A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles.
Why are some (4n+2)pi systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four-electron diatropic (aromatic) ring current for (4n+2)pi carbocycles and a two-electron paratropic (antiaromatic) current for (4n)pi carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier-orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen-localised currents in alternating azabora-heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic pi-conjugated systems. Ab initio valence-bond calculations confirm the localisation predicted by the naive model, and coupled-Hartree-Fock calculations give current-density maps that exhibit the predicted delocalised-to-localised/carbocycle-heterocycle transition.
Soncini
Alessandro
A
Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK.
Domene
Carmen
C
Engelberts
Jeroen J
JJ
Fowler
Patrick W
PW
Rassat
André
A
van Lenthe
Joop H
JH
Havenith
Remco W A
RW
Jenneskens
Leonardus W
LW