A gas-phase solvent effect: The role of water molecules in the conversion of the HOMg⁺/CO₂ adduct ion to the magnesium bicarbonate ion, Mg⁺O₂COH

Quantum chemical calculations at the CP-dG2thaw and MP2(thaw)/B4G levels of theory are reported for the bare HOMg⁺OCO ion resulting from addition of CO₂ to HOMg⁺, and to its mono-, di- and tri-hydrated forms. These calculations are used to determine bond dissociation energy (BDE) values for the (H₂O)_n(CO₂)_iHOMg⁺-OH₂ (n = 0-2; i = 0, 1) and (H₂O)_nHOMg⁺-OCO (n = 0-3) bonds, as well as to ascertain the relative energies for several key stationary points on each of the HOMg⁺·(H₂O)_n·CO₂ (n = 0-3) potential energy surfaces. Three principal findings emerge from these calculations. First, in contrast to the isoelectronic system NaOH + CO₂ → NaO₂COH held to play a leading role in noctilucent cloud nucleation, the reaction of HOMg⁺ + CO₂ does not result in formation of the magnesium bicarbonate cation Mg⁺O₂COH. Second, the cumulative Mg⁺-ligand bond energies for complexes of HOMg⁺ with several H₂O and CO₂ molecules rapidly approach, and then exceed, the available Mg⁺ recombination energy, indicating that dissociative recombination of HOMg⁺·(H₂O)_n·CO₂ (or its bicarbonate-containing isomer Mg⁺O₂COH·(H₂O)_n) is likely to result in the production of molecular Mg-containing neutrals. Third, we find that hydration exhibits a remarkable influence on the reactivity of HOMg⁺ with mesospheric CO₂: addition of CO₂ to bare HOMg⁺ does not result in bicarbonate formation, nor do the reactions of HOMg⁺·CO₂ and HOMg⁺·OH₂·CO₂ with H₂O, but the reaction of HOMg⁺·(OH₂)₂·CO₂ with H₂O leads to two possible bicarbonate-containing products (H₂O)₂·Mg⁺O₂COH and (H₂O)₃·Mg⁺O₂COH. The former product channel, which involves association followed by H₂O loss, is judged to be an unusual example of a catalytic process in which the principal contribution of the H₂O 'catalyst' is steric.