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

Abstract

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, τLW, and the MeSCN·H2O dissociation time constant, τWL, were determined. The observed τLW 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 τLW 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.

Experimental observation of nanophase segregation in aqueous salt solutions around the predicted liquid–liquid transition in water

“…This manifests itself as spectral diffusion or lineshape evolution as the waiting time approaches the time-scale for hydrogen-bond fluctuation. 45,46 Quantification of the lineshape evolution using the ellipticity of the lineshape 45 indicates a timescale of B500 fs, this timescale is similar to the value obtained at room temperature for methyl thiocyanate by Yuan et al 21,22 Such subpicosecond dynamics are consistent with previous 2D-IR studies on hydrated ionic systems. 46 In addition, a contribution from longer timescale dynamics is also evident from the fact that the lineshape evolution is not complete even at waiting times of 2 ps.…”

“…Yuan et al used a similar approach to use methyl thiocyanate as a probe of room temperature aqueous LiCl solutions. 21,22 Fourier-transform infrared spectroscopy (FTIR) and femtosecond infrared pump-probe spectroscopy 23,24 were used to measure the temperature-dependent IR spectrum and the vibrational excited-state lifetimes. Ultrafast optical Kerr-effect (OKE) spectroscopy [25][26][27][28] was used to measure the temperature-dependent depolarized Raman spectrum of a eutectic solution of LiSCN in water (LiSCN:5.8H 2 O).…”

Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes

“…The 2262 cm –1 band intensity decreases upon the addition of salts, and a new band associated with the cation-coordinated C≡N stretch emerges at higher frequencies ( Figure 2 b, Supporting Information, Figure S4 ). The upshift of the nitrile stretch upon hydrogen bonding 19 , 20 , 80 or ion coordination 38 , 79 , 81 is well documented and can be qualitatively explained by the ion-induced shift of nitrile electron density toward the nitrogen that partially removes its contribution to the antibonding molecular orbital in the C≡N moiety, thus strengthening the bond. This polarization is reflected in the increase of the C≡N transition dipole moment ( Figure S5 ).…”

Least-Squares Fitting of Multidimensional Spectra to Kubo Lineshape Models

“…We simulate the isotropic response of the C≡N stretch of MeSCN in H2O as characterized by Yuan and Fayer. [45][46][47] We add Gaussian noise (SNR ~600:1) to the free induction decay to simulate experimental data. Here we define signal in the SNR calculation as the peak magnitude of the 0-1 transition of the transient absorption spectrum at zero waiting time.…”

“…In reality, the Kubo time constants obtained from CLS fits are uncertain, with reported errors ranging from 5-50%. 45,46,50,51 This additional uncertainty must be accounted for when fitting the linear absorption spectrum if we hope to achieve accurate uncertainties for homogeneous dephasing and Kubo amplitudes. Accounting for this effect involves propagating the uncertainty of the Kubo time constants through the standard variance-covariance matrix of the linear absorption fit to obtain the modified variance-covariance matrix (Eq.…”