Dynamics of Water Molecules and Ions in Concentrated Lithium Chloride Solutions Probed with Ultrafast 2D IR Spectroscopy

Abstract

Water and ion dynamics in concentrated LiCl solutions were studied using ultrafast 2D IR spectroscopy with the methyl thiocyanate (MeSCN) CN stretch as the vibrational probe. The IR absorption spectrum of MeSCN has two peaks, one peak for water associated with the nitrogen lone pair of MeSCN (W) and the other peak corresponding to Li+ associated with the lone pair (L). To extract the spectral diffusion (structural dynamics) of W and L species, we developed a method that isolates the peak of interest by subtracting the 2D Gaussian proxies of multiple interfering peaks. Center line slope data (normalized frequency–frequency correlation function) for 2D bands from the W and L are fit with triexponential functions. The fastest component (1.1–1.6 ps) is assigned to local hydrogen bond length fluctuations. The intermediate timescale (∼4.0 ps) corresponds to the hydrogen bond network rearrangement. The slowest component decays in ∼40 ps and corresponds to ion pair and ion cluster dynamics. The very similar W and L spectral diffusion indicates that the motions of the water and ions are strongly coupled. Orientational relaxations of the W and L species were extracted using a new method to eliminate the effects of overlapping peaks. The results show that MeSCN bound to water undergoes orientational relaxation significantly faster than MeSCN bound to Li+. The orientational and spectral diffusion results are compared. A Stark coupling model is used to extract the root mean square average electric field caused by the ion clouds along the CN moiety as a function of concentration.

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).…”

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.…”

Cooperative CO2 absorption by amino acid-based ionic liquids with balanced dual sites

“…The 2D IR spectroscopy is a usual method to study the dynamics of interactions. 36,37 Thereinto, the IR spectra of ILs associated with the CO 2 content in 2D correction forms are shown in Fig. 5.…”