Powder ENDOR Spectra of p-Benzoquinone Anion Radical: Principal Hyperfine Tensor Components for Ring Protons and for Hydrogen-Bonded Protons

Article abstract of DOI:10.1021/ja00274a024

ENDOR spectra for the immobilized p-benzosemiquinone anion radical (BQ^.-) in disordered matrices are presented.Hyperfine interactions of the unpaired electron with three different classes of protons are apparent in the spectra and have been investigated: α-proton, hydrogen-bonded proton, and general matrix proton.Dipolar interactions are not averaged in powder samples, and first derivative ENDOR lines are observed for α- and hydrogen-bonded protons at frequencies which correspond to their principal hyperfine tensor values.Interpretation of the spectra has been facilitated by selective deuteration of the parent quinone and of the solvent.The g anisotropy of BQ^.-, although weak, allows orientation selection experiments at X-band which have provided information on the axis directions for the tensor components relative to the molecular structure.Hydrogen bonding of the BQ^.- carbonyl group to the alcohol hydroxyl group of the solvent is characterized by a purely dipolar interaction exhibiting axial symmetry.The hydrogen bond direction is in the plane of the quinone ring and the O...H bond distance is calculated to be 1.6 Angstroem.The principal hyperfine tensor components of the α-proton interaction are shown to depend critically on the nearest neighbor carbon spin density values which cause the principal values to deviate substantially from those expected for an isolated ^.C-H fragment.For the unpaired electron spin density distribution in BQ^.-, the α-proton hyperfine tensor acquires approximately axial character.In the matrix region, several classes of weakly interacting protons contribute to the structured ENDOR line shape observed; orientation selection and selective deuteration have been used to resolve the origin of some of the lines in this region.The observed ENDOR band shapes for each type of proton-electron interaction indicate that the nuclear relaxation probability (ω_n) is independent of orientation.