Diamagnetic susceptibility exaltation as a criterion of aromaticity

A simple model for counter-rotating ring currents in [n]circulenes

“…and for coronene (2, n )51.1 a.u.). The sign of the anisotropy term (Dn )71.7 a.u) for 3a is consistent with the diatropicity of the perimeter circulation and the classical r 2 scaling of n for a current loop [2]. The variation of Dn with n is consistent with the relative strengths of the opposing central current (1, Dn )61.8 a.u and 2, Dn )101.6 a.u [6]).…”

“…Ring currents in mono-and polycyclic conjugated systems, though not directly observable, are inferred through their manifestation in NMR spectroscopy and measurements of magnetic anisotropy [1,2]. Fortunately, modern ab initio distributed-origin methods at the coupled Hartree-Fock level, such as the ipsocentric [3] CTOCD-DZ (continuous transformation of the origin of current density; diamagnetic zero variant) method [4,5], now allow the direct visualisation of the induced ring current density in computations of modest cost.…”

The Phenalenyl Motif: A Magnetic Chameleon

“…Exaltation of magnetic susceptibility (L), [24] as estimated from Equation (1), picks up the difference between the carbocyclic systems with diatropic p current (positive values of L), and heterocyclic systems with paratropic p current (negative values of L), but again does not distinguish 12p/14p systems.…”

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

“…A widely accepted criterion of aromaticity for a cyclic system is its ability to sustain a diatropic ring current induced by the presence of a perpendicular external magnetic field [10][11][12][13][14][15][16]. Such currents can be inferred indirectly from their effects on observable magnetic properties (exaltation of magnetisability [17][18][19], downfield chemical shifts of external hydrogen nuclei [20]) and properties that are sub-observables, in Hirschfelder's sense [21], such as the characteristically negative NICS value at the ring centre or some other chosen point [22]. Inferences of this type all have difficulties associated with masking of ring-current effects by other contributions to the property [23][24][25][26][27], but ring-current aromaticity can in fact be deduced directly from calculation of the induced current itself, by solution of the Schrödinger equation for the molecule immersed in the magnetic field, using an appropriate distributed-origin method [28][29][30][31].…”