Magnetic anisotropy effects

The SH, SCHs, and weakly directing halogens cause small shifts, which to a large extent are determined by magnetic anisotropy effects, especially in the case of the halogens. Attempts have been made to estimate these effects for the other thiophenes. Except for orthohydrogens, these effects are usually very small. 

Pi-complexing is most commonly used to rationalize effects observed in aromatic solvents. The most frequent evidence cited is magnetic anisotropy effects on chemical shifts in the solute molecule. As was the case for hydrogen bonding no quantitative correlations with substantive parameters such as ultraviolet spectral shifts have been attempted. 

In early work, Spiesecke and Schneider (59) pointed out that inductive effects alone cannot account for a- and -signal shifts. They held diamagnetic neighbor-anisotropy effects (63) arising from anisotropic electron-charge distributions responsible for the deviations in the electronegativity correlations. For bonds with conical symmetry they applied McConnell s magnetic point-dipole approximation (64) for the estimation of this contribution, Act ... 

The correlation, especially when applied to a variety of parent molecules, is not as good as one might hope, however, indicating that other contributions must be operative. Spiesecke and Schneider (59) were the first to propose the existence of magnetic anisotropy effects (Section II-B-2) whereas Bucci (62) suggested an... 

Fig. 4.8. Substituent, electronegativities Ex versus 13C shifts (<5C in ppm) of substituted benzenoid carbons, corrected for magnetic anisotropy effects [386],...

The calculations confirm a visible anisotropy of the susceptibility components at low temperatures, and the overall magnetic productivity is decreased as a consequence of the decreased g-factors (compare the effective magnetic moments in Figs. 50 and 41). 

The calculations in a complete d5 space spanned by 252 functions are presented in Fig. 68. It can be seen that the magnetic anisotropy is very small and disappears above 2 K. The effective magnetic moment is constant down to a very low temperature. The calculated D-values are small. The calculations confirm that the ZFS model with S = 5/2 could be appropriate. 

The effective magnetic moment shows a broad maximum and resembles the octahedral pattern (Figs. 90,91, and 93). High magnetic anisotropy is seen at temperatures as high as 150 K with a sign opposite that of the compressed bipyramid. 

Shifts of the aromatic carbon atom directly attached to the substituent have been correlated with substituent electronegativity after correcting for magnetic anisotropy effects shifts at the para aromatic carbon have been correlated with the Hammett a constant. Ortho shifts are not readily predictable and range over about 15 ppm. Meta shifts are generally small-up to several parts per million for a single substituent. 

Figure 23 shows the coercivity dependence on temperature for FePt C cluster films with 45 vol. % C from 10 to 300 K. The films were annealed at different temperature for 10 min. The coercivity decreases with increase of measuring temperature, for example, dropping from 19 kOe at 10 K to about 13 kOe at 300 K for a film annealed at 750 °C for 10 min. This may be caused by a contribution of intrinsic temperature dependence of the anisotropy and magnetization, and thermal activation effects [49]. 

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