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.

The opposite effects of sodium and potassium cations on water dynamics

“…A similar fsIR experimental set up was used for a group of alkali thiocyanate solutions with a series of concentrations up to 15 M, 47 with the discussion focusing on how ion clustering, at high concentrations (>5 M), leads to segregation of the rotational dynamics between water and anions. A ps amplifier and a fs amplifier were synchronized with the same seed pulse from a 82 MHz oscillator.…”

Negligible cation effect on the vibrational relaxation dynamics of water molecules in NaClO4 and LiClO4 aqueous electrolyte solutions

“…The experimental setup for the ultrafast IR spectroscopy has been described elsewhere. 39,41,44,45 Briey, a ps amplier and a fs amplier are independently operated and synchronized with the same seed pulse from a Ti-sapphire oscillator. The ps amplier pumps an OPA to produce $1 ps Mid-IR pulses with a bandwidth $18 cm À1 at 1 kHz repetition rate.…”

“…39,40 We found that ions form significant amount of clusters in KSCN aqueous solutions. [41][42][43][44] We further investigated the different behavior of rotational dynamics of water molecules and anions due to the structural inhomogeneity and discussed the anion and cation effects. 43,44 To investigate the specic cation effect on the water dynamics in electrolyte aqueous solution, it is necessary to separate the contribution from dynamics of water in anionic hydration shell and water dynamics in the bulk liquid.…”

Direct Vibrational Energy Transfer in Monomeric Water Probed with Ultrafast Two Dimensional Infrared Spectroscopy

“…However, as the author pointed out, the two dimensional Infrared (2D IR) spectroscopy with access to all vibrational modes is needed for the comprehensive analysis of vibrational energy relaxation in monomeric water. In our previous studies, we developed the modespecific vibrational energy transfer method based on the 2D IR spectroscopy and successfully applied this method to the electrolyte aqueous solution systems [31][32][33][34][35][36]. Following the experimental and data analysis procedures established in our previous reports, we investigated the vibrational relaxation dynamics of monomeric water molecule dissolved in d-chloroform solution using the 2D IR spectroscopy.…”