Rzepa has extended the concept of Möbius aromaticity to homoaromaticity.1 1 is the homoaromatic analogue of the tropylium cation. Topoligical electron density analysis, also known as Atoms-In-Molecules (AIM), indicates no bond path connecting C1and C7. However, the NICS value at the ring critical point of 1 is -11.5 ppm, indicative of aromaticity. 2 is the potential Möbius aromatic analogue of 1. Unlike 1 which has a plane of symmetry, 2 has a C2 rotational axis of symmetry, as anticipated for a Möbius homoaromatic compound. However, there is no bond path connecting C1 with C9. But, the NICS value at the ring critical point of 2 is -11.3 ppm, supporting the notion of aromatic character! Suprisingly, the AIM analysis of the larger homologue 3 does have a bond path connecting C1 to C9, even though the distance separating these compounds is larger than in 2! Again the NICS value for 3 is negative (-9.8) and so it certainly appears to be Möbius homoaromatic.
The B3LYP/aug-cc-pVYZ structures of 1-3 are shown in Figure 1. As is Rzepa’s practice, he provides an extensive collection of data on the molecules he reports making great use of electronic depositories, and it looks like the ACS has now moved this “web-enhanced table” out into the open part of its web site: http://pubs.acs.org/doi/suppl/10.1021/ct8001915/suppl_file/index.html.
Figure 1. B3LYP/aug-cc-pVYZ optimized structures of 1-3.1
(1) Allan, C. S. M.; Rzepa, H. S., "Chiral Aromaticities. A Topological Exploration of Möbius Homoaromaticity," J. Chem. Theory Comput., 2008, 4, 1841-1848, DOI: 10.1021/ct8001915