Solvation effects on the chemistry of the gas-phase O•−(H2O) and OH−(H2O) cluster ions with molecular oxygen and carbon dioxide

Ion-molecule reactions in the gas phase are significantly influenced by hydration. Here we investigate the impact of hydration on the reactivity of two atmospherically relevant anions, O•− and OH−, with oxygen and carbon dioxide. A mixture of hydrated anions O•−(H2O)n and OH−(H2O)n, n < 60, is prepared in a laser vaporization source and reacted in a temperature-controlled ICR cell with O2 and CO2. While OH−(H2O)n does not react with O2, formation of hydrated ozonide O3•−(H2O)m is observed in the reaction of O•−(H2O)n with O2 for all studied cluster sizes. The reaction slows down with increasing cluster size, which compromises nanocalorimetry. Quantum chemical calculations show that ozonide formation is exothermic with ΔE0 = −52 kJ mol−1 for n ≈ 7–11, while O2 is very weakly bound to OH−(H2O)n. Observation of such a non-covalent (O2)OH−(H2O)m complex in a mass spectrometer might be possible at significantly lower temperatures than accessible in our experiment. For CO2, we observe reactions only in a narrow size regime, up to n ≈ 8 for O•−(H2O)n and n ≈ 6 for OH−(H2O)n, to form CO3•−(H2O)m and HCO3−(H2O)m, respectively. Calculations render both reactions substantially exothermic also for larger clusters, ruling out thermochemistry as an explanation for the size-dependent reactivity.