Magnetically anisotropic groups

A magnetically anisotropic group is one that is not equally magnetized along all three axes. The most common such groups are benzene rings (see p. 47) and triple bonds. 

LARMOR PRECESSION MAGNETICALLY ANISOTROPIC GROUP MAGNETOTACTIC Magnetic field strength,... 

Magnetically anisotropic groups can be considered to be surrounded by volumes of space in which protons will be shielded (-I-) or deshielded (—), that is, moved upheld or downfield, respectively. The best-established effects are associated with the groups shown in Figure 12.8. 

Figure 12.8. Shielding zones associated with some magnetically anisotropic groups.

Magnetically anisotropic substituents may obscure this rule. One of the most important exceptions are hydrogens in the a-position with respect to a carbonyl group in a-substituted cyclohexanones29 32, where AS = <5(Ha) — <5(He) > 0. 

A remarkable property of cyclopropanes is that they are magnetically anisotropic, with the protons coming into resonance in their NMR spectra at unusually high field, typically 1 ppm upfield of the protons of an open-chain methylene group. For 1H NMR spectroscopy, this is quite a large effect, and it is also strikingly in the opposite direction from that expected by the usual analogy... 

In their analysis of H-NMR spectra of various end-functionalised PIB, Jiaoshi and Kennedy [50] covered inductive effects (due to ert-chlorine-ended polyisobutylenes), magnetically anisotropic effects (dne to olefin gronps and phenyl rings) and allylic coupling (dne to olefinic end groups) ... 

Figure 1.13. The magnetic anisotropic effect of the carbon-carbon double bond, the carbonyl group, and the nitro group, and the resulting shielding (+) and deshielding (—) regions.

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