The hydrogen-bridged radical cation [H 2O⋯H⋯OCOH ·+: A combined experimental and theoretical study of its stability and dissociation chemistry

Abstract

Dissociative ionization of dihydroxyfumaric acid generated the hydrogen-bridged radical cation [H 2O⋯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 [H 2O⋯HO 2CH] ·+, 2/2a, [H 2O⋯HCO 2H] ·+, 3, and H 2O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 [H 2O⋯H⋯OCO] + and to the hydronium ion, H 3O +, as well as, upon collisional activation, to the dihydroxycarbene ion, HOCOH ·+.