The energy surface for isomeric [C2H3O]+ ions: Further experimental evidence

Mass spectra from collisionally activated dissociation (CAD) of C₂H₃O]⁺ ions, including isotopically labeled analogs, provide further information on the isomers CH₃C?O⁺] (a), CH₂?C?O⁺H] (b), ⁺CH₂CH?O] (c) and (d). Our data generally support the recent conclusions from theory by Radom and coworkers and from experiment by Terlouw, Holmes and coworkers. Most acetyl-containing molecular ions form a ions in high purity only at low energies, consistent with isomerization of higher energy molecular ions to form the more stable enol which dissociates to b. Isomer d, prepared from (CICH₂)₂CHOH, undergoes facile hydrogen scrambling, presumably through a degenerate 1,2-hydrogen shift. Theory suggests that c undergoes spontaneous isomerization to a and d; although C₂H₃O]⁺ ions from BrCH₂CHO appear to consist of a and ～15% d, the latter are formed without substantial hydrogen scrambling.