Do protons recombine with O2− and CO2− in water clusters?

- Uptake of HCl as well as HNO3 by gas-phase clusters O2‐(H2O)m and CO2−(H2O)m is efficient.
- Products Cl‐(HCl)n(H2O)m and NO3‐(HNO3)n(H2O)m are observed, which result from proton transfer in the cluster.
- The most plausible neutral products are HO2 and HOCO.
- These assignments are consistent with literature thermochemistry.

Gas phase reactions of O2−(H2O)m and CO2−(H2O)m with HCl as well as HNO3 are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. HCl and HNO3 are efficiently taken up by the clusters. In the reactions of O2−(H2O)m, a characteristic mass change is observed which is consistent with the evaporation of a hydroperoxyl radical HO2, most likely formed via proton transfer to O2−. Using CO2−(H2O)m as the ionic reactant, products indicating the formation of HOCO are hardly observed in the HCl reaction. With HNO3, however, evidence is found for elimination of HOCO, resulting from proton transfer to CO2−, when the clusters have lost almost all water molecules. Thermochemical cycles using literature thermochemistry show that the reactions are exothermic.