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Dynamics of Water Molecules and Ions in Concentrated Lithium Chloride Solutions Probed with Ultrafast 2D IR Spectroscopy
journal contribution
posted on 2019-08-22, 18:39 authored by Rongfeng Yuan, Michael D. FayerWater 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.