A quantum chemical investigation of structures,vibrational spectra and electron affinities of the radicals of quinone model compounds

In the present study, the first quantum chemical calculations of structures and vibrational spectra of radicals of 1,4-naphthoquinone and 2-methoxy-1,4-benzoquinone that account for electron correlation are presented. In the case of 1,4-naphthoquinone a good agreement between calculated vibrational frequencies and ¹⁸O-shifts of the 1,4-naphthoquinone radical (protonated radical anion) with experimental data of a species detected after irradiation of vitamin K₁ in solution is found. Our calculations, thus, support the previous assignment. In the case of 2-methoxy-1,4-benzoquinone we have localized the stable conformations with respect to the orientation of the methoxy group and we have determine the harmonic force fields for these structures. Our calculation suggest that, while the frequencies of the two conformers are similar, the ¹⁸O-shift of the most intensive absorptions in the spectral region between 1400 and 1700 cm^–1 of the two conformers differ significantly and might serve as a tool to distinguish between the two conformers. The applied DFT method is shown to predict electron affinities which are systematically underestimated by 10%.