The predominant dissociation process observed for metastable protonated oxalic acid ions HOOC-C(OH)2 + (generated by self-protonation) leads to H3O++ CO + CO2. We have traced the mechanism of this intriguing reaction using the CBS-QB3 model chemistry. Our calculations show that a unique ter-body complex, OCO⋯H3O+⋯CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H2O⋯H⋯OCO]+ and [H 2O⋯H⋯CO]+ which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer OCO⋯H+⋯CO which is formed by water extrusion from the ter-body complex. Arguments are provided that the ter-body complex is also generated in the ion source by the collision encounter between neutral and ionized oxalic acid.

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International Journal of Mass Spectrometry
Department of Neurology

Ervasti, H., Lee, R., Burgers, P., Ruttink, P., & Terlouw, J. (2006). Dissociation of protonated oxalic acid [HOOC-C(OH)2]+ into H3O+ + CO + CO2: An experimental and CBS-QB3 computational study. International Journal of Mass Spectrometry, 249-250, 240–251. doi:10.1016/j.ijms.2005.12.023