The components of nucleus-independent chemical shift (NICS) tensors for D_nhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICS_zz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICS_zz, which is dominated by contributions from the π system, characterizes NICS well.

Entirely unlike the aromatic closo B_nH_n^2- borane dianions, isoelectronic Si₆^2- and Si₁₂^2- are antiaromatic. Their O_h and I_h symmetries are responsible, as the other deltahedral silicon dianion clusters do not exhibit this behavior. These high symmetries prevent mixing among the degenerate lone pair and skeletal orbitals, leading to paratropic behavior.

Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO (“dissected NICSâ€�, LMO-NICS) method. Applications to (4n + 2) Ï€-electron systems, ranging from [n] annulenes to D_nh S₃, S₅, and N₆H₆²⁺ rings as well as to D_2h cyclobutadiene, show the extent to which their diatropic character results from the σ framework and from the Ï€ orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the σ orbitals, but is found only for “electron-richâ€� Ï€ systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size.

As shown by detailed nucleus-independent chemical shift (NICS) analyses of the contributions of each molecular orbital, the very recently reported gas-phase all-metal Al₄Li₃^- anion and its relatives (Kuznetsov, A.E.; Birch, K.A.; Boldyrev, A.I.; Li, X.; Zhai, A.I.; Wang, L.S. Science 2003, 300, 622) are aromatic rather than antiaromatic. The paratropic (antiaromatic) four-Ï€-electron contribution is overcome by the predominating diatropic effects of σ aromaticity. However, true antiaromatic all-metal clusters, such as Sn₆^2- (Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 297), do exist.