Unimolecular Chemistry of • CH 2 -O-C • =O (Methylenecarboxyldiradical) and HO—CH=C=O (Hydroxyketene) and of the Corresponding Radical Cations and Anions in the Gas Phase

The C₂H₂O₂^+• isomers ^•CH₂–O–C⁺=O (1^+•, methylenecarboxyl radical cation) and HO–CH=C=O^+• (2^+•, hydroxyketene radical cation) are produced in the gas phase and their spontaneous and collision-induced decompositions are compared to those of the known glyoxal radical cation, O=CH–CH=O^+• (3^+•). At threshold, all three ions yield CH₂=O^+• + CO via unique pathways. 1^+• undergoes direct CO rupture with substantial reverse-activation energy, 2^+•, after H-rearrangement to ^•O–CH₂–C⁺=O, loses CO without appreciable reverse-activation energy, and 3^+• eliminates CO via the ion–dipole complex ^+•O=CH₂•••CO. The fragmentations of collisionally-activated 1^+•–3^+• differ substantially, consistent with these ions being distinct C₂H₂O₂^+• radical cations. Charge reversal of 1^+•–3^+• shows that 1^−• and 2^−• are viable radical anions. The stabilities and reactivities of the corresponding neutral species are determined by neutralization of 1^+•–3^+• followed by reionization to either cations (⁺NR⁺) or anions (⁺NR⁻). Diradical 1 is found to be weakly bound by kinetic barriers and dissociates largely in the microsecond time scale to CH₂ + CO₂ and to CH₂=O + CO. In contrast, ketene 2 mainly survives intact within the same time window, decomposing only to a small extent to H^• + ^•O–CH=C=O. The extensive fragmentation observed upon ⁺NR⁺ of 3^+• is shown to occur in the reionization step and primarily reflects the low decomposition threshold of 3^+•.