Molecular-symmetry Reduction in CmHm−2 Cata-condensed Nonalternant Hydrocarbons

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On the basis of the second-order Jahn-Teller effect and the semiempirical SCF MO method, we examined the energetically most favorable molecular-symmetry groups and geometrical structures with respect to C–C bond lengths of the CmHm−2 cata-condensed nonalternant hydrocarbons. In the 4n π-electron systems the first excitation energies in the fully-symmetrical nuclear arrangements were predicted to be significantly smaller than a certain critical value, and a molecular-symmetry reduction accompanied by a marked double-bond fixation in the peripheral carbon skeleton to occur. On the other hand, in the 4n+2 π-electron systems, such energies were found to be considerably large for small members and to decrease rapidly with the number of carbon atoms, resulting in the molecular-symmetry reduction for members larger than a certain critical size. The electronic spectra were calculated using the most stable geometrical structures obtained by the SCF MO method.
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