On the dissolution processes of Na2l+ and Na3l2+ with the association of water molecules:: Mechanistic and energetic details

The sequential association energies for one through six water molecules clustering to Na2I+, as well as one and two water molecules clustering to Na3I2+, are measured. The association energies show a pairwise behavior, indicating a symmetric association of water molecules to the linear Na2I+ and Na3I2+ ions. This pairwise behavior is well reproduced by Density Functional Theory (DFT) calculations. DFT calculations also suggest that a significant separation of charge for the Na-I ion pair occurs when four or more water molecules cluster to a single sodium center. Two different solvent-separated ion pairs have been identified with the DFT calculations. Experiments also show that the dissolution processes, loss of a neutral NaI unit, occurs when six or more water molecules have been added to Na2I+ cluster. However, one or two water molecules are able to detach an NaI unit from the Na3I2+ cluster. The difference in solubility of the Na2I+ and Na3I2+ ions is due to the difference in the energies required to lose an NaI unit from these two species. The experiment also confirms that the loss of a neutral NaI unit, instead of an Na+ ion, occurs during the dissolution processes of Na3I2+. The microsolvation schemes proposed to explain our experimental observations are supported by DFT and phase space theory (PST) calculations.