Aromaticity is of fundamental importance to chemistry. Even after two centuries, the concept that electron delocalization in closed circuits endows molecules with special properties continues to increase in its scope of applicability and in its importance. Since aromaticity is not a directly measurable quantity, its magnitude is now generally evaluated in terms of structural, energetic, and magnetic criteria. However, magnetic properties are the most closely related to aromaticity, as they depend directly on the induced ring currents associated with cyclic electron delocalization.

Since its introduction in 1996 [1], NICS (nucleus-independent chemical shift) continues to gain popularity as an easily computed, generally applicable criterion to characterize aromaticity and antiaromaticity of rings, clusters, transition states, and transition-metal complexes. NICS is based on the magnetic shielding (with the sign reversed) computed at chosen points in the vicinity of molecules. Significantly negative (shielded) NICS values inside rings or cages are due to induced diatropic ring currents and denote aromaticity, whereas positive (deshielded) values denote paratropic ring currents and anti-aromatic behavior.

Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chemical shifts (NICS), were evaluated against a uniform set of aromatic stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)_πzz (based on the π contribution to the out-of-plane zz tensor component), performed best statistically (cc = 0.980) and in practice. The easily computable NICS(1)_zz index is a useful alternative (cc = 0.968) [2].

[1] P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R. v. E. Hommes, J. Am. Chem. Soc. 1996, 118, 6317.

[2] H. Fallah-Bagher-Shaidaei, C. S. Wannere, C. Corminboeuf, R. Puchta, P. v. R. Schleyer, Org. Lett. 2006, 8, 863-866.

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