title: Structural characterization of p-benzosemiquinone radical in a solid state: the radical stabilization by a low-barrier hydrogen bond.
abstract: Semiquinone (p-benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90 K) and EPR data support a dynamical disorder of the proton. A low-barrier O-H...O hydrogen bond stabilizes the radical. The C-O bond length is 1.297 (4) A, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs.
authors.affiliation: Rudjer Bosković Institute, POB 180, HR-10002, Zagreb, Croatia.
authors.forename: Kresimir
authors.initials: K
authors.lastname: Molcanov
authors.forename: Biserka
authors.initials: B
authors.lastname: Kojić-Prodić
authors.forename: Mario
authors.initials: M
authors.lastname: Roboz
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure; D012680:Sensitivity and Specificity
chemical_list: D016227:Benzoquinones; C025232:semiquinone radicals; C004532:quinone