jp512323k_si_001.pdf (340.14 kB)
Properties of Ammonium Ion–Water Clusters: Analyses of Structure Evolution, Noncovalent Interactions, and Temperature and Humidity Effects
journal contribution
posted on 2015-03-26, 00:00 authored by Shi-Tu Pei, Shuai Jiang, Yi-Rong Liu, Teng Huang, Kang-Ming Xu, Hui Wen, Yu-Peng Zhu, Wei HuangAlthough ammonium ion–water
clusters are abundant in the
biosphere, some information regarding these clusters, such as their
growth route, the influence of temperature and humidity, and the concentrations
of various hydrated clusters, is lacking. In this study, theoretical
calculations are performed on ammonium ion–water clusters.
These theoretical calculations are focused on determining the following
characteristics: (1) the pattern of cluster growth; (2) the percentages
of clusters of the same size at different temperatures and humidities;
(3) the distributions of different isomers for the same size clusters
at different temperatures; (4) the relative strengths of the noncovalent
interactions for clusters of different sizes. The results suggest
that the dipole moment may be very significant for the ammonium ion–water
system, and some new stable isomers were found. The nucleation of
ammonium ions and water molecules is favorable at low temperatures;
thus, the clusters observed at high altitudes might not be present
at low altitudes. High humidity can contribute to the formation of
large ammonium ion–water clusters, whereas the formation of
small clusters may be favorable under low-humidity conditions. The
potential energy surfaces (PES) of these different sized clusters
are complicated and differ according to the distribution of isomers
at different temperatures. Some similar structures are observed between
NH4+(H2O)n and M(H2O)n (where M represents
an alkali metal ion or water molecule); when n =
8, the clusters begin to form the closed-cage geometry. As the cluster
size increases, these interactions become progressively weaker. The
successive binding energy at the DF-MP2-F12/VDZ-F12 level is better
than that at the PW91PW91/6-311++G(3df, 3pd) level and is consistent
with the experimentally determined values.