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 x 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 X 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.