Vibrational Signatures of Solvent-Mediated Deformation of the Ternary Core Ion in Size-Selected [MgSO4Mg(H2O)n=4鈥?1]2+ Clusters

详细信息    查看全文

• 作者：Joseph W. DePalma ; Patrick J. Kelleher ; Christopher J. Johnson ; Joseph A. Fournier ; Mark A. Johnson
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：July 30, 2015
• 年：2015
• 卷：119
• 期：30
• 页码：8294-8302
• 全文大小：539K
• ISSN：1520-5215

文摘

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鈥?1]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鈥?/sup> 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 鈮?7 hydrates, which results in an incremental weakening of the interaction between the ions. Water molecules begin to interact with the sequestered SO₄2鈥?/sup> 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+鈥揙鈥揝 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 SO₄2鈥?/sup> oxygen atoms.}