On the antiaromatic-aromatic-antiaromatic transition of the stacked cyclobutadiene dimer

We have studied the changes in the aromatic nature of two cyclobutadiene (C4H4) molecules on decreasing the intermolecular distance and approaching the cubane structure in a face-to-face fashion. The analysis based on the calculations of the magnetically induced current density and the induced magnetic field shows that the aromaticity of the two C4H4 rings changes from a strongly antiaromatic character at long distances to an aromatic transition state of stacked C4H4 rings at intermediate internuclear distances when approaching the antiaromatic state of cubane.