe introduced as a Mossbauer probe. For example in amorphous DyFes, Hhf = 280 kOe and in crystalline DyFes, Hhf = 240 kOe leading to the conclusion that the net iron moment is increased from 1.6 to 1.9/u-B in going from the crystalline to the amorphous state. [Pg.285]

T) and a reduction in field magnitude is due to the influence of Beep and (here probably to a minor degree) of B,i,f as discussed for Eu compounds in section 5.1.1. In contrast, for Np + the orbital field dominates and all other contributions are of little importance. The average value of in the silicides is 0.6B f. This is quite close to what one expects if crystalline electric field interactions put the Ej level lowest. As shall be seen further below this crystalline field situation is strongly evidenced in the magnetic behavior of NpCr2Si2. [Pg.613]

Crystal field splittings in rare earth salts are known in fairly good detail from magnetic and optical studies and there is evidence from magnetic and thermal studies that crystalline fields affect the magnetic properties of rare earth metals, some metallic rare earth intermetallic compounds, and the behavior of rare earths as impurities in other metals. [Pg.179]

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. [Pg.80]


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See also in sourсe #XX -- [ Pg.192 ]




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