engleski

Structural characterization of p-benzosemiquinone radical in a solid state: the radical stabilisation by a low-barrier hydrogen bond

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 proton. A low-barrier O-H...O hydrogen bond stabilises the radical. The C-O bond length is 1.297(4) A, corresponding to bond order of about 1.5. The geometry of the radical implies an electron delocalisation throughout a 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.

semiquinone ; radical ; low-barrier hydrogen bond ; EPR spectroscopy ; variable-temperature X-ray diffraction

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