Unexpected Features of the Intramolecular Spin Exchange in Imidazoline Nitroxide Biradicals Dissolved in Ionic Liquids

Abstract

Three imidazoline-type nitroxide biradicals of the similar composition R₅^NO–CH=N–N=CH–R₅^N, B1, R₅^NO–CH=N–N=C(CH₃)–R₅^N, B2, and R₅^N–C(CH₃)=N–N=C(CH₃)–R₅^N, B3, with R₅^N and R₅^NO denoting, respectively, the nitroxide rings 1-oxyl-2,2,5,5-tetramethyl-3-imidazoline and 1-oxyl-2,2,5,5-tetramethyl-3-N–oxide imidazoline, have been studied by X-band electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in these biradicals dissolved in ethanol and the room temperature ionic liquid bmimBF₄ were characterized as a function of temperature by means of the analysis of the EPR lines shape. Thermodynamic parameters of the conformational rearrangements in ethanol were calculated. Analyzing the EPR spectra of these biradicals in bmimBF₄, it was revealed that the two-conformational model does not describe their conformational transitions. Moreover, the observed EPR spectra are not central symmetric especially at low temperatures that cannot be described and explained in the framework of the current theory of the intramolecular spin exchange. Probable reasons of this “strange” behavior are discussed.