Magnetically Induced Current Densities in Aromatic, Antiaromatic, Homoaromatic, and Nonaromatic Hydrocarbons

Abstract

The magnetically induced current densities for ring-shaped hydrocarbons are studied at the density functional theory (DFT) and second-order Møller-Plesset (MP2) levels using gauge-including atomic orbitals. The current densities are calculated using the gauge-including magnetically induced current approach. The calculations show that all studied hydrocarbon rings sustain strong diatropic and paratropic ring currents when exposed to an external magnetic field, regardless whether they are unsaturated or not. For nonaromatic rings, the strength of the paratropic current flowing inside the ring is as large as the diatropic one circling outside it, yielding a vanishing net ring current. For aromatic molecules, the diatropic current on the outside of the ring is much stronger than the paratropic one inside, giving rise to the net diatropic ring current that is typical for aromatic molecules. For antiaromatic molecules, the paratropic ring-current contribution inside the ring dominates. For homoaromatic molecules, the diatropic current circles at the periphery of the ring. The ring current is split at the CH(2) moiety; the main fraction of the current flow passes outside the CH(2) at the hydrogens, and some current flows inside the carbon atom. The diatropic current does not take the through-space short-cut pathway, whereas the paratropic current does take that route. Calculations of the ring-current profile show that the ring current of benzene is not transported by the pi electrons on both sides of the molecular ring. The strongest diatropic ring current flows on the outside of the ring and in the ring plane. A weaker paratropic current circles inside the ring with the largest current density in the ring plane. Due to the ring strain, small unconjugated and saturated hydrocarbon rings sustain a strong ring current which could be called ring-strain current. Nuclear magnetic shieldings calculated for 1,3,5-cycloheptatriene and homotropylium at the DFT and MP2 levels agree well with experimental values.

Ring Currents in Benzo‐ and Benzocyclobutadieno‐Annelated Biphenylene Derivatives

“…Jmax can be used as a measure of the bond current strength and was successfully applied in a series of recent studies. [41][42][43] Calculations of ring currents were performed using inhouse Fortran routines, GAMESS-UK [44] and SYSMO [45] .…”

Magnetically Induced Currents in [n]Cycloparaphenylenes, n = 6−11

“…24,34 There is a positive net current of the magnitude 2-5 nA/T in the phenylene rings of the nanohoops with n = 7-11. The phenylene rings are most planar in [11]CP, which can explain the improved electron delocalization in the phenylene moieties of the larger rings. Previously it was shown that benzene rings fused to antiaromatic molecules sustain a net current while the benzene rings fused to overall aromatic molecules only split the current which is delocalized over the whole molecule.…”

“…The other dianionic nanohoops with eight or more phenylene rings have a current susceptibility strength of about 35 nA/T and possess a similar structural stabilization, while [10]CP 2-forms an exception regarding both the structure and the ring current. The largest dianionic nanohoop [11]CP 2-does display a slightly smaller bond length alternation and smaller staggering angles between the phenylene rings as compared to the neutral [11]CP, and consequently the ring current is equally divided along the edges of the phenylene rings.…”

Modern Valence-Bond Description of Homoaromaticity

“…The various features of the SC(6) wavefunctions for 2 and 2-TS that are displayed in Figures 5 and 6 and in Table 4 indicate that the electronic structures of both of these systems resemble that of 1,3,5-hexatriene. This finding implies that both of 2 and 2-TS are non-aromatic whereas, according to Sundholm et al, 23,34 the ring-current strength in 2, calculated using the gauge-including magnetically induced current method (GIMIC), 34 is about half the benzene value, suggesting that 2 is half as aromatic as benzene. The SC results reported here show instead that the differences between the aromaticities of 2 and benzene are much more substantial.…”