Aromatic ring-current shielding

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... 

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. 

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]. 

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... 

Calculations show that only 60% of the chemical shift difference between and H is the result of the aromatic ring current, and that even Ha is shielded it would appear at 5 = w5.5 without the ring current Childs, R.F. McGlinchey, M.J. Varadarajan, A. J. Am. Chem. Soc., 1984, 106, 5974. 

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... 

Chemical shift is substantially determined by the electron population surrounding the nucleus in question and shielding it from the applied field. Chemical shifts are therefore used to probe the total electron population. The chemical shift range with protons is so small that aromatic ring currents and other anisotropic influences make such measurements using proton spectra unreliable, but the agreement between calculated and observed 13C chemical shifts is good. 

Shielding and deshielding of protons (a) shielding of proton due to induced diamagnetic electron circulation b) deshielding of protons in benzene due to aromatic ring currents. 

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