title: Bonding in nitroxide spin labels from 14N electric-quadrupole interactions.
abstract: Nitrogen nuclear electric quadrupole couplings from FT-EPR of nitroxide spin labels can be used to deduce the covalent transfer πc in the N–O π-bond, and ionicities iσ(NO) and iσ(NC) of the N–O and N–C σ-bonds, if they are combined with the unpaired spin density on the nitrogen ρπ(N) obtained from the dipolar hyperfine couplings. Application to EPR data from an analogue of the MTSSL nitroxide that is used in site-directed spin-labeling demonstrates how environmental polarity and hydrogen bonding are reflected in the bonding parameters of the C–NO–C spin label moiety. Several recent publications erroneously claim to have deduced three independent bonding parameters from nitrogen quadrupole couplings alone.
authors.affiliation: Max-Planck-Institut für biophysikalische Chemie , Am Fassberg 11, 37077 Göttingen, Germany.
authors.forename: Derek
authors.initials: D
authors.lastname: Marsh
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D006860:Hydrogen Bonding; D009584:Nitrogen / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*
chemical_list: D009589:Nitrogen Oxides; D009584:Nitrogen