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

Abstract

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 A above the ring plane NICS(1), aromatic ring current shielding (ARCS), and dissected NICS, i.e. NICS calculated from selected π orbitals NICSπ, again in the ring plane and 1 A 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.