Unimolecular Chemistry of •CH2-O-C•=O (Methylenecarboxyldiradical) and HO—CH=C=O (Hydroxyketene) and of the Corresponding Radical Cations and Anions in the Gas Phase

Abstract

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+•.