Ion-Molecule Complex Dissociation and Formation Dynamics in LiCl Aqueous Solutions from 2D IR Spectroscopy

Ion-molecule complex dynamics as well as water dynamics in concentrated lithium chloride (LiCl) solutions are examined using ultrafast two-dimensional infrared (2D IR) spectroscopy with the CN stretching mode of methyl thiocyanate (MeSCN) as the vibrational probe. In pure water, MeSCN has a narrow symmetric absorption line shape. 2D IR spectral diffusion measurements of the CN stretch give the identical time dependence of water dynamics, as previously observed using the OD stretch of HOD in H₂O. In concentrated LiCl solutions, the IR absorption spectrum of MeSCN displays two distinct peaks, one corresponding to water H-bonded to the N lone pair of MeSCN (W) and the other corresponding to Li⁺ associated with the N (L). These two species are in equilibrium, and switching of the CN bonding partner from Li⁺ to H₂O and vice versa was observed and explicated with 2D IR chemical exchange spectroscopy. The MeSCN·Li⁺ complex dissociation time constant, τ_LW, and the MeSCN·H₂O dissociation time constant, τ_WL, were determined. The observed τ_LW chemical exchange dissociation time constant changes from 60 to 40 ps as the LiCl concentration decreases from ∼10.7 to ∼7.7 M, mainly due to the increase of the water concentration as the LiCl concentration is reduced. The observed time constants are independent of the model for the chemical reaction. With the assumption of a simple chemical equation, MeSCN·Li⁺ + H₂O ⇄ MeSCN·H₂O + Li⁺, the equilibrium equation rate constants were obtained from the observed chemical exchange time constants. It was determined that the equilibrium rate constants barely change even though the viscosity changes by a factor of 2 and the ionic strength changes by a factor of 1.4. Extrapolation to dilute LiCl solution estimates the τ_LW to be ∼30 ps. The orientational relaxation (anisotropy decay) of both the W and L complexes was measured using polarization selective 2D IR experiments. The lithium-bonded species undergoes orientational relaxation ∼3 times slower than the water-bonded species in each LiCl solution studied. The difference demonstrates the distinct interactions with the medium experienced by the neutral and charged species in the concentrated salt solutions.