Quantum Chemical Insights into the Dissociation of Nitric Acid on the Surface of Aqueous Electrolytes Himanshu Mishra, [a,b,c] Robert J. Nielsen, [b] Shinichi Enami, [d] Michael R. Hoffmann, [c] Agustı ´n J. Colussi, [c] and William A. Goddard, III* [a,b] 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 þ ---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. V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24151 How gas-phase nitric acid reacts on environmental aqueous surfaces has important implications in atmospheric chemistry. Nitrate (rather than nitric acid) is a sink for the nitrogen oxides polluting urban air, whence it is removed via wet or dry depo- sition. The stratospheric clouds produced by condensation of gaseous nitric acid play a key role in the Antarctic ‘ozone hole’ by recycling active chlorine. [1–4] Thus, the issue of whether nitric acid, a strong acid in bulk water, dissociates on the surface of airborne ice, aerosol, and water particles has recently received much attention, both experimentally and theoretically. However, most reports on the relevant HNO 3 interactions with water in gas–liquid encounters are mainly theoretical. [5–7] Experimental studies on this issue have focused on the extent of dissociation of dissolved HNO 3 (aq) at the air– water interface. [8,9] A few reports explored the dissociation of HNO 3 (g) on wet salts [10,11] and other solids. [10,12] The more re- alistic case in which HNO 3 (g) collides with the surface of dilute ( \1 mM) electrolytes have received less attention. It has been predicted that HNO 3 (g) could dissociate yielding contact ion pairs on discrete (n  4) water clusters. [2,13,14] Accurate theo- retical descriptions of the air–water interface, however, remain a challenge. [15–17] ‘Surface-specific’’ spectroscopies do provide information on interfacial structures whose identities, however, are subject to interpretational ambiguities. [18] Some reports predicted a kinetic barrier toward HNO 3 dissociation at the air–water interface, [6,7,19–23] whereas others did not. [8,17] Previ- ous experiments involving the interaction of HNO 3 (g) on wet salts, as surrogates of marine aerosols, have provided evidence of NO 3  formation. [10,24] Thus, our understanding of the fate of gaseous nitric acid at aqueous surfaces remains unclear. In this article we reinvestigate these issues, focusing on the dis- sociation of HNO 3 (g) on the surface of dilute electrolyte solu- tions. We ask whether HNO 3 (g) dissociates upon colliding with the surfaces of (i) neat water and (ii) aqueous electrolytes. Before describing our computational model, we summarize the phenomenon of ion-partitioning at aqueous interfaces. It has been shown by a variety of experimental [25–29] and theo- retical [26,27,30,31] methods that ions, especially large monovalent anions, tend to partition to the air–water interface. Although the forces driving this phenomenon are not fully under- stood, [32,33] it can be safely assumed that a fraction of dis- solved Cl  is present at the aerial interface of aqueous NaCl solutions. Herein, we investigate using quantum mechanics (QM) whether those interfacial ions participate in the dissocia- tion of HNO 3 (g) molecules during collisions with the surface of water. We use density functional theory (DFT) at the B3LYP level to analyze interfacial ( (if) ) dissociation of nitric acid on model surfaces of pure water and aqueous electrolyte. [a] H. Mishra, W. A. Goddard, III Department of Materials Science, California Institute of Technology, California 91125 E-mail: wag@wag.caltech.edu [b] H. Mishra, R. J. Nielsen, W. A. Goddard, III Materials and Process Simulation Center, California Institute of Technology, California 91125 [c] H. Mishra, M. R. Hoffmann, A. J. Colussi Ronald and Maxine Linde Center for Global Environmental Science, California Institute of Technology, California 91125 [d] S. Enami The Hakubi Center, Kyoto University, Kyoto 606-8302, Japan Contract grant sponsor: National Science Foundation; contract grant number: AGS-964842. . V C 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2012, DOI: 10.1002/qua.24151 1 WWW.Q-CHEM.ORG RAPID COMMUNICATION