Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO₄Mg(H₂O)_{n =4-11}]²⁺ Clusters

Elucidation of the molecular-level mechanics underlying the dissolution of salts is one of the long-standing, fundamental problems in electrolyte chemistry. Here we follow the incremental structural changes that occur when water molecules are sequentially added to the ternary [MgSO₄Mg]²⁺ ionic assembly using cryogenic vibrational predissociation spectroscopy of the cold, mass-selected [MgSO₄Mg(H₂O)_n=4-11]²⁺ cluster ions. Although the bare [MgSO₄Mg]²⁺ ion could not be prepared experimentally, its calculated minimum energy structure corresponds to a configuration where the two Mg²⁺ ions attach on opposite sides of the central SO₄^2- ion in a bifurcated fashion to yield a D_2d symmetry arrangement. Analysis of the observed spectral patterns indicate that water molecules preferentially attach to the flanking Mg²⁺ ions for the n ≥ 7 hydrates, which results in an incremental weakening of the interaction between the ions. Water molecules begin to interact with the sequestered SO₄^2- anion promptly at n = 8, where changes in the band pattern clearly demonstrate that the intrinsic bifurcated binding motif among the ions evolves into quasilinear Mg²⁺-O-S arrangements as water molecules H-bond to the now free SO groups. Although condensed-phase MgSO₄ occurs with a stable hexahydrate in which water molecules lie between the ion pairs, addition of a sixth water molecule to one of the Mg²⁺ ions in the n = 11 cluster occurs with the onset of the second hydration shell such that the cation remains coordinated to one of the SO₄^2- oxygen atoms.