Energy Component Analysis of the Pseudo-Jahn−Teller Effect in the Bicyclic Nonalternant Hydrocarbons: The Pentalenoid and Heptalenoid Systems

Abstract

To gain insight into the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis has been carried out for the bicyclic nonalternant hydrocarbons termed pentalenoid and heptalenoid systems by using the ab initio RHF method with 6-31G(d) basis set. Inspection of the energy component comprised in the total energy reveals that the stability of a less symmetrical nuclear configuration is largely responsible for the decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to σ electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT distortion. Another energy component also plays an essential role in the pseudo-JT stabilization: For the examples, the preference for the C2h structure rather than the D2h one for the pentalene arises from a decrease in the interelectronic repulsion energy due to π electrons, while that for heptalene results from a lowering of the nuclear−electron attraction energy due to π electrons. This sharp distinction between the pentalenoid and heptalenoid systems is accounted for in terms of an electrostatic interaction combined with a charge relaxation attributed to π electrons.