A Unified Orbital Model of Delocalised and Localised Currents in Monocycles, from Annulenes to Azabora‐heterocycles

Abstract

Why are some (4n+2)π 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)π carbocycles and a two‐electron paratropic (antiaromatic) current for (4n)π 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 π‐conjugated systems. Ab initio valence‐bond calculations confirm the localisation predicted by the naïve model, and coupled‐Hartree–Fock calculations give current‐density maps that exhibit the predicted delocalised‐to‐localised/carbocycle–heterocycle transition.

The Phenalenyl Motif: A Magnetic Chameleon

“…Subtleties of the adaptation of the pseudo-p method for hetero-A C H T U N G T R E N N U N G atoms have been discussed elsewhere. [27] In passing, we may note that the ipsocentric method gives a direct interpretation of the maps as they are seen, that is, it interprets the perimeter current. In contrast, graph-theoretical models that decompose the current into circuit contributions [28, 8b] would interpret the plotted perimeter current as a summation of superimposed diatropic benzene-and naphthalene-circuit contributions that cancel out everywhere except on the perimeter, plus a weak paratropic perimeter-circuit contribution.…”

Efficient mapping of ring currents in fullerenes and other curved carbon networks

“…As will be explained below, this model relies on a matching in symmetry between the carbon π orbitals responsible for ring currents and the σ orbitals of a set of hydrogen atoms that has the same connectivity. In previous work, the pseudo-π model has been shown to reproduce qualitatively and even semi-quantitatively the patterns of induced current density in planar and near-planar aromatic, antiaromatic and non-aromatic systems, giving yes/no answers to the question of aromaticity and an account of the origins of the ring current (if any) in terms of orbital contributions, all at a tiny fraction of the cost of a full ab initio treatment [36][37][38][39][40].…”

BN versus CC: How Similar Are They?

“…While many earlier studies suggested that borazine is highly aromatic, the most recent theoretical results suggest just the opposite. [10,11] Thus, the matter is still one of debate and study.…”