Ring-current shielding

If six electrons in a ring constitutes aromaticity, then furan is aromatic. But such aromaticity is only formal and has no meaning unless it can be shown that furan has properties modified beyond what can be expected from a molecule that can be variously regarded as a divinyl ether and a cyclopentadiene derivative as circumstances demand. No such demonstration exists, with the possible exception of the NMR ring current shielding... 

The nucleus experiences a ring current shielding of approximately 0.85 ppm. 

Recent theoretical studies have resulted in establishing appropriate criteria for homoaromaticity of carbocalions. These include aromatic ring-current shielding (ARCS), nucleus-independent chemical shifts (NICS), bond-length alternations, and NMR shielding. On the basis of such criteria, cations CgHg, C.J I],-), and were found to be the most homoaromatic. The... 

Sundholm = aromatic ring-current shielding (ARCS) M... 

In Figure U of Hai Hallion (1972), the predicted "ring-current" shielding (ftram the classical and quantum-mechanical theories)... 

Very recently Juselius and Sundholm studied the aromatic character of magnesium porphyrins by performing aromatic ring current shieldings (ARCS) and NICS calculations. Although the NICS method cannot be applied to the molecule as a whole because of the location of the magnesium atom in the molecular center, they determined NICS values at the centers of the pyrrolic rings. This shows that NICS values can be used for the study of local aromaticity in polycyclic compounds. 

Comparison of the external annulene Ca for 4, 93, and 95 shows a 2.1 ppm difference between the Me, Et, and Pr annulenes and of the internal bridge carbon Cb a 5.1 ppm difference between 4 (CH3 attached) and 93 (CH2 attached) and a 0.2 ppm difference between 93 and 95 (both CH2 attached). The ring current shielding of Ca is thus not significant. The bridge carbons Cb are better placed to experience the ring current and shielded 9—10 ppm. Comparisons between different systems can thus be made, but they must be carefully chosen to ensure... 

Magnetic Criteria This has been the main characterization criterion for the aromatic molecules from the theoretical standpoint. There are various methods, which have been developed that can describe the magnetic properties for aromatic compounds. Some of them are, multiple aromaticity (simultaneous presence of o- and Jt-aromaticity) [15], mapping ring currents, aromatic ring current shielding (ARCS) [16], nuclear-independent chemical shift (NICS) [17], bifurcation analysis of the electron localization function (ELF) [18] and many more. For a comprehensive review of the various techniques and the developments in the field of aromaticity, the readers are referred to two volumes of Chemical Reviews that were edited by Professor P. v. R. Schleyer [1,11]. 

The magnitude of the effect of ring current shielding can be calculated for a nucleus at a given distance, d, from the center of the rapidly tumbling ring. The original calculation of Pople (1956) is of the form... 

---