Symmetry Rule for Predicting Bond Distortions in Conjugated Hydrocarbons

A symmetry rule for predicting molecular shapes which is based on the second-order Jahn-Teller effect has been applied to the predictions of bond distortions in a number of nonalternant hydrocarbons in their ground and electronically excited states. In spite of the very crude approximation that only the lowest-lying excited state plays a dominant role in determining the energetically most favorable nuclear displacement, a fairly clear-cut criterion for molecular-symmetry reduction was obtained. The actual types of the most soft bond distortions, determined by examining the two-center components of transition densities, are in good agreement with the results of variable bond-length SCF MO calculations and available experimental results. The problem of symmetry reductions possible in large cyclic polyenes C4n+2H4n+2 is briefly discussed.