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

The acid dissociation of a nitric acid HNO3 molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO3 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 HNO3 center dot H2O pair. Four key structural features for the HNO3 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 (H3O+), rather than the Zundel cation (H5O2+), is found to be predominant next to the NO3- ion in contact ion pairs in all cases.