Energy Component Analysis of the Pseudo-Jahn−Teller Effect in the Ground State of the Triafulvalene Anion, Pentafulvalene Cation, and Heptafulvalene Anion Radicals

To understand the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis of the total energy was carried out in the ground state of the titled ion radicals by using the MCSCF method with 6-31G(d) basis set. Examination of the energy components comprising in the total energy reveals that in the radicals the stability of a less symmetrical nuclear configuration ( C 2 v ) is attributable commonly to the energy lowering of the internuclear repulsion term and the kinetic and interelectronic repulsion terms due to σ electrons. These observations are consistent with an expansion of the molecular skeleton brought about by the pseudo-JT distortion. In the triafulvalene anion radical, it is further found that the nuclear−electron attractive and interelectronic repulsive terms due to π electrons also contribute to the stability of the C 2 v structure. In the pentafulvalene cation and heptafulvalene anion radicals, on the other hand, the interelectronic repulsive and nuclear−electron attractive terms due to π electrons contribute to the stability of the C 2 v structure, respectively. These differences are accounted for in terms of a charge polarization attributed to the migration of π electrons. Moreover, characteristic electronic properties inherent in the radicals are discussed with much attention to the charge and unpaired spin-density distributions in the distorted C 2 v structure.