Magnetic anisotropy prediction

Figure 4.34. Time-independent part of the orientation parameter as a function of the field frequency for different values of the internal magnetic anisotropy of the particles. The ratio e = TdAb = 1CT4 curves correspond to a = 100 (1), a = 10 (2), o = 5 (3), a = 2 (4), a = 0.1 (5). Thin lines a and b resemble the limiting behavior predicted by the rigid and soft dipole models, respectively. Note that at this graph the lines a and 1 visually coincide.

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. 

A quantitative measure of electron delocalization in a planar, cyclic molecule may be obtained by comparing the measured out-of-plane magnetizability component or magnetic anisotropy with the value predicted for a hypothetical structure in which the electron distribution is completely localized. The difference between the observed and calculated values, Ayani and is the estimate of the extent of electron delocalization and of the relative... 

One alternative is to construct molecular clusters with already preformed mononuclear or polynuclear species with well-known electronic structure, so that the nature of the ground state and even in some cases the magnitude and the nature of the magnetic anisotropy may be easier to predict. Such a step by step approach was first used by Kahn and coworkers to prepare the very... 

The molecular clusters based on cyanide offer a simple example to predict the nature and the magnitude of the magnetic anisotropy of the ground spin state. 

Thus, the chemical shift of a proton attached to an organic compound is known to be affected by primary structure, i.e., the identity of the atom to which it is directly attached, the identities of those one atom removed, etc., by hybridization changes on these atoms, by van der Waals interactions with nearby nonbonded atoms, by electric Helds associated with permanent moments in the molecule, and by neighboring magnetic anisotropy Chemical shifts of substances in the liquid state may also be affected sizably by the medium. The magnitudes of these effects are comparable, and each is not readily calculated. Thus only a bold NMR spectroscopist will be conHdent in predicting the proton chemical shift to within 0.5 ppm. 

Nevertheless, it is worth noticing that the latter allows us to describe all the situations, whatever the degree of orbital degeneracy of the interacting ions, and thus to predict the magnetic anisotropy of real systems. 

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