Evaluation of aromaticity: A new dissected NICS model based on canonical orbitals

Several methods to address aromaticity in terms of nucleus-independent chemical shifts (NICS) are compared. These include NICS at the ring centre NICS(0), NICS 1 Å above the ring plane NICS(1), aromatic ring current shielding (ARCS), and dissected NICS, i.e. NICS calculated from selected π orbitals NICS_π, again in the ring plane and 1 Å above. The methods are tested on the basis of density-functional theory (DFT) and the individual gauge for local orbitals (IGLO) technique. Applications include simple organic rings (C₄H₄, C₄H₄²⁺, C₆H₆, C₅H₅^-, C₇H₇⁺ and transition metal carbonyl complexed molecules Fe(CO)₃C₄H₄ and Cr(CO)₃C₆H₆.