Structure and Proton Reactivity of the Semiquinone Anion and Dianion of Biphenol in Water

Whether the redox intermediate state of 4,4‘-biphenol in water, C12H8O2 -•, has a planar or staggered molecular geometry twisted about the inter-ring bond has been examined using time-resolved resonance Raman spectroscopy and electronic structure computation. Hydrogen-bonding effects on the p,p‘-biphenylsemiquinone radical anion (C12H8O2 -•) structure were incorporated in the calculations by associating two water molecules with each oxygen site. The predicted structure, with inter-ring CC bond length of 1.455 Å and dihedral angle of 17.7°, provides a good description of the spectroscopic observations. Hydrogen bonding slightly elongates the CO bonds, but its effect on the inter-ring twist is negligible. Nonequivalence of the two phenyl rings, which are noncoplanar, is indicated by the resonance Raman spectra of C12H8O2 -• in liquid water as well as in ethanol matrix at low temperatures. The π-conjugation between the rings is small in the radical anion, but greater than in its reduced dianion (C12H8O2 2-) state, which signifies the role of torsional distortion of the molecular geometry in redox reactions of biphenyl systems. The change in the calculated CO bond length from the parent dianion to the p,p‘-biphenylsemiquinone radical anion (0.031 Å) is significantly smaller than that for the analogous p-benzosemiquinone radical anion system (0.049 Å) and correlates well with the change in acid−base equilibria. The predictive value and limitations of the theoretical structure calculations of molecular ions and radicals containing the X−C6H4O- (X ≠ O) moiety, which occurs in a wide variety of chemical and biochemical redox systems, are examined.