Collision-induced dissociation of stable nitroxides: A combined tandem mass spectrometry and computational study of TEMPO • and SG1 •

The dissociation behavior of two stable nitroxides, namely 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO^•) and N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide (SG1^•), subjected as protonated molecules to collisional activation was investigated using a combination of different mass spectrometry experiments and theoretical calculations. Elemental composition of reaction products was derived from accurate mass data measured in high resolution tandem mass spectrometry experiments, primary fragments were distinguished from secondary ions based on both breakdown curves and MS³ data, and H/D exchange experiments were performed to support proposed structures. Postulated fragmentation pathways were then studied in terms of energetic, using the standard B3LYP/6-31G(d) method. While protonation of TEMPO^• mainly occurred on the oxygen atom of the nitroxyl function, a series of protomers were found for SG1^• with the adducted proton preferentially located onto the P=O group of this phosphorylated species. For both protonated nitroxides, major product ions measured in tandem mass spectrometry arose from reactions occurring at low energy costs via elimination of radical species. Formation of secondary fragments that were detected with low abundance when raising the activation level of both precursor ions could be rationalized with pathways proceeding via high energy transition states.