posted on 2017-01-01, 00:00authored byJinfei Ling, Xunlei Ding, Zhenyu Li, Jinlong Yang
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.