Local diamagnetic current

Circulation of electrons around a nucleus can be viewed as being similar to the flow of an electric current in an electric wire. From physics, we know that the flow of a current through a wire induces a magnetic field. In an atom, the local diamagnetic current generates a secondary, induced magnetic field which has a direction opposite that of the applied magnetic field. 

We have seen that the magnetic field R, required to obtain the resonance crmdition for nucleus i at a particular irradiating rf field (Ri) is not equal to the applied static field Rq, but is instead R, = Ro(l — a) [see Eq. (20.9)], where the nuclear screening constant, a, depends on the chemical structural environment of nucleus /. The local electron density in the vicinity of the nucleus shields it from the applied field Bo by producing small local magnetic fields (diamagnetic currents). Any structural feamre that alters the electronic environment of a nucleus will affect its screening constant a and lead to an alteration in its resonance frequency or chemical shift 5,. 

As noted earlier, chemical shifts arise from the secondary magnetic fields produced by the circulation of electrons in the molecule. These so-called local diamagnetic currents are induced by the fixed magnetic field and result in secondary fields that may either decrease or enhance the field to which a given proton responds. The effects are complex, and we eonsider only the major aspects of the phenomenon here. More complete treatments can be found in several reference works. ... 

The NMR spectrum of the syn isomer shows evidence of a diamagnetic ring current, based on both the relatively low-field position of the vinylic hydrogens and the upfield shift of the methylene hydrogens. The anti isomer shows much less pronounced shifts. The X-ray crystal structure of the syn isomer shows a moderate level of bond alternation, ranging ftom 1.36 to 1.45 A (Fig. 9.4A). In the anti isomer, bond alternation is more pronounced, vith the double bond in the center ring being essentially a localized double bond (Fig. 9.4B). 

As shown by Pascal,64 Lonsdale,63 and Dauben,159 the A quantity for aromatic molecules is exalted, namely, the delocalized molecule is more diamagnetic than the localized reference A is a negative quantity. The exalted diamagnetism was interpreted to result from the ring currents carried by the delocalized electrons which circulate under the influence of the... 

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