Generation and spectroscopic characterization of the 2,3,5,6-tetramethoxy-1,4-benzosemiquinone reactive intermediate

The 2,3,5,6-tetramethoxy-1,4-benzosemiquinone transient radical is generated from 2,3,5,6-tetrafluoro-1,4-benzoquinone and is detected by EPR spectroscopy. The average isotropic hyperfine coupling constant of its 12 protons, 〈aiso(1H)〉, is very small and results in well resolved spectra. The spectra require unconventional low frequency and amplitude magnetic field modulation to preserve the number of hyperfine splittings and line shapes. The small magnitude of the aiso(1H) is verified by computing its hyperfine tensors using hybrid density functional (HDF) methods. Deuterium isotopic substitution, mass spectroscopy and HDF computations all confirm the structural assignment of this novel radical intermediate.