Density functional study on the structures and vibrational spectra of the radical anion and cation of biphenyl

Density functional theory using the 6-31G* basis set and two non-local exchange-correlation functionals (B-YLP and B3-LYP) has been applied to the calculations of the structures and vibrational spectra of the radical anion and cation, of biphenyl. The B3-LYP dihedral angles between the two phenyl rings are 5.8° and 19.5° for the radical anion and cation, respectively. Upon ionization, structural changes occur in the direction from benzenoid to quinoid. The unscaled vibrational frequencies calculated at the B-YLP/6-31G* level are mostly in agreement with the frequencies in the Raman spectra of the radical anion and cation of the normal and perdeuterated species reported in the literature. The calculations predict that both the radical anion and cation give rise to a very strong infrared band, and that this band arises from a mode in which the benzenoid-to-quinoid deformation occurs out of phase in the two phenyl rings.