Ab initio studies on structure and vibrational spectra of ubiquinone and its radical anion

Abstract

The structure and vibrational spectra of ubiquinone (UQ) and its radical anion (UQ^• has been studied by ab initio MO calculations. For UQ, as in the case of its unsubstituted counterpart p-benzoquinone (p-BQ), calculations show extensive coupling between the C=C and C=O stretching local modes giving rise to four frequencies in 1600-1750 cm⁻¹ region. However, the vibrational structure of these modes are found to be different from that of p-BQ because of a near complete decoupling between the two C=O stretching local modes. For UQ^•, these frequencies are shifted significantly to lower wave numbers. The ring C=C stretching frequencies show noticeable increase. These shifts in the frequencies are explained on the basis of the structural changes associated with the reduction of UQ to UQ^•. Assignment of the vibrational frequencies are made on the basis of PEDs, isotopic shifts and comparison with the experimental results.