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

Abstract

To elucidate the nature of the pseudo-Jahn-Teller (JT) effect, an energy component analysis has been carried out for the ground and electronically 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 planar 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 energy due to σ electrons. These observations are consistent with a totally symmetric expansion of the carbon skeleton brought about by the pseudo-JT distortion. Concomitantly, a contraction of the π electron cloud takes place by polarization of the bond charges, and the nuclear-electron attraction energy of π electrons also plays an important role in the pseudo-JT stabilization. Further, the stability of a nonplanar tub structure in cyclooctatetraene results from a lowering of the nuclear-electron attraction energy. In the excited states examined, the situation differs from molecule to molecule. In the lowest excited singlet state of cyclobutadiene, the stability of a rhombic structure originates from the energy lowerings of the nuclear-electron attractive term of σ electrons and the kinetic term of π electrons. While in the lowest excited triplet state of benzene, the stability of a quinoid structure arises from the energy lowerings of the internuclear repulsive term, the interelectronic repulsive term of σ electrons, and the nuclear-electron attractive term of π electrons.

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

“…The data available for these structures are not extensive, but our values are in good agreement with the MCSCF values of Koseki and Toyota. [52] The frequency analysis revealed that 6 t is a true minimum, while 6 t' has three imaginary frequencies and lies 0.3 kcal mol À1 above. We only discuss the ELF p results of 6 t herein.…”

How do the Hückel and Baird Rules Fade away in Annulenes?

“…The former is characterized by two clear double bounds separated by two radical C atoms, whereas the AQ shows two allylic structures merged by two single C-C bonds. The energy difference between the two conformations is below 1 kcal/mol and they are connected by a transition state with an energy barrier of 2.5 kcal/mol [78]. Both conformers are expected to be antiaromatic, displaying an energy destabilization that is responsible for their prominent photochemical reactivity [79].…”

The excited state antiaromatic benzene ring: a molecular Mr Hyde?

“…3 1-Q is lower than 3 1-AQ by merely 0.7 kcal mol À1 , and the D 6h symmetric structure which is the transition state for the interconversion is 2.5 kcal mol À1 above 3 1-Q at the MCSCF/6-31G(d) level. 59 In the S 1 state it has been found experimentally that benzene still has D 6h symmetry but with CC bonds which are elongated to 1.432-1.435 Å. 60,61 Interestingly, the drive to these structural changes is also notable in the extent of electron sharing between pairs of C atoms when benzene is calculated in the T 1 and S 1 states at the hexagonal structure (vide infra).…”