Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy
Ion/Water Network Structural Dynamics in Highly Concentrated Lithium Chloride and Lithium Bromide Solutions Probed with Ultrafast Infrared Spectroscopy
复制标题
用超快红外光谱探测高浓度氯化锂和溴化锂溶液中的离子/水网络结构动力学
DOI：
10.1021/acs.jpcb.2c08792
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发表时间：
2023
期刊：
影响因子：
--
通讯作者：
Fayer, Michael D.
中科院分区：
文献类型：
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作者：
Roget, Sean A.;Heck, Tristan R.;Carter-Fenk, Kimberly A.;Fayer, Michael D.
利用超快偏振选择性泵浦探测（PSPP）实验，在1-4 ~ 1-16个水分子/离子对的范围内，对高浓度LiCl和LiBr水溶液的结构动力学进行了研究。在这些高盐浓度下，存在具有复杂结构动力学的扩展离子/水网络。从PSPP实验人口衰减突出两个不同的水成分。从衰减的频率依赖性的振幅，光谱的羟基绑定到卤化物和水氧，这是不能观察到的FT-IR光谱。PSPP实验还测量频率相关的水取向弛豫。在短时间内，在一个有限的角锥内发生摆动动力学。在高浓度下，锥角取决于频率（氢键强度），但在较高的水浓度（每个离子对>10个沃茨）下，不存在频率依赖性。平均锥角随离子浓度的降低而增大。完全HOD取向弛豫的慢时间常数与浓度无关，但在LiCl中比在LiBr中慢。从文献中的结构MD模拟LiCl的比较表明，损失的锥角的波长依赖性和增加的锥角的浓度降低时发生的患病率大的离子/水簇让位于接触离子对。
The structural dynamics of highly concentrated LiCl and LiBr aqueous solutions were observed from 1–4 to 1–16 water molecules per ion pair using ultrafast polarization-selective pump–probe (PSPP) experiments on the OD stretch of dilute HOD. At these high salt concentrations, an extended ion/water network exists with complex structural dynamics. Population decays from PSPP experiments highlight two distinct water components. From the frequency-dependent amplitudes of the decays, the spectra of hydroxyls bound to halides and to water oxygens are obtained, which are not observable in the FT-IR spectra. PSPP experiments also measure frequency-dependent water orientational relaxation. At short times, wobbling dynamics within a restricted angular cone occurs. At high concentrations, the cone angles are dependent on frequency (hydrogen bond strength), but at higher water concentrations (>10 waters per ion pair), there is no frequency dependence. The average cone angle increases as the ion concentration decreases. The slow time constant for complete HOD orientational relaxation is independent of concentration but slower in LiCl than in LiBr. Comparison to structural MD simulations of LiCl from the literature indicates that the loss of the cone angle wavelength dependence and the increase in the cone angles as the concentration decreases occur as the prevalence of large ion/water clusters gives way to contact ion pairs.