Correlations in the Solute-Solvent Dynamics Reach Beyond the First Hydration Shell of Ions.

While the real-space structure of solvation shells has been explored for decades, a dynamical perspective that directly relies on changes in the H-bond network became accessible more recently mainly via far-infrared (THz) spectroscopies. A remaining key question is how many hydration shells are affected by ion-induced network perturbations. We disclose that theoretical THz difference spectra of aqueous salt solutions can be deciphered in terms of only a handful of dipolar auto- and cross-correlations, including the second solvation shell. This emphasizes the importance of cross-correlations being often neglected in multicomponent models. Analogously, experimental THz responses of simple ions can be deciphered in a similar way. Dramatic intensity cancellations due to large positive and negative contributions are found to effectively shift intensity maxima. Thus, THz spectroscopy provides an unprecedented view on the details of hydration dynamics, which can be understood by a combination of experiment and theory.