An ab Initio and Raman Investigation of Magnesium(II) Hydration

Abstract

The weak polarized Raman band assigned to the ν1-MgO6 mode of the hexaaquo Mg(II) ion has been studied over the temperature range 25 to 125 °C. The 356 cm-1 stretching mode frequency decreases by about 3 cm-1 but broadens by 13 cm-1 over a 100 °C temperature range. A depolarized mode at 235 cm-1 could be assigned to ν2. These data suggest that the hexaaquo Mg(II) ion is thermodynamically stable in perchlorate and chloride solutions. In sulfate solutions, an equilibrium exists between the hexaaquo ion and an inner-sphere sulfato complex. Ab initio geometry optimizations of Mg(H2O)6 2+ were carried out at the Hartree−Fock and Møller−Plesset levels of theory, using various basis sets up to 6-31+G*. Frequency calculations confirm that the T h structure is a minimum. The unscaled frequencies of the MgO6 unit are lower than the experimental frequencies, and scaling only marginally improves the agreement. The theoretical binding enthalpy for the hexaaquo Mg(II) ion accounts for about 70% of the experimental hydration energy of Mg(II). A comparison of three models for the second hydration sphere is presented, and the most suitable is found to be one of lower symmetry T, in which alternate faces of the MgO6 octahedron are H-bonded to water trimers. The unscaled Hartree−Fock frequencies agree very well with our experimental observations, giving nearly exact agreement with experiment.

Comment on “Dynamic ion association in aqueous solutions of sulfate” [J. Chem. Phys. 123, 034508 (2005)]

“…1, confirm that the 1 ͑a 1 ͒ mode of free SO 4 2− at ϳ982 cm −1 possesses a very small depolarization ratio of 0.0035± 0.0010. 6,9 However, the same value of is observed in ͑NH 4 ͒ 2 SO 4 ͑aq͒ solutions, 10 which are generally considered to be fully dissociated. This suggests that the very small but finite depolarization of this mode arises from a slight departure of the free hydrated sulfate ion from ideal T d symmetry, consistent with theoretical calculations.…”

“…This suggests that the very small but finite depolarization of this mode arises from a slight departure of the free hydrated sulfate ion from ideal T d symmetry, consistent with theoretical calculations. 9,10 Furthermore, in the anisotropic Raman spectrum of MgSO 4 ͑aq͒ a weak contribution at 993 cm −1 with = 0.006± 0.002 has also been observed, 8 although it is difficult to quantify at room temperature ͑cf. Fig.…”

Tracing Mineral Reactions Using Confocal Raman Spectroscopy

“…This Raman band profile is composed of contributions from the symmetric ν 1 (H 2 O), the antisymmetric ν 3 (H 2 O) stretching vibration, as well as from a 2ν 2 combination mode [33]. There are many examples of Raman spectroscopy being used to identify hydrated ions [34,35], oxyanions [36,37], and ion complex formation [38][39][40], including oligomers of the same ions [41,42]. Formation of ion complexes is particularly interesting as this can aid precipitation if the different components of a mineral phase interact in solution.…”

A repulsion mechanism explains magnesium permeation and selectivity in CorA

“…However, although CorA has been crystallized in a wide range of conditions, so far all available CorA structures seem to correspond to nonconductive conformations, which obviously limits the basic mechanistic insights regarding Mg 2+ selectivity and translocation that can be derived from these high-resolution structures. Computational analyses, together with NMR, X-ray absorption, and Raman spectroscopy studies, have established that Mg 2+ holds to its first hydration shell much more tightly than any other physiological cation (8)(9)(10)(11); this implies that any Mg 2+ -selective transport system must either compensate for the high hydration energy (and accommodate the invariable octahedral geometry of this hexacoordinated ion) or establish a selectivity mechanism able to discriminate a hydrated or partially hydrated Mg 2+ ion from monovalent and other divalent cations. Several hypotheses have been postulated to explain CorA's function, including its role as a Mg 2+ -selective channel (12), a Co 2+ transporter (13), and even as an exporter of divalent cations (14).…”