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The acid dissociation of a nitric acid HNO3molecule located at various depths in a water slab is investigated via Car−Parrinello molecular dynamics simulations. HNO3is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociatealthough not alwaysby transferring a proton to a water molecule within a few picoseconds when embedded at various depths within the aqueous layer. The acid dissociation events are analyzed and discussed in terms of the proton donor−acceptor O−O hydrogen bonding distance and the configurations of the nearest-neighbor solvent waters of an HNO3·H2O pair. Four key structural features for the HNO3acid dissociation are identified and employed to analyze the trajectory results. Key solvent motions for the dissociation include the decrease of the proton donor−acceptor O−O hydrogen bonding distance and a 4 to 3 coordination number change for the proton-accepting water. The Eigen cation (H3O+), rather than the Zundel cation (H5O2+), is found to be predominant next to the NO3−ion in contact ion pairs in all cases. [ABSTRACT FROM AUTHOR]

The infrared signatures of nitric acid HNO 3and its conjugate anion NO 3−at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+∗ level of theory on the HNO 3·(H 2O) 3→ NO 3−·H 3O +·(H 2O) 2model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species’ intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts. [ABSTRACT FROM AUTHOR]

The issue of acid dissociation of nitric acid at an aqueous surface is relevant in various portions of the atmosphere in connection with ozone depletion. This proton-transfer reaction is studied here via electronic structure calculations at the HF/SBK(d) level of theory on the HNO3·(H2O)3model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters with an increasing degree of solvation of the nitrate group. Free energy estimates for all the cases examined favor undissociated, molecular nitric acid over the 0−300 K temperature range, including that relevant for the upper troposphere, where it is connected to the issue of the mechanism of nitric acid uptake by water ice aerosols. The presence of molecular HNO3at 300 K at the surface is further supported by vibrational band assignments in good agreement with a very recent surface-sensitive vibrational spectroscopy study of diluted HNO3/H2O solutions. [ABSTRACT FROM AUTHOR]