Kinetics of the Reaction of CO₃^•–(H₂O)n, n = 0, 1, 2, with Nitric Acid, a Key Reaction in Tropospheric Negative Ion Chemistry

CO₃^.– is abundantly found in the troposphere (0.9-2.3%).1 Recently, the CLOUD collaboration at CERN has shown that such anions play important roles in the tropospheric aerosol nucleation and cloud formation.2 Formation of CO₃^.– in the troposphere via the catalytic free electron reaction with O₂ and CO₂ is well studied decades ago,3 but the gas-phase reactions of CO₃^•– are still remaining unclear. Herein, we demonstrate the gas-phase reactions of CO₃^.–(H₂O)0–2 with nitric acid by combination of laboratory experiments (under ultra-high vacuum conditions) and quantum chemical calculations. Experimental findings show that bare CO₃^.– produces NO₃^-(OH^•) and NO₃^- as primary products but at relatively slow reaction rate. CO₃^.–(H₂O) affords the stabilization of the NO₃^–(HCO₃^•) collision complex and accelerates the formation of NO₃-(OH^•), but these processes have slower rates for CO₃^.–(H₂O)₂. Quantum chemical calculations suggest that these reactions proceed through an initial proton-transferred NO₃^-(HCO₃^•) collision complex, which can rearrange to NO₃^-(OH^•)(CO₂) and further evaporate CO₂ and OH^• to produce NO₃^-. Water evaporation energy of hydrated CO₃^•– is significantly lower than the transition states. Therefore, for CO₃^.–(H₂O), the reaction pathway to NO₃^-(OH^•) is barely accessible (exothermic by 9 kJ mol^-1), quenching the formation of NO₃^–. For CO₃^•–(H₂O)2, only the formation of NO₃^-(HCO₃^•) is energetically allowed with a moderate exothermicity of -30 kJ mol^-1. The reaction mechanism of CO₃^•– with HNO₃ is very similar to that with HCl as recently reported.