Solvation effects on the chemistry of the gas-phase O•−(H2O)n and OH−(H2O)n 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.