Interaction between Molecular Oxygen and Nitroxide Radicals: A Search for a Reversible Complex

Dedicated to the memory of Professor Hanns Fischer At high concentrations of oxygen, the EPR spectrum of the nitroxide radical 4-oxo-TEMPO (=4-oxo-2,2,6,6-tetramethylpiperidin-1-yloxy) is found to broaden significantly. In addition to the expected broadening, double integration of the EPR signals indicates that a significant fraction of the nitroxide spins has BdisappearedC. In perfluoro(2-butyltetrahydrofuran) at 273 K, the extent of diminution of the EPR signal intensity is ca. 20%. The results are analyzed in terms of collision and supramolecular com-plexes between oxygen and 4-oxo-TEMPO. It is concluded that a supramolecular complex is responsible for the observed phenomenon. Introduction. – Molecular oxygen exists as a ground-state triplet and possesses a low-lying singlet state [1]. The interaction of triplet O2 as well as singlet O2 in solution with nitroxide radicals, a well-studied class of persistent paramagnetic species [2] [3], has been investigated in great detail in recent years. For example, the EPR spectros-ACHTUNGTRENNUNGcopy of the interaction of triplet O2 with nitroxide radicals has been extensively studied