Frontier Orbital Views of Stacked Aromaticity

Recent studies have theoreticallyand experimentallydemonstratedthat antiaromatic molecules with 4n pi electrons exhibit stackedaromaticity according to pi-pi stacking when arrangedin a face-to-face manner. However, the mechanism of its occurrencehas not been clearly studied. In this study, we investigated the mechanismof stacked aromaticity using cyclobutadiene. When the antiaromaticmolecules are stacked in a face-to-face manner, the orbital interactionsbetween the degenerate singly occupied molecular orbitals (SOMOs)of the monomer unit cause a larger energy gap between the degeneratehighest-occupied molecular orbitals (HOMOs) and the lowest-unoccupiedmolecular orbitals (LUMOs) of the dimer. However, the antiaromaticmolecules are more stable in less symmetric conformations, mainlybecause of pseudo-Jahn-Teller distortions. In the case of cyclobutadiene,the two SOMOs of the monomer unit split into HOMO and LUMO becauseof the bond alternation. When the molecules are stacked in a face-to-facemanner, the HOMO-LUMO gap of the dimer is smaller than thatof the monomer due to the interactions between the HOMOs and LUMOsof the two monomer units. When the monomer units are within a specificdistance of each other, the HOMO and LUMO of the dimer, which correspondto antibonding and bonding between the units, respectively, are interchanged.This alternation of molecular orbitals may result in an increase inthe bond strength between the monomer units, exhibiting stacked aromaticity.We demonstrated that it is possible to control the distance exhibitedby stacked aromaticity by engineering the HOMO-LUMO gap ofthe monomer units.

Automated summary

Recent studies have shown that antiaromatic molecules with 4n pi electrons exhibit stacked aromaticity when arranged in a face-to-face manner. In this study, the mechanism of stacked aromaticity was investigated using cyclobutadiene. It was found that the orbital interactions between the singly occupied molecular orbitals (SOMOs) of the monomer unit play a role in the occurrence of stacked aromaticity. Additionally, the stability of antiaromatic molecules in less symmetric conformations was attributed to pseudo-Jahn-Teller distortions.