Ab initio study of hydrated cesium iodide dimer (CsI)2-/0(H2O)0-6 and the cation size effect on (MI)2-/0(H2O)0-6 (M = Li, Na, K, Cs).

The structures of microsolvated (CsI)2-/0(H2O)0-6 clusters were determined using ab initio calculations. Our studies show that one Cs atom at the apex was firstly separated from the pyramid-shaped (CsI)2- unit when the water number reaches 3, whereas CsI distances did not increase significantly from n = 0 to 6 for neutrals. Additionally, the atomic charge and reduced density gradient analyses were carried out; the results reveal that the extra electrons are almost entirely localized on terminal Cs atom and the Cs+-water interactions dominate in (CsI)2-(H2O)0-6. The water-water interactions show up at n = 5. The comparison of (CsI)2-/0(H2O)n with (MI)2-/0(H2O)n (M = Li, Na, K) shows that neutral (CsI)2 is the most difficult to be separated, which matches the law of matching water affinity. As for anions, the most difficult separation occurs in the case of small size (LiI)2- due to the effect of extra electrons, and (MI)2- with larger size cation is more likely to interact with water to form a pyramid structure.