Origin of the Negative Shift of Half-Wave Reduction Potentials of Aromatic Polynuclear p-Quinones with Increasing Conjugation

Abstract

The half-wave reduction potentials (E^red_1/2) of aromatic polynuclear p-quinones are negative shifted with increasing π-electron conjugation, although the E^red_1/2 values of usual organic substances are positive shifted with the above structure change. The main reason for this exceptional behavior of the p-quinones has been discussed in detail by applying the composite system method (linear combination of molecular orbitals (LCMO) approximation). For example, after dividing naphthoquinone into benzoquinone and cis-butadiene we consider the mutual interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the former and those of the latter. It was concluded that the interaction between the LUMO of the former and the HOMO of the latter plays an important role. so that the LUMO of benzoquinone is destabilized by going to naphthoquinone, leading to the negative shift of E^red_1/2 value.