A Quantum Mechanical Viewpoint on Structural Stability of the Smallest Hydrated Sulfate Cluster [SO4(H2O)3]2-

Abstract

Being the most kosmotropic type anion, SO 4 2 - shows a very strong affinity towards sequential hydration, and hence undergoes immediate stabilization via differently sized hydrated clusters [SO 4 (H 2 O) n ] 2− , n ≤ 80 in solution state. Interestingly, the smallest yet stable hydrated cluster detected through the advanced experimental techniques is [SO 4 (H 2 O) 3 ] 2− with hydration number n = 3. Despite its ubiquity in many sulfate enriched aqueous systems, the concerned electronic structure and structural stability reported so far are mostly based on the position of atomic nuclei in the theoretically produced low energy molecular geometry, which is, however, not regarded as a quantitative approach for depicting chemical bonding and structural entity in respect to electron density distributions around atoms and bonds between the atoms. In this study, the molecular orbitals (MOs) and Mulliken population analysis methods are used to extract chemical bond related information of [SO 4 (H 2 O) 3 ] 2− from its DFT derived molecular wavefunctions ( ) and then total molecular electron density surface (TMEDS), MOs iso-surfaces, and contour lines are deeply analyzed prior to describe how these EDSs actually provide detailed insights into the orbital interactions, atomic bonding, and structural stability. It is found that [SO 4 (H 2 O) 3 ] 2− ion stabilizes with (a) a closed 3D structural entity and a realistic distribution of electron cloud around the specific atoms and bonds, (b) a delocalized type electron density, and (c) an orbital type bonding interaction between the atoms. Besides these, a quantitatively measured electron amplitudes and the specific type bonding interactions make that hydrated structure quantum mechanically feasible.