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 …