Unimolecular dissociation of anions derived from succinic acid (H 2 Su) in the gas phase: HSu − and ClMgSu − . Relationship to CO 2 fixation

Electrospray ionization of mixtures of succinic acid (here denoted H₂Su) and magnesium chloride in water/methanol give rise to ions of the type ESu⁻ (E = H or ClMg). The unimolecular dissociation of these ions was studied by collisionally induced dissociation mass spectrometry and interpreted by quantum chemical calculations (density functional theory and the composite Gaussian-4 method) of relevant parts of the potential energy surfaces. The major dissociation pathways from HSu⁻ were seen to be dehydration and decarboxylation, while ClMgSu⁻ mainly undergoes decarboxylation. The latter reaction proceeds without barrier for the reverse reaction; addition of CO₂ to a Grignard type structure ClMg(CH₂CH₂CO₂)^–. In contrast, addition of CO₂ to the analogous H(CH₂CH₂CO₂)^– ion has a substantial barrier. Dehydration of HSu⁻ gives rise to deprotonated succinic anhydride via a transition state for the key intramolecular proton transfer having an entropically favorable seven-member ring structure. The succinate systems studied here are compared to the previously reported analogous maleate systems, providing further insight to the structure–reactivity relationship.