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 HNO 3 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 HNO 3 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 HNO 3 embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO 3 − . Our findings suggest that nearby anions catalyze HNO 3 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 × 10 4 water molecules) covered by our experiments if weakly adsorbed HNO 3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water.