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

Waiting time dependent rotationalanisotropies of SCN–anions and water moleculesin alkali thiocyanate (XSCN, X = Li,Na, K, Cs) aqueous solutions at various concentrations were measuredwith ultrafast infrared spectroscopy. It was found that cations cansignificantly affect the reorientational motions of both water moleculesand SCN–anions. The dynamics are slower in a solutionwith a smaller cation. The reorientational time constants follow theorder of Li+> Na+> K+≃Cs+. The changes of rotational time constants of SCN–at various concentrations scale almost linearly withthe changes of solution viscosity, but those of water molecules donot. In addition, the concentration-dependent amplitudes of dynamicalchanges 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 ultrafastvibrational energy exchange method and molecular dynamics simulationsprovide an explanation for the observations: the observed rotationaldynamics are the balanced results of ion clustering and cation/anion/waterdirect interactions. In all the solutions at high concentrations (>5M), substantial amounts of ions form clusters. The structural inhomogeneityin the solutions leads to distinct rotational dynamics of water andanions. The strong interactions of Li+and Na+because of their relatively large charge densities with water moleculesand SCN–anions, in addition to the likely geometricconfinements because of ion clustering, substantially slow down therotations of SCN–anions and water molecules insidethe ion clusters. The interactions of K+and Cs+with water or SCN–are much weaker. The rotationsof water molecules inside ion clusters of K+and Cs+solutions are not significantly different from those of otherwater species so that the experimentally observed rotational relaxationdynamics are only slightly affected by the ion concentrations. [ABSTRACT FROM AUTHOR]