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

We have studied the changes in the aromatic nature of two cyclobutadiene (C 4 H 4 ) 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 C 4 H 4 rings changes from a strongly antiaromatic character at long distances to an aromatic transition state of stacked C 4 H 4 rings at intermediate internuclear distances when approaching the antiaromatic state of cubane.