title: Cation effects on rotational dynamics of anions and water molecules in alkali (Li+, Na+, K+, Cs+) thiocyanate (SCN-) aqueous solutions.
abstract: 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.
authors.affiliation: Department of Chemistry, Rice University, Houston, Texas 77005, USA.
authors.forename: Hongtao
authors.initials: H
authors.lastname: Bian
authors.forename: Hailong
authors.initials: H
authors.lastname: Chen
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhang
authors.forename: Jiebo
authors.initials: J
authors.lastname: Li
authors.forename: Xiewen
authors.initials: X
authors.lastname: Wen
authors.forename: Wei
authors.initials: W
authors.lastname: Zhuang
authors.forename: Junrong
authors.initials: J
authors.lastname: Zheng
mesh_terms: D000468:Alkalies / Q000737:chemistry; D000838:Anions / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D002586:Cesium / Q000737:chemistry*; D008094:Lithium / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D011188:Potassium / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013861:Thiocyanates / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D000468:Alkalies; D000838:Anions; D002412:Cations; D013861:Thiocyanates; D014867:Water; D002586:Cesium; D008094:Lithium; D012964:Sodium; C031760:thiocyanate; D011188:Potassium