Structure Making and Breaking Effects of Cations in Aqueous Solution: Nitrous Oxide Pump-Probe Measurements

Ultrafast IR pump-probe responses resonant with the ν₃ asymmetric stretch of nitrous oxide (N₂O) at ∼2230 cm^-1 are reported for 2 M aqueous salt solutions of MgCl₂, CaCl₂, NaCl, KCl, and CsCl at room temperature. The solvated cations of these chloride solutions span the range from strongly to weakly hydrating ions, and correspondingly are often categorized as structure makers and structure breakers, respectively. The observed salt dependent trends of the N₂O ν₃ vibrational energy relaxation (VER) and rotational reorientation anisotropy (R(t)) decays are consistent with the categorization of these cations as structure breakers or makers, and show evidence of effects on the water hydrogen bonding network beyond the first solvation shell of these ions. This N₂O mode is resonant with the H₂O bend-libration band region. The corresponding FTIR is fitted well by a two Gaussian plus sloping continuum baseline model that allows a framework for characterizing the salt perturbations of the solvent spectral density in the ν₃ resonant region. Both coupling strengths and density of states effects appear to contribute the systematic cation dependent T₁ effects reported here. R(t) decays follow bulk viscosity values. These results are contrasted with previous IR pump-probe studies predominantly based on the relaxation dynamics of the OH/OD vibrational stretch of HOD hydrogen bonded to anions in salt solutions.