Aromaticity in macrocycles has been less studied than aromaticity in small polycyclic aromatic hydrocarbons (PAHs). In the work carried out by Dr. Álvaro Muñoz Castro from the Grupo de Química Inorgánica y Materiales Moleculares of the Universidad Autónoma de Chile and by Sílvia Escayola, Dr. Albert Poater, and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona, the authors analyze the most efficient circuits for π-delocalization in a grossly warped nanographene (C80H30), containing five- and seven-membered rings inserted into a six-membered mesh. DFT calculations of different aromaticity indices (FLU, HOMA, EDDB, and ring currents) indicate that one of the two most favorable circuits for π-electron delocalization formally has 50 π-electrons abiding by Hückel’s rule, whereas the second one formally has 75 π-electrons and, remarkably, it does not follow any of the known rules of aromaticity. The nanographene studied display both local aromaticity in the external six-membered rings and macrocyclic aromaticity in the 50 amd 75 π-electrons circuit. This is the first time that a π-electronic circuit has an odd number of electrons, and also that the circuit involves cross-conjugated pathways (ie they do not have alternating single and double bonds).
This finding has been highlighted by Chemistry World, as can be seen in the following link.
S. Escayola, A. Poater, A. Muñoz-Castro* and M. Solà*. An unprecedented π-electronic circuit involving an odd number of carbon atoms in a grossly warped non-planar nanographene. Chem. Commun., 2021, DOI: 10.1039/D1CC00593F