ENERGY COMPONENT ANALYSIS OF THE PSEUDO-JAHN-TELLER EFFECT IN THE GROUND AND ELECTRONICALLY EXCITED-STATES OF THE CYCLIC CONJUGATED HYDROCARBONS - CYCLOBUTADIENE, BENZENE, AND CYCLOOCTATETRAENE

To elucidate the nature of the pseudo-Jahn-Teller (JT) effect, an ener gy component analysis has been carried out for the ground and electron ically excited states of the titled cyclic polyenes by using the MCSCF method with 6-31G(d) basis set. Examination of the energy components comprised in the total energy reveals that in the ground state of plan ar cyclobutadiene and cyclooctatetraene molecules, the stability of a bond-alternated structure is largely attributable to a decrease in the internuclear repulsion energy and the interelectronic repulsion energ y due to a electrons. These observations are consistent with a totally symmetric expansion of the carbon skeleton brought about by the pseud o-JT distortion. Concomitantly, a contraction of the pi electron cloud takes place by polarization of the bond charges, and the nuclear-elec tron attraction energy of pi electrons also plays an important role in the pseudo-JT stabilization. Further, the stability of a nonplanar tu b structure in cyclooctatetraene results from a lowering of the nuclea r-electron attraction energy. In the excited states examined, the situ ation differs from molecule to molecule. In the lowest excited singlet state of cyclobutadiene, the stability of a rhombic structure origina tes from the energy lowerings of the nuclear-electron attractive term of a electrons and the kinetic term of pi electrons. While in the lowe st excited triplet state of benzene, the stability of a quinoid struct ure arises from the energy lowerings of the internuclear repulsive ter m, the interelectronic repulsive term of sigma electrons, and the nucl ear-electron attractive term of pi electrons.