How do dimethyl oxalate ions CH3O-C(=O)-C(=O)-OCH3·+ break in half? Loss of CH3· + CO2 versus CH3O-C-O·

The interesting unimolecular dissociation chemistry of dimethyl oxalate (DMO) ions, CH3O-C(=O)-C(=O)-OCH3·+, has been studied by vacuum ultraviolet photoionization and tandem mass spectrometry based experiments. The measured appearance energy (AE) for the generation of CH3O-C=O+ (10. 5 eV) is not compatible with a simple bond cleavage involving the cogeneration of the radical CH3O-C=O· whose calculated AE is 11 kcal/mol higher. However, because the CH3O-C=O· radical is thermodynamically less stable than its dissociation products CH3· and CO2, by 19 kcal/mol, a two-step dissociation of ionized DMO into CH3O-C=O+ + CH3· + CO2 is energetically feasible. Collision induced dissociative ionization experiments clearly show that low energy DMO ions dissociate into CH3· + CO2 without the intermediacy of CH3O-C=O·. Experiments using a charged collision chamber further indicate that CO2 is released first, followed by loss of CH3· and not vice versa and a mechanism is proposed. The measured AE, which we assign to the two-step process, is 8 kcal/mol higher than the calculated value. This could be due to a competitive shift caused by a prominent low energy decarbonylation reaction yielding the hydrogen bridged radical cation CH2=O … H … O=C-OCH3·+. However, from metastable ion observations and AE measurements on deuterium labeled DMO ions, it follows that there is no competitive shift and that the elevated AE for the two-step process corresponds to the barrier for the first step, loss of CO2. Finally, neutralization-reionization experiments on ionized DMO and CH3O-C=O+ provide evidence for the existence of CH3O-C=O· as a kinetically stable radical.