TETRAMETHYLENE DICATIONS (CH2)4(2+) - ARE THE NORBORNADIENE DICATION, THE PAGODANE DICATION, AND RELATED SYSTEMS AROMATIC

The (CH2)4(2+) potential energy hypersurface has been investigated by semiempirical and ab initio calculations. The global minimum is an acyclic structure of C(i) symmetry. Harmonic vibrational analysis of the rectangular cyclobutane dication (D2h), which serves as a model for ''sigma-aromatic'' pagodane and norbornadiene dications, indicates three imaginary frequencies. Nonetheless, strong evidence that the pagodane dication has a nonclassical, ''aromatic'' structure of D2h symmetry is presented. A rapid equilibrium of two classical C2v structures and a diradical D2h species, in which both are consistent with the experimental NMR spectroscopic data, can be excluded on the basis of our theoretical investigations. The norbornadiene dication is also predicted to be stabilized by aromaticity, whereas structures with larger distances between the two formal ethylene radical cations are predicted to exhibit increasing diradicaloid character.