ENERGY COMPONENT ANALYSIS OF THE PSEUDO-JAHN-TELLER EFFECT IN THE BICYCLIC NONALTERNANT HYDROCARBONS - THE PENTALENOID AND HEPTALENOID SYSTEMS

To gain insight into the nature of the pseudo-Jahn-Teller (JT) effect, an energy component analysis has been carried out for the bicyclic no nalternant 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 st ability of a less symmetrical nuclear configuration is largely respons ible for the decrease in the internuclear repulsion energy and the int erelectronic repulsion energy due to sigma electrons. These observatio ns are consistent with an expansion of the carbon skeleton brought abo ut 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 C-2h structure rather than the D-2h one for the pen talene arises from a decrease in the interelectronic repulsion energy due to pi electrons, while that for heptalene results from a lowering of the nuclear-electron attraction energy due to pi electrons. This sh arp distinction between the pentalenoid and heptalenoid systems is acc ounted for in terms of an electrostatic interaction combined with a ch arge relaxation attributed to pi electrons.