10.1021/jp512323k.s001 Shi-Tu Pei Shi-Tu Pei Shuai Jiang Shuai Jiang Yi-Rong Liu Yi-Rong Liu Teng Huang Teng Huang Kang-Ming Xu Kang-Ming Xu Hui Wen Hui Wen Yu-Peng Zhu Yu-Peng Zhu Wei Huang Wei Huang Properties of Ammonium Ion–Water Clusters: Analyses of Structure Evolution, Noncovalent Interactions, and Temperature and Humidity Effects American Chemical Society 2015 PW 91PW 3 pd noncovalent interactions binding energy High humidity Noncovalent Interactions water molecules ammonium ions energy surfaces growth route PES cluster growth Structure Evolution NH cluster size increases size clusters alkali metal ion 2015-03-26 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Properties_of_Ammonium_Ion_Water_Clusters_Analyses_of_Structure_Evolution_Noncovalent_Interactions_and_Temperature_and_Humidity_Effects/2182438 Although 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 NH<sub>4</sub><sup>+</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> and M­(H<sub>2</sub>O)<sub><i>n</i></sub> (where M represents an alkali metal ion or water molecule); when <i>n</i> = 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.