Hydration Leads to Efficient Reactions of the Carbonate Radical Anion with Hydrogen Chloride in the Gas Phase.

The carbonate radical anion CO3•- is a key intermediate in tropospheric anion chemistry. Despite its radical character, only a small number of reactions have been reported in the literature. Here we investigate the gas-phase reactions of CO3•- and CO3•-(H2O) with HCl under ultrahigh vacuum conditions. Bare CO3•- forms OHCl•- with a rate constant of 4.2 × 10-12 cm3 s-1, which corresponds to an efficiency of only 0.4%. Hydration accelerates the reaction, and ligand exchange of H2O against HCl proceeds with a rate of 2.7 × 10-10 cm3 s-1. Quantum chemical calculations reveal that OHCl•- is best described as an OH• hydrogen bonded to Cl-, while the ligand exchange product is Cl-(HCO3•). Under tropospheric conditions, where CO3•-(H2O) is the dominant species, Cl-(HCO3•) is efficiently formed. These reactions must be included in models of tropospheric anion chemistry.