Microsolvation of LiI and CsI in Water: Anion Photoelectron Spectroscopy and <i>ab initio</i> Calculations

In order to understand the microsolvation of LiI and CsI in water and provide information about the dependence of solvation processes on different ions, we investigated the LiI­(H₂O)_<i>n</i>^– and CsI­(H₂O)_<i>n</i>^– (<i>n</i> = 0–6) clusters using photoelectron spectroscopy. The structures of these clusters and their corresponding neutrals were investigated with <i>ab initio</i> calculations and confirmed by comparing with the photoelectron spectroscopy experiments. Our studies show that the structural evolutions of LiI­(H₂O)_<i>n</i> and CsI­(H₂O)_<i>n</i> clusters are very different. The Li–I distance in LiI­(H₂O)_<i>n</i>^– increases abruptly at <i>n</i> = 3, whereas the abrupt elongation of the Li–I distance in neutral LiI­(H₂O)_<i>n</i> occurs at <i>n</i> = 5. In contrast to the LiI­(H₂O)_<i>n</i>^– clusters, the Cs–I distance in CsI­(H₂O)_<i>n</i>^– increases significantly at <i>n</i> = 3, reaches a maximum at <i>n</i> = 4, and decreases again as <i>n</i> increases further. There is no abrupt change of the Cs–I distance in neutral CsI­(H₂O)_<i>n</i> as <i>n</i> increases from 0 to 6. Water molecules interact strongly with the Li ion; consequently, water molecule(s) can insert within the Li⁺–I^– ion pair. In contrast, five or six water molecules are not enough to induce obvious separation of the Cs⁺–I^– ion pair since the Cs–water interaction is relatively weak compared to the Li–water interaction. Our work has shown that the structural variation and microsolvation in MI­(H₂O)_<i>n</i> clusters are determined by the delicate balance between ion–ion, ion–water, and water–water interactions, which may have significant implications for the general understanding of salt effects in water solution