Shuzhi Wang

Depth-Dependent Dissociation of Nitric Acid at an Aqueous Surface: Car−Parrinello Molecular Dynamics

The acid dissociation of a nitric acid HNO(3) molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO(3) is 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 HNO(3).H(2)O pair. Four key structural features for the HNO(3) acid 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 (H(3)O(+)), rather than the Zundel cation (H(5)O(2)(+)), is found to be predominant next to the NO(3)(-) ion in contact ion pairs in all cases.

Theoretical Studies of the Dissociation of Sulfuric Acid and Nitric Acid at Model Aqueous Surfaces

Abstract A brief review is given of recent electronic structure calculations addressed to the acid dissociation at aqueous surfaces of sulfuric acid H 2 SO 4 and nitric acid HNO 3 , proton transfer reactions which are important in various atmospheric chemistry contexts. Two of many examples of their atmospheric relevance are sulfate aerosol surfaces acting as heterogeneous reaction sites for reactions related to ozone depletion in the mid-latitude stratosphere, and the uptake of gas phase HNO 3 by ice aerosols in the upper troposphere to provide surfaces for heterogeneous reactions, again related to ozone depletion. Despite the fact that these acids are usually regarded as strong and readily dissociate in the more familiar room temperature, bulk water solution context, it is found that both—particularly HNO 3 —can remain molecular at an aqueous surface under a wide range of atmospherically relevant conditions.

Nitric Acid Dissociation at an Aqueous Surface: Occurrence and Mechanism

Here we briefly review some highlights of our recent work on the acid dissociation of nitric acid HNO3 at an aqueous surface, a proton transfer reaction of interest not only from a fundamental perspective, but also in connection with heterogeneous chemistry in a wide range of atmospheric contexts. Two types of studies of the potential acid dissociation are discussed, quantum chemical reaction path calculations to assess the reaction free energy and Car-Parrinello molecular dynamics simulations to assess the reaction feasibility. We discuss the agreement and disagreement between the predictions of these two calculations as a function of the initial location of the HNO3 molecule, ranging from a positioning on top of the aqueous surface to one several water layers below the surface. Special attention is given to the four key water solvent motions found to be necessary for the proton transfer reaction to occur. Finally, an Eigen cation, rather than a Zundel cation, is in all cases found to be predominant next to the nitrate ion in contact ion pairs produced in the acid dissociation. This predominance remains, although diminished, for solvent-separated ion pairs.