Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Möbius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins

An atoms-in-molecules (AIM) and electron localization function
(ELF) critical point analysis is reported for two types of lemniscular
system, each of which exhibits double-half-twist Möbius topology.
This reveals that this type of conformation for [14]annulene 1 has, in addition to the obvious bond critical points (BCPs),
two weaker transannular points in the central cross-over region. These
can be interpreted in terms of local rings showing single-half-twist
Möbius homoaromaticity in addition to the double-half-twist
aromaticity revealed by the annulene as a whole. Another example of
a single-half-twist Möbius homoaromatic 9 is suggested
here to show aromatic properties as strong as its nonhomoaromatic
analogue 8. The AIM critical points in 1 are relatively insensitive to the ring size (varied from 12 to 16),
and only small changes are seen in the critical point properties when
the π-electron count is incremented from 4n+2 to 4n by dianion
formation. These results are discussed in terms of the reported transformation
of the 14-π-electron octalene 10 by reduction/alkylation
into 12, an isomer of 1. Another class of
molecule that exhibits lemniscular topology is the phyrins. A transannular
BCP in the central cross-over region for the double-half-twist aromatic
[26]hexaphyrin 3 is revealed, which is not present for
the double-half-twist antiaromatic [28]hexaphyrin 2.
The NICS(rcp) for the former indicates strong Möbius homoaromaticity.