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

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journal contribution
posted on 03.07.2013, 00:00 authored by Hongtao Bian, Hailong Chen, Qiang Zhang, Jiebo Li, Xiewen Wen, Wei Zhuang, Junrong Zheng
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.