The Molecular-Symmetry Reduction in Open-Shell Conjugated Hydrocarbons

On the basis of the second-order Jahn-Teller theorem, we propose a criterion for predicting the stable groundstate geometrical structures of open-shell conjugated hydrocarbons. It is found that in contrast with the cases of the parent closed-shell hydrocarbons no molecular symmetry reduction occurs in the ion radicals of the CmHm–2 catacondensed nonalternant hydrocarbons, such as pentalene and heptalene. On the other hand, it is revealed that the ion radicals of fulvalene systems suffer the molecular symmetry reduction from D2h to C2v. In addition, the electronic spectra were calculated using the stable geometrical structures obtained by use of the semiempirical open-shell SCF MO method.