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Aromaticity, the Hückel 4 n+2 Rule and Magnetic Current
Author(s) -
Zhao Lili,
GrandeAztatzi Rafael,
ForoutanNejad Cina,
Ugalde Jesus M.,
Frenking Gernot
Publication year - 2017
Publication title -
chemistryselect
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.437
H-Index - 34
ISSN - 2365-6549
DOI - 10.1002/slct.201602080
Subject(s) - aromaticity , delocalized electron , ring current , ring (chemistry) , chemistry , diamagnetism , ab initio , computational chemistry , benzene , chemical shift , electromagnetic shielding , molecule , chemical physics , molecular physics , physics , magnetic field , organic chemistry , quantum mechanics , earth's magnetic field
Quantum chemical calculations using density functional theory and correlated ab initio methods of the 10 π‐electron systems (N 6 H 6 ) 2+ and C 2 N 4 H 6 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 I NG suggests that π‐delocalization in planar (N 6 H 6 ) 2+ and C 2 N 4 H 6 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.