Aromatic rings magnetic anisotropic effect

These hybridisation variations are caused by anisotropy within the chemical bonds. This is due to the non-homogeneous electronic distribution around bonded atoms to which can be added the effects of small magnetic fields induced by the movement of electrons (Fig. 9.12). Thus, protons on ethylene are deshielded because they are located in an electron-poor plane. Inversely, protons on acetylene that are located in the C-C bond axis are shielded because they are in an electron-rich environment. Signals related to aromatic protons are strongly shifted towards lower fields because, as well as the anisotropic effect, a local field produced by the movement of the aromatic electrons or the ring current is superimposed on the principal field (Fig. 9.12). 

Either chloroform or carbon tetrachloride also form the inclusion complexes as shown in Fig. 2. The formation of 2 in Fig. 2 is further confirmed by the fact that the H chemical shift of chloroform is shifted toward lower magnetic field by 0.14 ppm, which is due to the anisotropic shielding effect by the aromatic ring of phenolate ion. 

In Section 16.5, we first discussed the anisotropic eflFects of an aromatic ring. Specifically, the motion of the it electrons generates a local magnetic field that effectively deshields the protons connected directly to the ring. 

Fig. 1.2. The field induced magnetic moment is depicted schematically in this drawing. This effect is most pronounced in aromatic molecules such as fluorobenzene, where comparatively strong electron ring currents may be induced, leading to a field induced, molecular magnetic dipole moment which opposes the exterior field. Trying to align the induced moment, the exterior field will exert a torque ind X H on the molecule and will thus perturb the overall rotation. This perturbation is seen as a splitting in the rotational spectra. Since there will be a torque only in the case that md and H are not aligned, i.e., if Jg is anisotropic, only the anisotropies of the molecular susceptibility tensor can be obtained from the splittings of the rotational lines...

In conclusion, measuring aromaticity by comparison of chemical shifts as representations of ring current is possible, provided the examples are chosen carefully. The protons to be compared must be located in similar regions of the magnetic field the molecules must have similar planarity and not be subjected to strong anisotropic or charge effects. 

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