Paramagnetic Resonance of Alkyl Nitroxides

Abstract

Methods of preparation and the electron paramagnetic resonance spectra are given for the three prototype alkyl nitroxide free radicals: H2NO, CH3(H)NO, and (CH3)2NO. The sigma—pi interaction theory of Karplus and Fraenkel for 13C, which has been applied extensively to 14N hyperfine splitting, is examined critically by the application of linear regression to 41 14N-containing free radicals. Three different sets of 14N sigma—pi interaction parameters are found which reproduce the experimental 14N coupling constants to an equally high degree of accuracy. Consequently, the prediction of experimental coupling constants within the order of magnitude of solvent effects (5%—10%), should not be considered a satisfactory test of a set of theoretical 14N interaction parameters, since the parameter values can fall within a wide range and still fulfill the test requirement. The parameter SN does not appear to play an important role in the sigma—pi interaction of these radicals.