Aromaticity, the Hückel 4 n+2 Rule and Magnetic Current

Abstract

Quantum chemical calculations using density functional theory and correlated ab initio methods of the 10 π‐electron systems (N6H6)2+ and C2N4H6 show that the planar forms are no minima on the potential energy surfaces. The twisted ring structures of the two species are energy minima, but acyclic isomers are much lower in energy. The planar geometries sustain strong diamagnetic ring current comparable with that of benzene. In contrast, the calculated multicenter normalized Giambiagi electron delocalization index ING suggests that π‐delocalization in planar (N6H6)2+ and C2N4H6 is much weaker than in benzene. Since aromaticity is synonymous for a particular stability of cyclic delocalized systems, it may be stated that calculation or measurement of magnetic chemical shifts due to induced ring currents is not a reliable method to ascertain the aromatic character of a molecule. Aromatic compounds exhibit ring current induced magnetic shielding, but the reverse conclusion that ring current induced magnetic shielding identifies aromaticity is not justified. Furthermore, the 4n+2 rule as indicator of aromatic stabilization should only be used in conjunction with the ring size; the nature of the occupied π orbitals must always be examined.

The electron density of delocalized bonds (EDDB) applied for quantifying aromaticity

“….a multicenter delocalization is a necessary condition for a diatropic ring current to exist, provided that there are proper virtual molecular orbitals to excite to.''). [28][29][30][31][32] Finally, the DI-based indices (especially MCDI), unlike the aromaticity measures described above, enable one to study most of the types of aromaticity that can be found in literature. 6 Also, the MCDIs are the only descriptors that passed a set of rigorous tests for aromaticity quantifiers.…”

Why Aromaticity Is a Suspicious Concept? Why?

“…Available indices of aromaticity do not always give consistent results among themselves (Balaban, 1980; Lloyd, 1996; Poater et al, 2004; Zhao et al, 2017). Thus, for instance, predictions based on magnetic criteria of aromaticity often deviates from those based on energetic grounds (Aihara, 2006).…”

The vorticity of the current density tensor and 3D‐aromaticity

“…This latter group is peculiar, it is the only one that depends on the properties of a perturbed system. The electron distribution of the ground state of a system is significantly different from the magnetic perturbed one, because of this, the magnetic criteria for aromaticity has been recently criticized . However, these magnetic schemes have been widely used because they are based in a well‐known idea for chemist.…”

“…Even for a very symmetric carbocage, the current strength differs depending on which symmetry axis is chosen for the direction of B . It has also been reported that the high energy planar N 6 H 6 2+ has a similar current strength as benzene, which indicate that the ring currents parallel to the molecular plane do not possess all the electron delocalization information to discern between these two molecules. Another complication is that the analysis of the topology of a vector field, as J ( r ), is more cumbersome.…”