Short-Range Cooperative Slow-down of Water Solvation Dynamics Around SO 4 2- -Mg 2+ Ion Pairs.

The presence of ions affects the structure and dynamics of water on a multitude of length and time scales. In this context, pairs of Mg 2+ and SO 4 2- ions in water constitute a prototypical system for which conflicting pictures of hydration geometries and dynamics have been reported. Key issues are the molecular pair and solvation shell geometries, the spatial range of electric interactions, and their impact on solvation dynamics. Here, we introduce asymmetric SO 4 2- stretching vibrations as new and most specific local probes of solvation dynamics that allow to access ion hydration dynamics at the dilute concentration (0.2 M) of a native electrolyte environment. Highly sensitive heterodyne 2D-IR spectroscopy in the fingerprint region of the SO 4 2- ions around 1100 cm -1 reveals a specific slow-down of solvation dynamics for hydrated MgSO 4 and for Na 2 SO 4 in the presence of Mg 2+ ions, which manifests as a retardation of spectral diffusion compared to aqueous Na 2 SO 4 solutions in the absence of Mg 2+ ions. Extensive molecular dynamics and density functional theory QM/MM simulations provide a microscopic view of the observed ultrafast dephasing and hydration dynamics. They suggest a molecular picture where the slow-down of hydration dynamics arises from the structural peculiarities of solvent-shared SO 4 2- -Mg 2+ ion pairs.