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

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Authors

ZHAO Lili GRANDE-AZTATZI Raphael FOROUTANNEJAD Cina UGALDE Jesus FRENKING Gernot

Year of publication 2017
Type Article in Periodical
Magazine / Source ChemistrySelect
MU Faculty or unit

Central European Institute of Technology

Citation
Web http://onlinelibrary.wiley.com/doi/10.1002/slct.201602080/full
Doi http://dx.doi.org/10.1002/slct.201602080
Field Physical chemistry and theoretical chemistry
Keywords Aromaticity; Ring Current; Huckel Rule
Description Quantum chemical calculations using density functional theory and correlated ab initio methods of the 10 pi-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 pi- 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 pi orbitals must always be examined.
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