b Analytische Chemie, Universität-GH Siegen, W-5900 Siegen, Germany
The m/z 69 ion generated by electron ionization of some aryl-substituted ß-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. Until recently, it appears to have received little experimental or theoretical attention. For the proposed structure, OCCHCO+, its resistance to unimolecular dissociation to HCCO+ and CO, combined with available thermochemical data, places the upper limit for its
Hf at ~987 kJ/mol, and a lower limit for the proton affinity (PA) of C3O2 at 453 kJ/mol. However, based on comparisons with representative molecules, the PA of C3O2 is certainly much higher, i.e. ~750 - 900 kJ/mol at 298 K, and, therefore, for OCCHCO+, Hf(gas, 298 K) is in the range 538 - 663 kJ/mol. This range of values leads to favorable thermochemistry for its production in the EI mass spectrum of acetylacetone, but, irrespective of its actual Hf, its formation is thermochemically favored over the observed ions in the mass spectrum.
In the case of aryl-substituted ß-diketones also containing a CF3 substituent (isobaric with OCCHCO+) the OCCHCO+ ion still dominates. A similar observation applies to the EI mass spectra of transition metal aryl-substituted ß-diketonates.
These observations will be discussed in relation to empirical estimates and computational quantum chemistry calculations of the thermochemistry of the ion formation and decomposition reactions, and an interpretation of the results will be offered.