Initial hydration behavior of sodium iodide dimer: photoelectron spectroscopy and ab initio calculations.

We investigated (NaI)2(-)(H2O)n (n = 0-6) clusters to examine the initial solvation process of (NaI)2 in water, using negative ion photoelectron spectroscopy and theoretical calculations. The structures of these clusters and their neutrals were determined by comparing ab initio calculations with experimental results. It is found that bare (NaI)2(-) is a L-shaped structure and the corresponding neutral is a rhombus. In (NaI)2(-)(H2O), the water molecule prefers to interact with the middle Na atom of the L-shaped (NaI)2(-). For (NaI)2(-)(H2O)n clusters with n = 2-3, two types of structures are nearly degenerate in energy: one is L-shaped and the other is pyramid-shaped. As for (NaI)2(-)(H2O)n with n = 4-6, the dominant structures are pyramid-shaped. For the anionic clusters, one of the Na-I distances increases abruptly when n = 2; for the neutral clusters, rapid lengthening of the Na-I distances occurs when n = 4. Additionally, analyses of the reduced density gradient were carried out, and the results reveal that Na(+)-water interactions dominate in (NaI)2(-)(H2O)n for n≤ 4, whereas I(-)-water and water-water interactions are significantly enhanced when n increases to 5.