The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations.

Cyclooligomerisation to squarate is initiated by the coupling of two CO units in MgCl−.

The unimolecular dissociation of hydrogen squarate and the squarate radical anion has been studied by electrospray ionization mass spectrometry (including collisionally induced dissociation) and quantum chemical calculations, providing consistent reaction models. In both cases, consecutive decarbonylations are observed as the dominating fragmentations. The reverse of these reactions corresponds to the successive cyclooligomerization of CO, which constitutes the most atom-efficient route to the cyclic oxocarbons. The reaction models indicate moderate barriers for CO addition to HC n O n – and C n O n •–, respectively, being larger for the former than for the latter. Cyclooligomerization leading to a neutral product is endothermic, while the analogous one-electron reductive coupling is exothermic. The analysis shows that the addition of an electron is essential for cyclooligomerization to give the cyclic four-CO squarate structure.

Electrospray ionization of mixtures of succinic acid (here denoted HSu) 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 CO to a Grignard type structure ClMg(CHCHCO). In contrast, addition of CO to the analogous H(CHCHCO) 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.

We present a computational survey of the reduction of CO2 to CO by alkali metal and alkaline earth metal chloride anions in the gas phase, uncovering also mechanistic aspects on the selective tuning between oxalate and carbonate products relevant to chemical or electrochemical processes. The reduction of a single CO2 molecule is typically endothermic, whereas the corresponding disproportionation reaction involving two molecules is exothermic. Our computational results suggest consistent periodic trends with reaction energies being highest for elements toward the center of each group. The factors governing these trends are discussed, in particular, the covalent contributions to bonding in these highly ionic species.