Aromatic Pathways in Twisted Hexaphyrins

Abstract

The aromatic pathways and the degree of aromaticity of expanded porphyrins have been determined by explicit calculations of the routes and strengths of the magnetically induced currents using the gauge-including magnetically induced current (GIMIC) approach. Density functional theory calculations show that the doubly twisted hexaphyrins fulfilling Hückel's (4n + 2) pi-electron rule for aromaticity and those obeying the 4n pi-electron rule for antiaromaticity are aromatic and antiaromatic, respectively. The investigated [26]hexaphyrin (2) and (3) and [30]hexaphyrin (5) isomers are aromatic, and [28]hexaphyrin (4) is antiaromatic. The formally antiaromatic [24]hexaphyrin (1) does not sustain any strong ring current and must be considered nonaromatic. A detailed analysis of the current pathways of the hexaphyrins is presented. It was found that the current pathways of the investigated aromatic hexaphyrins are not always dominated by the flow along the inner route through the non-hydrogenated C-N-C moieties, as previously proposed. The current flow is often split into two branches at the pyrrole rings, but sometimes it takes the outer route via the C=C bond of the pyrrole. The current pathway of the weak paratropic ring current of [24]hexaphyrin is dominated by the outer C=C route. The calculations show that the routes of the current transport cannot be assessed merely by inspection or from nucleus independent chemical shifts; explicit calculations of the current pathways are compulsory. The current-density studies also show that the pyrrole rings do not sustain any strong ring currents of their own.

Hydrogen–hydrogen bonding: The current density perspective

“…The choice of the orientation of the field is rather critical. In a study on systems with a net ring current strength, the orientation of the field has been chosen to maximize or minimize the ring current strength itself . The choice of the orientation of the field for the characterization of pair current strengths in acyclic moieties, presumably characterized by incoming and outgoing local currents of equal magnitude, should be such that no nearby atoms perturb significantly the pair‐localized circulation.…”

The vorticity of the current density tensor and 3D‐aromaticity

“…Normally, B is chosen parallel to the main symmetry axis, if it exists. However, this selection is arbitrary and plenty different directions of B should be evaluated to find the field that produces the strongest ring currents . Matías et al found difficulties to find significant differences on current density maps of two isomers (one 3D‐aromatic and the non 3D‐aromatic) of fullerene C 50 because of the difficulty of finding the plane with the most important currents .…”

The influence of benzene rings on aromatic pathways in the porphyrins

“…However, carrying out the explicit integration of the magnetically induced ring‐current density over the cut plane area, crossing the bonds of interest normally, the ring‐current can be determined . This technology makes it possible to determine current pathways in large macrocycles as well . Recently, the calculations of the current density for Möbius twisted hexaphyrins demonstrated that the aromatic and antiaromatic properties of the molecules depend on the geometries and the total number of π electrons .…”

“…This technology makes it possible to determine current pathways in large macrocycles as well . Recently, the calculations of the current density for Möbius twisted hexaphyrins demonstrated that the aromatic and antiaromatic properties of the molecules depend on the geometries and the total number of π electrons . In the work of Fliegl et al, the calculations of the current densities of transporphyrins gave some new information about possible current pathways.…”

“…In this work, the calculation of current density confirmed the antiaromatic nature of isophorins because the contribution of diamagnetic current dominates in their total ring current. It should be noted that the density functional theory (DFT), where B3LYP exchange‐correlation (XC) functional does not depend on the current‐density, is used in the works . However, in the presence of a magnetic field, the XC functional also becomes dependent on the current‐density, and in such situations, current‐DFT (CDFT) is most appropriate .…”