Detection and identification of various carbon-centred free radicals using N-arylketonitrones: a spin trapping/EPR/MS study

Three N-aryl-C,C-dimethoxycarbonylnitrones were shown to be efficient tools to detect and identify carbon-centred free radicals by coupling the spin trapping method with both EPR detection and MS/MS structure elucidation. The fragmentation pathway of these ketonitrones was first studied by MS/MS. Then, these compounds were used to trap a series of four carbon-centred free radicals and the so-formed spin adducts were analysed by the means of two techniques. EPR spectroscopy allowed to easily detect their presence in the medium, while their structure elucidation was performed using tandem mass spectrometry. The tert-butyl radicalspin adducts showed very weak EPR signals and could not be detected by MS, probably because of a sensitivity issue. In the other cases, collision-induced dissociation of the various spin adducts mainly proceeds via three pathways, consisting of the elimination of the arylnitroso fragment, the radical initially trapped, or the methoxycarbonyl radical. The relative rate of these dissociations was observed to highly depend on the nature of the radical trapped. MS/MS analysis of the spin adducts allows unambiguous identification of the addends. This study also showed an unexpected reactivity of these ketonitrones towards ˙CH3, resulting in the formation of EPR-silent methoxyamines. All these results demonstrate the potential of N-arylketonitrones in the identification of short-lived free radicals by the means of the spin trapping/EPR/MS technique.