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

The interesting unimolecular dissociation chemistry of dimethyl oxalate (DM O) ions, CH3O-C(=O)-C(+O)-OCH3.+, has been studied by vacuum ultraviolet ph otoionization and tandem mass spectrometry based experiments. The measured appearance energy (AE) for the generation of CH3O-C=O+ (10.5 eV) is not com patible with a simple bond cleavage involving the cogeneration of the radic al CH3O-C=O-. whose calculated AE is 11 kcal/mol higher. However, because t he CH3O-C=O-. radical is thermodynamically less stable than its dissociatio n 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 induce d dissociative ionization experiments clearly show that low energy DMO ions dissociate into CH3. + CO2 without the intermediacy of CH3O-C=O-.. Experim ents using a charged collision chamber further indicate that CO, is release d first, followed by loss of CH3. and not vice versa and a mechanism is pro posed. 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 sh ift caused by a prominent low energy decarbonylation reaction yielding the hydrogen bridged radical cation CH2=O ... H ... O=C-OCH3.+. However, from m etastable ion observations and AE measurements on deuterium labeled DMO ion s, it follows that there is no competitive shift and that the elevated AE f or the two-step process corresponds to the barrier for the first step, loss of CO2. Finally, neutralization-reionization experiments on ionized DMO an d CH3O-C=O+ provide evidence for the existence of CH3O-C=O-. as a kinetical ly stable radical. (J Am Soc Mass Spectrom 1999, 10, 869-877) (C) 1999 Amer ican Society for Mass Spectrometry.