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.

中文翻译：

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稳定氮氧化物的碰撞诱导解离：TEMPO• 和 SG1• 的组合串联质谱和计算研究

两种稳定的氮氧化物，即 2,2,6,6-四甲基哌啶-1-氧基 (TEMPO•) 和 N-叔丁基-1-二乙基膦酰基-2,2-二甲基丙基硝基氧化物 (SG1•) 的解离行为使用不同质谱实验和理论计算的组合研究了质子化分子对碰撞活化的影响。反应产物的元素组成来源于高分辨率串联质谱实验中测量的准确质量数据，基于击穿曲线和 MS3 数据区分初级碎片与次级离子，并进行 H/D 交换实验以支持提议的结构。然后使用标准 B3LYP/6-31G(d) 方法在能量方面研究假定的碎片途径。虽然 TEMPO• 的质子化主要发生在硝酰基官能团的氧原子上，但发现了 SG1• 的一系列原体，加合质子优先位于该磷酸化物质的 P=O 基团上。对于这两种质子化氮氧化物，串联质谱法测量的主要产物离子来自通过消除自由基物种以低能量成本发生的反应。在提高两种前体离子的活化水平时检测到的低丰度次级碎片的形成可以通过高能量过渡态进行的途径合理化。在串联质谱中测量的主要产物离子来自通过消除自由基物种以低能量成本发生的反应。在提高两种前体离子的活化水平时检测到的低丰度次级碎片的形成可以通过高能量过渡态进行的途径合理化。在串联质谱中测量的主要产物离子来自通过消除自由基物种以低能量成本发生的反应。在提高两种前体离子的活化水平时检测到的低丰度次级碎片的形成可以通过高能量过渡态进行的途径合理化。