Hydrated Sulfate Clusters SO42–(H2O)n (n = 1–40): Charge Distribution Through Solvation Shells and Stabilization

Investigations of inorganic anion SO₄^2- interactions with water are crucial for understanding the chemistry of its aqueous solutions. It is known that the isolated SO₄^2- dianion is unstable, and three H₂O molecules are required for its stabilization. In the current work, we report our computational study of hydrated sulfate clusters SO₄^2-(H₂O) _n ( n = 1-40) in order to understand the nature of stabilization of this important anion by water molecules. We showed that the most significant charge transfer from dianion SO₄^2- to H₂O takes place at a number of H₂O molecules n ≤ 7. The SO₄^2- directly donates its charge only to the first solvation shell and surprisingly, a small amount of electron density of 0.15| e| is enough to be transferred in order to stabilize the dianion. Upon further addition of H₂O molecules, we found that the cage effect played an essential role at n ≤ 12, where the first solvation shell closes. During this process, SO₄^2- continues to lose density up to 0.25| e| at n = 12. From this point, additional water molecules do not take any significant amount of electron density from the dianion. These results can help in development of understanding how other solvent molecules could stabilize the SO₄^2- anion as well as other multicharged unstable anions.