Five different anionic [C 3 ,H 4 ,O] •Ϫ isomers, i.e. the radical level of theory. For some species, notably the singlet state of oxyallyl, the theoretical treatment is complemented by G2, anions of acrolein, acetyl carbene, formyl methyl carbene, methoxy vinylidene, and oxyallyl are generated in an ion CASSCF, and MR-CI calculations. Theory and experiment are in good agreement in that at the neutral stage (i) acrolein beam mass spectrometer and subjected to neutralizationreionization (NR) mass spectrometric experiments including does not react within the µsec timescale, (ii) acetyl and formyl methyl carbenes isomerize to methyl ketene, (iii) methoxy neutral and ion decomposition difference (NIDD) mass spectrometry; the latter allows for the examination of the neutralsЈ vinylidene rearranges to methoxy acetylene, (iv) singlet 1 A 1 oxyallyl undergoes ring closure to cyclopropanone, and (v) unimolecular reactivity. Further, the anionic, the singlet and triplet neutral, and the cationic [C 3 ,H 4 ,O] •Ϫ/0/•+ potential-triplet 3 B 2 oxyallyl may have a lifetime sufficient to survive a NR experiment. energy surfaces are calculated at the B3LYP/6-311++G (d,p)