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+ + CO2does 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.