AB-INITIO MCSCF STUDY ON ELECTRONICALLY EXCITED SINGLET-STATES OF FULVALENE SYSTEMS - ENERGY COMPONENT ANALYSIS OF THE PSEUDO-JAHN-TELLER EFFECT

Geometry optimizations of the lowest (second) excited singlet states o f the fulvalene systems were carried out by using the ab initio MCSCF method with the 6-31G(d) basis set. The electronically excited molecul es are found to undergo a pseudo-Jahn-Teller (JT) distortion from D-2h to C-2 nu On the basis of the results obtained, an energy component a nalysis of the total energy has been carried out to elucidate the phys ical picture of the pseudo-JT effect. Inspection of the energy compone nts comprised in the total energy reveals that the stability of a less symmetric structure (C-2 nu) is attributable commonly to the energy l owerings of the intenuclear repulsion term and the interelectronic rep ulsive and kinetic terms due to sigma electrons. These observations ar e consistent with an expansion of the carbon skeleton brought about by the pseudo-JT bond distortion. It is further found that in triafulval ene and heptafulvalene the nuclear-electron attractive and interelectr onic repulsive terms due to pi electrons contribute to the pseudo-JT s tabilization, respectively. In pentafulvalene and 7,7'-bis(bicyclo[4. 1.0]hepta-1,3,5-trienylidene), on the other hand, both the nuclear-ele ctron attractive and interelectronic repulsive terms due to pi electro ns contribute to the energetic stability of the C-2 nu structure. Subs equently, heptafulvalene undergoes a pseudo-JT effect from the planar C-2 nu to a nonplanar C-s structure. Interestingly, the stability of n onplanar C-s structure results from the energy lowering of the nuclear -electron attractive term. Moreover, the structural and electronic pro perties inherent in the less symmetric C-2 nu and C-s nuclear arrangem ents of the electronically excited molecules are discussed.