Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy

The structural dynamics of highlyconcentrated LiCl and LiBr aqueoussolutions were observed from 1-4 to 1-16 water moleculesper ion pair using ultrafast polarization-selective pump-probe(PSPP) experiments on the OD stretch of dilute HOD. At these highsalt concentrations, an extended ion/water network exists with complexstructural dynamics. Population decays from PSPP experiments highlighttwo distinct water components. From the frequency-dependent amplitudesof the decays, the spectra of hydroxyls bound to halides and to wateroxygens are obtained, which are not observable in the FT-IR spectra.PSPP experiments also measure frequency-dependent water orientationalrelaxation. At short times, wobbling dynamics within a restrictedangular cone occurs. At high concentrations, the cone angles are dependenton frequency (hydrogen bond strength), but at higher water concentrations(>10 waters per ion pair), there is no frequency dependence. Theaveragecone angle increases as the ion concentration decreases. The slowtime constant for complete HOD orientational relaxation is independentof concentration but slower in LiCl than in LiBr. Comparison to structuralMD simulations of LiCl from the literature indicates that the lossof the cone angle wavelength dependence and the increase in the coneangles as the concentration decreases occur as the prevalence of largeion/water clusters gives way to contact ion pairs.

Automated summary

The structural dynamics of highly concentrated LiCl and LiBr aqueous solutions were studied using ultrafast polarization-selective pump-probe experiments. Results show that at high salt concentrations, an extended ion/water network exists with complex structural dynamics. Two distinct water components were observed from population decays, and hydroxyl spectra bound to halides and water oxygens were obtained. Frequency-dependent water orientational relaxation and wobbling dynamics within a restricted angular cone were also measured. The cone angles were found to be dependent on frequency at high concentrations, but not at higher water concentrations, indicating a transition from large ion/water clusters to contact ion pairs.