Solvent Shared Ion Pairs and Direct Contacted Ion Pairs in LiCl Aqueous Solution by IR Ratio Spectra

The local structure and molecular interactions of Li+ salt in aqueous solutions is important in many fields. However, whether solvent shared ion pairs and the direct contact ion pairs exist in aqueous LiCl solutions or not, and the details about these ion pairs are still under debate. Here, we proposed a novel IR ratio method. Using this method, the hydration spectra of Cl− in LiCl, NaCl, and KCl aqueous solutions were measured from the diluted concentration to the highly concentrated solution. Hydration number of Cl− from the hydration spectra was determined to be 2 in the aqueous LiCl. These data demonstrated that about 3–4 Li+ replaced some water molecules in the first hydration shell of Cl−. As the concentration of LiCl increased, an abnormal increase in the hydration number was observed. This is because the water molecule that bridges Li+ and Cl− in the solvent-shared ion pair are particularly stable, which was directly proven by the red shift of the hydration spectra of Cl− in the O–H stretching region. All the hydration spectra and hydration numbers not only applied to uncover the solvent shared ion pairs and direct contacted ion pairs in LiCl aqueous solution, but also can be employed to the benchmark of force fields in the classical molecular dynamics simulations.