We report vibrational spectra of the H₂-tagged, cryogenically cooled X⁻ · HOCl (X = Cl, Br, and I) ion–molecule complexes and analyze the resulting band patterns with electronic structure calculations and an anharmonic theoretical treatment of nuclear motions on extended potential energy surfaces. The complexes are formed by “ligand exchange” reactions of X⁻ · (H₂O)_n clusters with HOCl molecules at low pressure (∼10⁻² mbar) in a radio frequency ion guide. The spectra generally feature many bands in addition to the fundamentals expected at the double harmonic level. These “extra bands” appear in patterns that are similar to those displayed by the X⁻ · HOD analogs, where they are assigned to excitations of nominally IR forbidden overtones and combination bands. The interactions driving these features include mechanical and electronic anharmonicities. Particularly intense bands are observed for the v = 0 → 2 transitions of the out-of-plane bending soft modes of the HOCl molecule relative to the ions. These involve displacements that act to break the strong H-bond to the ion, which give rise to large quadratic dependences of the electric dipoles (electronic anharmonicities) that drive the transition moments for the overtone bands. On the other hand, overtone bands arising from the intramolecular OH bending modes of HOCl are traced to mechanical anharmonic coupling with the v = 1 level of the OH stretch (Fermi resonances). These interactions are similar in strength to those reported earlier for the X⁻ · HOD complexes.

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. The critical step in the reductive process is the coordination of CO₂ to an alkali metal anion, forming a metal carbonite MCO₂ ⁻ able to subsequently receive a second CO₂ molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO₂ in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li–Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO₂, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C₂O₄ ²⁻ moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.