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

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)62+ were carried out at the Hartree−Fock and Moller−Plesset levels of theory, using various basis sets up to 6-31+G*. Frequency calculations confirm that the Th 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 hydrat...