title: Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4-11](2+) Clusters.
abstract: 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 Mg(2+) ions attach on opposite sides of the central SO4(2-) ion in a bifurcated fashion to yield a D2d symmetry arrangement. Analysis of the observed spectral patterns indicate that water molecules preferentially attach to the flanking Mg(2+) 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 Mg(2+)-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 Mg(2+) 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.
authors.affiliation: Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Joseph W
authors.initials: JW
authors.lastname: DePalma
authors.affiliation: Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Kelleher
authors.affiliation: Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Johnson
authors.affiliation: Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Fournier
authors.affiliation: Sterling Chemistry Laboratory, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Johnson