Electron-acceptor effect of aminoxyl radicals

Abstract

Acidic properties of several compounds with aminoxyl-based spin labels were found to be abnormally strong when compared with those of diamagnetic analogues. Analysis of this phenomenon in terms of both Taft constants and Hinze–Jaffe–Huheey group electronegativities enabled us to attribute it to the strong electron-acceptor effect of the aminoxyl group that results in high electronegativity of aminoxyl-containing substituents. Quantitative estimates of this effect for two spin labels were obtained. Group electronegativity of imidazoline-based spin label is close to that of the -CF₃ substituent, while the electron-acceptor effect of piperidine-based spin label is intermediate between those of -CHF₂ and -CH₂F groups. The strong electron-acceptor effect of aminoxyl spin labels should be taken into account when using them in biological and other studies.