Properties of Imidazoline Nitroxides

NMR Spectra. Regularities of Long-Range Hyperfine Interactions 

It should be noted that long electron spin-lattice relaxation times (Ti 10 to 10 s) impede recording of the NMR spectra of radicals. Therefore lines in the NMR spectra of radicals become observable only at high rates of electron spin exchange. Spin exchange correlation time Tex is directly proportional to the viscosity rj and inversely proportional to the temperature and radical concentration [R] rj/T[R ]. Therefore, 

Stable imidazoline nitroxides, because of their high solubility in organic solvents, are rather convenient for investigation by NMR. A considerable number of works are devoted to the study of the NMR spectra of these radicals (Sagdeev et a/., 1968, 1973 Volodarsky et a/., 1968, 1969 Molin and Sagdeev, 1972 Sadykov et a/., 1974, 1977). The parameters of the NMR spectra and calculated values of the HFI constants for a large series of imidazoline nitroxides are presented in Table I. 

To a positive paramagnetic shift there corresponds a downfield shift. 

37c and 37c(D), whereas the signal from the other group is almost not shifted at all. 

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