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 center dot- and OH-, with oxygen and carbon dioxide. A mixture of hydrated anions O center dot-(H2O)(n) and OH-(H2O)(n), n < 60, is prepared in a laser vaporization source and reacted in a temperature-controlled ICR cell with O-2 and CO2. While OH-(H2O)(n) does not react with O-2, formation of hydrated ozonide O-3(center dot-)(H2O)(m) is observed in the reaction of O center dot-(H2O)(n) with O-2 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 Delta E-0 = -52 kJ mol(-1) for n approximate to 7-11, while O-2 is very weakly bound to OH-(H2O)(n). Observation of such a non-covalent (O-2)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 approximate to 8 for O center dot-(H2O)(n) and n approximate to 6 for OH-(H2O)(n), to form CO3 center dot-(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.