Quantum chemical insights into the dissociation of nitric acid on the surface of aqueous electrolytes

Recent experiments in our laboratory have shown that the probability of gaseous HNO₃ deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO ₃ molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO₃ always binds to the outermost water molecules both via donating and accepting hydrogen-bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl^-. As the dissociation of HNO₃ embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO₃^-. Our findings suggest that nearby anions catalyze HNO₃ dissociation by preorganizing interfacial water and drawing the proton - away from the incipient [H⁺ - -NO ₃^-] close ion-pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl^- range (1 Cl ^- in ∼5.5 × 10⁴ water molecules) covered by our experiments if weakly adsorbed HNO₃ were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water.