Spectroscopic Studies of Some Semiquinone Radical Ions

Abstract

Previous ESR and UV spectroscopic studies of semiquinone cations are reviewed. Cations of 4,4'-bipheno-semiquinones prepared by chemical oxidation of the corresponding diols and reduction of the quinones in sulphuric acid, were studied using ESR and UV spectroscopy. Oxidation of biphenyl-4,4'-diol in sulphuric acid leads to either the 4,4'-biphenosemiquinone cation or it's 3,3'-disulphonated derivative, whereas, oxidation in aluminium chloride-nitromethane yields only the parent cation. Reduction of 4,4'-biphenoquinone in dideutero-sulphuric acid initially gives the 4,4'-biphenos emiquinone cation which subsequently undergoes a stepwise exchange of the protons in the 3,3',5 and 5' positions for deuterons. A variable temperature study of this process is reported and discussed. Similar patterns of behaviour are observed in the 1,4-dihydroxybenzene-1,4-benzogquinone system, The 3,3',5,5'—te tra-t—bu ty 1-4, 4'-biphenosemiquinone cation under certain conditions undergoes dealkylation to give secondary radicals. Anions of some alkylated 4,4'-biphenosemiquinones were prepared by the air oxidation of the diols in a variety of solvent mixtures and studied by ESR. The solvent dependance of the hyperfine coupling constants for the 3,3',5,5'-tetramethyl- derivative are reported and discussed in terms of an equilibrium between semi-quinone anions and hydrogen-bonded complexes of such anions with the hydroxylic solvent. The theory of Gendell, Freed and Fraenkel is shown to be quantitatively in agreement with the results. A correlation between the electron-donating power of the solvent and the equilibrium constant for the dissocation of the hydrogen-bonded complexes is also discussed. Molecular orbital calculations were performed for the 4,4'-biphenosemiquinone system and it's 3,3',5,5'- tetramethyl-derivative. Changes in the hyperfine coupling constants between the anion and the cation of 4,4'-biphenosemiquinone are consistent with these qualitative predictions. Several electron donor compounds were treated with 3,3',5,5'-tetrachloro- and 3,3',5,5'-tetrabromo—4,4'— biphenoquinones. Paramagnetic products were only detected in the case of phenothiazine.