Magnetic ring current

Fig. 6. The local magnetic ring current field Hr of an aromatic molecule. Ho is the external polarizing field. The field strength at a point P is determined by the position relative to the aromatic ring and by the size of Hr. Hr depends on the total number of ji-electrons in the aromatic molecule...

Analysis of this data reveals that the parameters are very similar to those of the monocyclic model compound A -carboxamidopyrazole, but there is a small shielding effect due to contribution of the resonance structures (see Scheme 1), which may be indicative of a small magnetic ring current. The H coupling constants are of similar magnitude to those of simple pyrazoles. The chemical shift of the carbonyl carbon appears in the amide rather than in the ketone region. 

There are other ways to decide if a compound is aromatic, of which the magnetic ring current from NMR, and various other spectroscopic methods were all examined. From each of these methods, the result was the same, namely any homoaromatic resonance present in triquinacene was too small to measure. 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Ring Currents Aromatic and Antiaromatic Magnetic Resonance Imaging Spectra by the Thousands Gas Chromatography GC/MS and MS/MS... 

A dramatic decrease in the magnitude of the magnetic susceptibility anisotropy is observed on going from thiirane to the open-chain analog, dimethyl sulfide, and has been attributed to non-local or ring-current effects (70JCP(52)5291). The decrease also is observed to a somewhat lesser extent in oxirane relative to dimethyl ether. 

Figure 15.14 The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized tt electrons circulating in the molecular orbitals of the aromatic ring.

Induced magnetic field because of ring current... 

Ring current (Section 15.8) The circulation of tt electrons induced in aromatic rings by an external magnetic field. This effect accounts for the downfield shift of aromatic ring protons in the lH NMR spectrum. 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

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