Contrasting Behavior of Pentafluorophenoxyacetone and Pentafluorobenzyloxyacetone in Electron Impact and Electron Capture Mass Spectrometry

Towards the goal of finding new ketone electrophores suitable as molecular labels for electrophoric release tags, pentafluorophenoxyacetone (1) and pentafluorobenzyloxyacetone (2) were prepared. Both ketones were evaluated by electron capture (EC) and electron impact (EI) modes of mass spectrometry (MS). By EC-MS, 1 nearly gave a single ion (as desired), whereas 2 gave many ions. This behavior was completely reversed in EI-MS. To account for certain ion fragments in the EC mass spectrum of 2, an anion radical McLafferty-type rearrangement and loss of a carbene neutral were postulated. Electron impact of 1 gave an abundant ion at m/z 117 (C5F3+), which was suggested to be a diyne cation.