Nonresonant and Resonant Mode-Specific Intermolecular Vibrational Energy Transfers in Electrolyte Aqueous Solutions

Abstract

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermi's Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.

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

“…Figure 6 displays the vibrational energy exchange spectra of MSCN/MS 13 C 15 N = 1/1 (M = Li, Na, K, and Cs) at the ion concentration of 5 mol/kg at various waiting times. As elaborated in our previous publications, 30,31,39 at 0 ps vibrational energy transfers between SCN − and S 13 C 15 N − have not occurred so that only two pairs of diagonal peaks (peaks 1−4) show up in the spectrum. Peaks 1 and 2 are the CN stretch 0−1 and 1−2 transitions, respectively, and peaks 3 and 4 are the 13 C 15 N stretch 0−1 and 1−2 transitions, respectively.…”

“…As previously measured, 30 significantly slower than those (faster than 30 ps) of the thiocyanate anion rotations in the solutions. The resonant energy transfer time in a pure KSCN saturated aqueous solution is 3 ps, 30,31 indicating that the resonant energy transfer time in the 2% sample for S 13 C 15 N − will be 150 ps which is significantly slower than the rotations.…”

“…20,27 The effects of ion/ion interactions were almost completely ignored, though in the studies the ion concentrations were typically high (>2 M) in the solutions where substantial amounts of ion pairing and clustering are anticipated from the predictions of MD simulations. 28,29 Recently, by applying the ultrafast vibrational energy exchange method, 30,31 we found that ions form significant amounts of clusters in MSCN (M = K, NH 4 , Na, Cs, and Li) aqueous solutions (≥1 M). 32,33 Combining vibrational energy exchange measurements with ultrafast anisotropy decay measurements, we also found that in the KSCN aqueous solutions the structural inhomogeneity because of ion clustering leads to distinct rotational dynamics of water molecules and SCN − anions.…”

Ion Segregation in Aqueous Solutions

“…The detailed energy transfer kinetics and data analysis for the series of KSCN solutions are provided in the Supporting Information, Figures S6−S12. As elaborated in previous publications, 31,38 according to the dipole/dipole approximation of which the energy transfer rate is inversely proportional to the sixth power of donor/acceptor distance, 45 slower energy transfer results from a decrease in the population of thiocyanate anions in close contact (clustered) in the solution, if the transition dipole moments of the energy donor and acceptor and the local refractive index remain constant. Experiments show that the latter two factors change for less than 2% by the addition of 5 M KI (see Supporting Information Tables S3 and S4).…”

“…By monitoring the vibrational energy transfer kinetics 30,31,37,38 between SCN − and S 13 C 15 N − , we were able to determine the relative anion distances in the solutions. We obtained through these experiments clear evidence that both dynamical segregations and ion clusters exist in the unsaturated strong electrolyte aqueous solutions.…”