First-Principles Study of Molecular Clusters Formed by Nitric Acid and Ammonia.

Molecular clusters formed by m nitric acid molecules and n ammonia molecules are studied with density functional theory. For smaller clusters with m, n ≤ 4, all possible combinations of m and n are considered, while for larger clusters in the 5 ≤ m, n ≤ 8 range we only consider the possibilities with |m - n| ≤ 1. Hydrogen bond network formation is an important stabilization mechanism in these clusters. At the same time, proton transfer is generally preferred except in the smallest clusters. Nitric acid and ammonia evaporation rates of these clusters are calculated with both collision activation barriers and reaction thermodynamics explicitly considered. However, unlike in the case of cluster growth from sulfuric acid and ammonia, activation barriers do not play an important role here. If m and n are unequal, evaporation of the abundant species is always preferred. For clusters with m = n > 2, ammonia evaporation is faster than nitric acid. Stabilities of all clusters can be quantitatively evaluated by the evaporation rate of the preferred species. Larger clusters are generally more stable. However, exceptions can occur at structure motif transition point. Deviation from the stoichiometry of m = n significantly lowers the cluster stability. For a cluster pair formed by the same number of molecules, the nitric acid abundant one is more stable, which determines the growth pathway of these clusters.