Dissociation of protonated oxalic acid [HOOC-C(OH)2]+into H3O++CO+CO2:: An experimental and CBS-QB3 computational study

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, O=C=O center dot center dot center dot H3O+center dot center dot center dot CO, plays a key role in the rearrangement process. This complex can also dissociate to the proton bound dimers [H2O center dot center dot center dot H center dot center dot center dot O=C=O] (+) and [H2O center dot center dot center dot H center dot center dot center dot CO](+) which are minor processes observed in the metastable ion mass spectrum. A further minor process leads to the proton bound dimer O=C=O center dot center dot center dot H+center dot center dot center dot 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. (c) 2005 Elsevier B.V. All rights reserved.