Electron density analysis of bent aromatic molecules: intramolecular interactions in small paracyclophanes

The electron distribution of molecules with distorted aromatic rings was analyzed using theoretical methods. The molecular geometries of Dewar benzene, the series of [n] paracyclophanes (n = 2,..., 6), and [1.1] and [2.2] paracyclophane were optimized at the QCISD/631++G(d, p) approximation. The partition of three-dimensional space provided by the quantum theory of atoms in molecules was applied using the electron densities obtained at this highly correlated level of theory. The analysis shows that Dewar benzene and [2] paracyclophane belong in a separate family. In the other cases, the distortion of the benzene moiety provokes a charge transfer to the phenylene group and induces a moderate single bond-double bond alternation that yields some decrease in the aromaticity of the carbon backbone. The charge concentrations accounted for by the Laplacian of the electron density and the quadrupole polarization of the ipso C atoms provide an explanation for their reactivity in electrophilic substitution reactions. The steric strain in paracyclophanes was analyzed in terms of the forces exerted on the electron density, the Ehrenfest forces. This analysis did not provide any evidence of repulsive forces taking place in the molecules. In particular, the aromatic rings in [2.2] paracyclophane are highly aromatic, with an important electron delocalization between aromatic rings and forces that are always attractive.