The hydrogen-bridged radical cation [H2O⋯H⋯OCOH·+: A combined experimental and theoretical study of its stability and dissociation chemistry

Dissociative ionization of dihydroxyfumaric acid generated the hydrogen-bridged radical cation [H2O⋯H⋯OCOH·+, 1, as shown by a combination of tandem mass spectrometric techniques (metastable ion, collision-induced dissociation, and MS/MS/MS experiments) and computational chemistry (using ab initio MO and density functional theories). This hydrogen-bonded radical cation is predicted by theory to be more stable than the isomeric ions [H2O⋯HO2CH]·+, 2/2a, [H2O⋯HCO2H]·+, 3, and H2OC(OH)·+2, 4/4a, while HC(OH)·+3 is not stable at all. The heat of formation of the most stable conformer of 1 was estimated as 73 kcal mol−1 (with an assigned uncertainty of ± 4 kcal mol−1) and its isomers 2, 2a, 3, 4 and 4a are calculated (Becke3LYP/6-31G∗∗ + ZPE) to be higher in energy by 10.0, 9.4, 13.3, 18.1 and 19.1 kcal mol−1, respectively. Accordingly, the MP4SDTQ/6-31G∗∗//MP2/6-31G∗∗ + ZPE relative energies of 2a, 4 and 4a are 11.5, 19.2 and 21.3 kcal mol−1. Mass spectrometric experiments on isotopically labelled di[18O]hydroxyfumaric acid indicated that two 1,5-hydrogen transfers and sequential expulsions of CO lead, via an intermediate dihydroxyketene-water type of ion-neutral complex, to the hydrogen-bridged product ion 1. Unimolecular metastable dissociations of the latter lead to the proton-bound ion [H2O⋯H⋯OCO]+ and to the hydronium ion, H3O+, as well as, upon collisional activation, to the dihydroxycarbene ion, HOCOH·+.