Dissociation of nitric acid at an aqueous surface: Large amplitude motions in the contact ion pair to solvent-separated ion pair conversion

Abstract

Beyond its fundamental interest, the acid dissociation of nitric acid (HNO₃) at an aqueous interface is of importance in a wide variety of atmospheric contexts. Here we present a Car–Parrinello molecular dynamics (CPMD) study of the second step of this process, the formation, via proton transfer (PT), of a solvent-separated ion pair (SSIP) from a contact ion pair (CIP) of the hydronium (H₃O⁺) and the nitrate (NO⁻₃) ions. This reaction represents an extension of our earlier CPMD study of the first PT step to produce the CIP from molecular HNO₃ at various locations at and below the aqueous surface (S. Wang, R. Bianco and J. T. Hynes, J. Phys. Chem. A, 2009, 113, 1295); it is important in establishing the ionic distribution in the aqueous interfacial region, with potential consequences for heterogeneous reactions occurring in that region. We focus on the large amplitude, microscopic level motions—such as the hydrogen-bonding coordination number changes around the proton-donating and -accepting species—which are key for the CIP → SSIP PT conversion.