Generation of the m/z 69 ion C3HO2+ by electron ionization of some aryl-substituted β-diketones

The m/z 69 ion, prominent in the electron ionization (EI) mass spectra of some aryl-substituted β-diketones, has been shown to have the composition C3HO2+ by accurate mass determination, identification of its precursor ions, and of its decomposition product ions. In contrast, it is a minor ion in the EI mass spectrum of acetylacetone, which lacks an aryl substituent but otherwise has the correct structural features to produce this ion. Until recently, this species appears to have attracted little experimental or theoretical attention. The proposed structure, OCCHCO+, is supported by its collisionally activated dissociation to m/z 41, HCCO+, and by ab initio quantum chemical calculations. Available thermochemical data predict the thermo-chemistry to be unfavorable for its production in the EI mass spectrum of benzoylacetone, but favorable in the case of acetylacetone, contrary to the experimental observations. Even in the case of aryl-substituted β-diketones also containing a CF3 substituent (isobaric with OCCHCO) the m/z 69 ion is predominantly OCCHCO+. Since this ion is an inherently stable species it is evident that its ready formation requires precursors having suitable structural features; a mechanistic pathway to its formation from aryl-substituted β-diketones is suggested.