Characterization of Free Radical Spin Adducts of 5-Diisopropyloxy-Phosphoryl-5-Methyl-1-Pyrroline-N-Oxide Using Mass Spectrometry and 31P Nuclear Magnetic Resonance

Abstract

5-Diisopropyloxy-phosphoryl-5-methyl-1-pyrroline- N-oxide (DIPPMPO) was used to trap a variety of free radicals and the stable compounds generated by the natural decomposition of the initially formed spin adducts were characterized by 31P nuclear magnetic resonance (NMR) and mass spectrometry. Initially, the starting spin trap DIPPMPO was completely characterized using GC-MS and its fragmentation pathway was studied in detail. Then, DIPPMPO was used to trap an oxygen-centered free radical (the hydroxyl radical •OH) and two carbon-centered free radicals (methyl •CH3 and 1-phenyl-ethanol-1-yl •CCH3(OH)Ph radicals). The 31P NMR signals were thus assigned and the structures of adducts were studied and confirmed by mass spectrometry. Overall, the fragmentation pathways of the radical adducts proceed mainly via the loss of the diisopropyloxy(oxido)phosphoranyl radical. For the specific case of trapping·OH radicals, it is possible to visualize the rearrangement of the nitroxide radical adduct to its nitrone form as invoked in the literature. This spin trapping technique, coupled with 31P NMR and MS, provides a tool for the identification of short-lived and low molecular weight free radicals present in a variety of processes.