Determination of the Spin Distribution in Nitronylnitroxides by Solid-State 1H, 2H, and 13C NMR Spectroscopy

Nitronylnitroxide radicals substituted by 2‘-hydroxy-, 3‘-hydroxy-, 4‘-hydroxy-, and 4‘-methoxyphenyl groups and by a methyl group (2, 3, 4, 5, and 6, respectively) have been investigated in the solid state with 1H, 2H, and 13C NMR spectroscopy under magic angle spinning (MAS) and in solution with 1H and 2H NMR spectroscopy. Well-resolved 13C MAS NMR spectra have been recorded which show signals in ranges up to almost 1900 ppm. The 1H NMR signal spread does not exceed 60 ppm (except for a unique methyl signal of 6 near −230 ppm), and the resolution is worse than that of 13C NMR spectra. Narrower signals have been obtained with 2H NMR spectroscopy. The signals have been assigned with the help of various criteria including the results of ab initio calculations. From the NMR data it has been concluded that the nitronylnitroxide five-ring is puckered, thus rendering all substituents inequivalent. Signal coalescence at elevated temperatures has shown that these inequivalencies are partly averaged out while the puckering is maintained. Dynamics of this sort have not been observed when OH···ON bridges render the lattice more rigid. The spin densities at the carbon and hydrogen atoms have been obtained from the NMR data. They are in accord with theoretical results in the limit of approximation. The spin maps so obtained confirm that the polarization mechanism determines the spin distribution in 2−6. The distribution is modulated by hyperconjugation and competing polarization paths. It is concluded that solid-state NMR spectroscopy may be an alternative to polarized neutron diffraction in evaluating the spin distribution in radicals. The weak and strong points of the method are discussed.