Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study

The dynamics of aqueous magnesium chloride solutions, from relatively dilute (0.5 m) to near saturated (4.2 m) concentrations, were investigated using ultrafast two dimensional infrared and polarization selective pump-probe spectroscopies. The experiments were performed on two spectrally distinct nitrile stretch frequencies of the selenocyanate vibrational probe, corresponding to the CN nitrogen lone pair being associated with water and with Mg²⁺. No chemical exchange of the two species was observed over the experimental time scale (∼100 ps), enabling straightforward analysis of their dynamics. The dynamics reported by the Mg²⁺-associated peak are slower than those of the water-associated peak, suggesting that the immediate environment of the hydrated Mg²⁺ is different from the rest of the solution. Notably, the Mg²⁺-associated peak displays three spectral diffusion time scales, the slowest being ∼30 ps, while the water-associated peak decays as a faster biexponential. From the complete orientational relaxation time and hydrodynamic theory, a magnesium hydration number of six was obtained, which is in good agreement with NMR and X-ray diffraction studies. This hydration number holds for all concentrations until near saturation, when the linewidths and the dynamics deviate from linear trends, indicative of Mg²⁺ solvation structure changes resulting from a shortage of water molecules needed for full solvation.