Diagnosis of the σ-, π- and (σ plus π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules

The NICSzz-scan curves of aromatic organic, inorganic and all-metal molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the sigma-, pi- and/or double (sigma + pi)-aromaticity. The NICSzz-scan curves of sigma-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 angstrom with the induced diatropic ring current arising from T-x,T-y-allowed transitions involving exclusively sigma-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 angstrom) characterize double (sigma + pi)-aromaticity, the chief contribution to the induced diatropic ring current arising from T-x,T-y-allowed transitions involving both sigma- and pi-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (sigma + pi)-aromatics where the pi-diatropic ring current overwhelms the sigma-type one. In the absence of any contribution from the sigma-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure paromaticity.