ESR and 1H NMR studies of a new class of nitroxyl, nitronylnitroxyl and iminonitroxyl radicals

Abstract

AbstractA new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H‐imidazol di‐N‐oxides with O2 or PbO2 in the presence of nucleophilic reagents. This method allowed the preparation of radicals which could not be synthesized by known procedures. These radicals were studied by ESR and 1H NMR spectroscopy. The hyperfine interaction constants of the NNR were measured and assigned (in some cases radicals containing 15N were used). The influence of protonation and deprotonation of functional groups (OH, NH2) in the NNR on their ESR spectra was investigated and the pK values of these radicals were measured by ESR.Differences between the ESR spectra of two isomeric INR were found, and the structures of these radicals were established by 1H NMR. Changes in spin density delocalization in 3‐imidazoline 3‐oxide nitroxyl radicals were studied, and the spin density was found to be ‘pushed out’ to the peripheral fragments of these radicals on increasing the number of methoxy groups in the α‐position to the N—O. fragment. Hindered rotation around the σ‐bond of a bulky substituent in the α‐position to the N—O. fragment was detected by ESR. The activation energy of this hindered rotation was ΔE = 30±1.7 kJ mol−1.