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

Abstract

A new synthesis of nitronylnitroxyl (NNR), iminonitroxyl (INR) and nitroxyl radicals (NR) is suggested, involving oxidation of 4H-imidazol di-N-oxides with 0, or PbO, 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 ' H 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, NH,) 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 ' H 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 a-position to the N-0' fragment. Hindered rotation around the a-bond of a bulky snbstituent in the a-position to the N-0' fragment was detected by ESR. The activation energy of this hindered rotation was AE = 30 f 1.7 kJ mo1-l.

EPR and Quantum Chemical Studies of the pH-sensitive Imidazoline and Imidazolidine Nitroxides with Bulky Substituents

“…A similar observation of proton hyperfine splitting was published by Khramtsov et al [1] for 5,5-dimethyl-2-methoxy-2-(2-methoxycarbonylethyl)-4-phenyl-2,5-dihydroimidazole-1-oxyl 3-oxide [4]. The authors [1] were able to measure the splitting constant of about 0.07 mT assigned to hyperfine interaction with one of the methylene protons of the (CH) 2 COOCH 3 group.…”

“…The nitroxide radicals of imidazoline and imidazolidine types were shown to reveal the changes in their electron paramagnetic resonance (EPR) spectra due to reversible protonation of N-3 nitrogen [17] of the radical heterocycle and are widely used as pH-probes [4, 5, 14, 15]. However, instability of the paramagnetic N–O· fragment towards chemical reduction is an unavoidable factor that significantly limits their application, particularly in biological systems.…”

Reversible Reactions of Thiols and Thiyl Radicals with Nitrone Spin Traps

“…Nevertheless, the analogous breakdown of the series of 3-imidazoline-3-oxide nitroxides with formation of dinitrone 4H-imidazole di-N-oxides was previously described. 52,53 In another report, Karoui et al 16 explained a time-dependent decrease in DEPMPO/GS • adduct with a parallel increase in DEPMPO/CO 2 • adduct upon the photolysis of GSNO, by the formation of CO 2 •radical in the reaction (7) of formate with GS • . The simple kinetic estimates show that in the presence of 20 mM DEPMPO and 400 mM formate, the latter reaction can hardly compete with GS • scavenging by DEPMPO (the ratio k 7 [HCOO -]/k 9b [DEPMPO] ≈ 1.3 × 10 -3 if k 7 ) 10 4 M -1 s -1 , 16 or it is ≈0.1 if k 7 ) 8 × 10 5 M -1 s - 1 23 ).…”

2 mm Electron paramagnetic resonance studies of the new types of imidazoline nitroxide radicals

“…The appearance of such groups at the carbon a-atom causes a substantial change in the isotropic hfi constant with the nitrogen nucleus and a change in the radical g-factor determined from EPR spectra of radicals in solution. 6 More detailed information about the influence of the 1 5 ~…”