Fragmentation of the enolate ion of 2,3-butanedione. Characterization of the acetyl anion by charge inversion mass spectrometry

The unimolecular and collision-induced fragmentation reactions of the enolate ion of 2,3-butanedione, CH₃COCOCH₂]^?, have been studied, Unimolecular fragmentation on the metastable ion time-scale forms HCCO]^?, C₂H₃O]^?, C₃H₅O]^? and CH₃CO₂]^?. Charge inversion mass spectrometry shows that the C₂H₃O]^? ion is the acetyl anion while the C₃H₅O]^? product is the acetone enolate ion; formation of the latter product involves a large release of kinetic energy (T _1/2 = 0.99 eV). The fragmentation reactions occurring following collisional activation have been determined for 8 keV collisions and over the range 1.5–30 eV center-of-mass collision energy. Formation of HCCO]^? and CH₃CO]^? are of the most important reactions following collisional activation and it is concluded that the two reactions have similar critical reaction energies even though formation of HCCO]^? is favored thermochemically.