Ion-Specific Perturbations to the Collective Hydrogen-Bonding Network in Liquid Water: A Hyper-Raman Spectroscopic Study of the Librational Motions

We directly observe the librational band of liquid water and aqueous alkali halide solutions using hyper-Raman (HR) spectroscopy. While the libration, an intermolecular hindered rotational motion of water, reflects the collective nature of the hydrogen-bonding network in liquid water, its direct observation across the full spectral range remains challenging with conventional vibrational spectroscopy. We systematically examine ion-specific perturbations to the hydration structure by analyzing HR spectra in the librational region. We found that halide anions dominate the librational response rather than alkali metal cations. Cl – and Br – induce low-frequency shifts due to the formation of weaker ion–water hydrogen bonds. In stark contrast, F – induces a pronounced high-frequency shift of the librational band, indicating the formation of strong electrostatic F – –water bonds. Temperature-dependent measurements further revealed that the dense F – –water network in highly concentrated solutions is resistant to thermal disruption. These results offer new insights into the unique nature of fluoride hydration, highlighting HR spectroscopy as a sensitive probe of the collective nature of the hydrogen-bonding network in the vicinity of ions.