Electronic structures of some cata-condensed nonalternant hydrocarbons: pentalenopentalenes, azulenoazulenes, and pentalenoazulenes

On the basis of the second-order Jahn-Teller theory and the semiempirical SCF MO method, we have examined the energetically most favorable ground-state geometrical structures with respect to C-C bond lengths of some cata-condensed nonalternant hydrocarbons consisting of five- and seven-membered rings. It is predicted that both the geometrical valence isomers of pentalenopentalenes, 1 and 2, should suffer molecular-symmetry reduction, exhibiting a marked double-bond fixation in the peripheral carbon skeleton, while in azulenoazulenes, 3 and 4, no molecular-symmetry reduction occurs. Tne electronic spectra have been calculated using the energetically favorable geometrical structures obtained by the SCF MO calculations. The theoretical values for azuleno[1, 2-b]azulene (3) are in good agreement with experimental data.