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

The carbonate radical anion CO 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 CO and CO(HO) with HCl under ultrahigh vacuum conditions. Bare CO forms OHCl with a rate constant of 4.2 × 10 cm s, which corresponds to an efficiency of only 0.4%. Hydration accelerates the reaction, and ligand exchange of HO against HCl proceeds with a rate of 2.7 × 10 cm s. Quantum chemical calculations reveal that OHCl is best described as an OH hydrogen bonded to Cl, while the ligand exchange product is Cl(HCO). Under tropospheric conditions, where CO(HO) is the dominant species, Cl(HCO) is efficiently formed. These reactions must be included in models of tropospheric anion chemistry.