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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 ρπN 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.
