| Abstract
| - To elucidate the nature of the pseudo-Jahn−Teller (JT) effect,an energy component analysis has been carriedout for the ground and electronically excited states of the titledcyclic polyenes by using the MCSCF methodwith 6-31G(d) basis set. Examination of the energy componentscomprised in the total energy reveals thatin the ground state of planar cyclobutadiene and cyclooctatetraenemolecules, the stability of a bond-alternatedstructure is largely attributable to a decrease in the internuclearrepulsion energy and the interelectronic repulsionenergy due to σ electrons. These observations are consistentwith a totally symmetric expansion of the carbonskeleton brought about by the pseudo-JT distortion. Concomitantly,a contraction of the π electron cloudtakes place by polarization of the bond charges, and thenuclear−electron attraction energy of π electronsalso plays an important role in the pseudo-JT stabilization.Further, the stability of a nonplanar tub structurein cyclooctatetraene results from a lowering of the nuclear−electronattraction energy. In the excited statesexamined, the situation differs from molecule to molecule. In thelowest excited singlet state of cyclobutadiene,the stability of a rhombic structure originates from the energylowerings of the nuclear−electron attractiveterm of σ electrons and the kinetic term of π electrons. Whilein the lowest excited triplet state of benzene,the stability of a quinoid structure arises from the energy loweringsof the internuclear repulsive term, theinterelectronic repulsive term of σ electrons, and thenuclear−electron attractive term of π electrons.
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