Cation Effects on Rotational Dynamics of Anions and Water Molecules in Alkali (Li⁺, Na⁺, K⁺, Cs⁺) Thiocyanate (SCN^–) Aqueous Solutions

Waiting time dependent rotational anisotropies of SCN^– anions and water molecules in alkali thiocyanate (XSCN, X = Li, Na, K, Cs) aqueous solutions at various concentrations were measured with ultrafast infrared spectroscopy. It was found that cations can significantly affect the reorientational motions of both water molecules and SCN^– anions. The dynamics are slower in a solution with a smaller cation. The reorientational time constants follow the order of Li⁺ > Na⁺ > K⁺ ≃ Cs⁺. The changes of rotational time constants of SCN^– at various concentrations scale almost linearly with the changes of solution viscosity, but those of water molecules do not. In addition, the concentration-dependent amplitudes of dynamical changes are much more significant in the Li⁺ and Na⁺ solutions than those in the K⁺ and Cs⁺ solutions. Further investigations on the systems with the ultrafast vibrational energy exchange method and molecular dynamics simulations provide an explanation for the observations: the observed rotational dynamics are the balanced results of ion clustering and cation/anion/water direct interactions. In all the solutions at high concentrations (>5 M), substantial amounts of ions form clusters. The structural inhomogeneity in the solutions leads to distinct rotational dynamics of water and anions. The strong interactions of Li⁺ and Na⁺ because of their relatively large charge densities with water molecules and SCN^– anions, in addition to the likely geometric confinements because of ion clustering, substantially slow down the rotations of SCN^– anions and water molecules inside the ion clusters. The interactions of K⁺ and Cs⁺ with water or SCN^– are much weaker. The rotations of water molecules inside ion clusters of K⁺ and Cs⁺ solutions are not significantly different from those of other water species so that the experimentally observed rotational relaxation dynamics are only slightly affected by the ion concentrations.