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In general, intermolecular distancesin condensed phases at theangstrom scale are difficult to measure. We were able to do so byusing the vibrational energy transfer method, an ultrafast vibrationalanalogue of Förster resonance energy transfer. The distancesamong SCN–anions in KSCN crystals and ion clustersof KSCN aqueous solutions were determined with the method. In thecrystalline samples, the closest anion distance was determined tobe 3.9 ± 0.3 Å, consistent with the XRD result. In the 1.8and 1 M KSCN aqueous solutions, the anion distances in the ion clusterswere determined to be 4.4 ± 0.4 Å. The clustered anion distancesin aqueous solutions are very similar to the closest anion distancein the KSCN crystal but significantly shorter than the average aniondistance (0.94–1.17 nm) in the aqueous solutions if ion clusteringdid not occur. The result suggests that ions in the strong electrolyteaqueous solutions can form clusters inside of which they have directcontact with each other. [ABSTRACT FROM AUTHOR]

Interactions between model molecules representing buildingblocksof proteins and the thiocyanate anion, a strong protein denaturantagent, were investigated in aqueous solutions with intermolecularvibrational energy exchange methods. It was found that thiocyanateanions are able to bind to the charged ammonium groups of amino acidsin aqueous solutions. The interactions between thiocyanate anionsand the amide groups were also observed. The binding affinity betweenthe thiocyanate anion and the charged amino acid residues is about20 times larger than that between water molecules and the amino acidsand about 5–10 times larger than that between the thiocyanateanion and the neutral backbone amide groups. The series of experimentsalso demonstrates that the chemical nature, rather than the macroscopicdielectric constant, of the ions and molecules plays a critical rolein ion/molecule interactions in aqueous solutions. [ABSTRACT FROM AUTHOR]

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]