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, 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 [H2O⋯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.