Theoretical Study of the Dissociation of Nitric Acid at a Model Aqueous Surface

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)3 model 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
HNO3 at 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.