Carbon dioxide in an ionic liquid: Structural and rotational dynamics
Carbon dioxide in an ionic liquid: Structural and rotational dynamics
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DOI：
10.1063/1.4943390
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发表时间：
2016-03-14
影响因子：
4.4
通讯作者：
Fayer, Michael D.
中科院分区：
文献类型：
--
作者：
Giammanco, Chiara H.;Kramer, Patrick L.;Fayer, Michael D.
离子液体（ILs），具有广泛的可调结构基序和分子间相互作用的溶质，已被提出作为可能的碳捕获介质。为了告知最佳离子液体系统的选择，了解溶解气体（特别是二氧化碳（CO2））在这些独特组织的材料中存在的复杂溶剂化结构内在基本时间尺度（飞秒至皮秒）上的动力学和相互作用的细节可能是有用的。利用超快红外光谱研究了室温离子液体1-乙基-3-甲基咪唑双（三氟甲基磺酰基）酰亚胺（EmimNTf（2））中CO2的转动和局域结构涨落动力学.偏振选择性泵浦探测测量产生的CO2振动跃迁偶极子的取向相关函数。发现二氧化碳的重定向发生在3个时间尺度上：0.91 +/- 0.03，8.3 +/- 0.1，54 +/- 1 ps。最初的两个是由于限制摆动运动起源于门控的CO2运动的IL阳离子和阴离子。最后（最慢）的衰减对应于完全的取向随机化。二维红外振动回波（2D IR）光谱提供了结构重排的信息，这会导致光谱扩散，通过2D线形的时间依赖性。对随时间变化的二维红外光谱进行分析，得到频率-频率相关函数（FFCF）。偏振选择性的二维红外实验上进行的CO2不对称拉伸的平行和平行泵浦的几何形状产生显着不同的FFCF由于一种现象被称为重定向诱导光谱扩散（RISD），揭示了强大的矢量相互作用与液体结构，慢慢演变（独立测量）旋转时间尺度。为了将RISD对FFCF的贡献从结构光谱扩散贡献中分离出来，将先前开发的一阶Stark效应RISD模型重新公式化以描述二阶（二次）Stark效应-一阶Stark效应消失，因为CO2不具有永久偶极矩。通过这种分析，我们表征了CO2在离子液体溶剂化环境中的结构波动，其分离成仅大小和组合的大小和方向相关的液体的随时间变化的电场。这种新的方法将使各种离子液体系统中的CO2动力学之间的比较非常精辟。(C)2016 AIP Publishing LLC.
Ionic liquids (ILs), which have widely tunable structural motifs and intermolecular interactions with solutes, have been proposed as possible carbon capture media. To inform the choice of an optimal ionic liquid system, it can be useful to understand the details of dynamics and interactions on fundamental time scales (femtoseconds to picoseconds) of dissolved gases, particularly carbon dioxide (CO2), within the complex solvation structures present in these uniquely organized materials. The rotational and local structural fluctuation dynamics of CO2 in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf(2)) were investigated by using ultrafast infrared spectroscopy to interrogate the CO2 asymmetric stretch. Polarization-selective pump probe measurements yielded the orientational correlation function of the CO2 vibrational transition dipole. It was found that reorientation of the carbon dioxide occurs on 3 time scales: 0.91 +/- 0.03, 8.3 +/- 0.1, 54 +/- 1 ps. The initial two are attributed to restricted wobbling motions originating from a gating of CO2 motions by the IL cations and anions. The final (slowest) decay corresponds to complete orientational randomization. Two-dimensional infrared vibrational echo (2D IR) spectroscopy provided information on structural rearrangements, which cause spectral diffusion, through the time dependence of the 2D line shape. Analysis of the time-dependent 2D IR spectra yields the frequency-frequency correlation function (FFCF). Polarization-selective 2D IR experiments conducted on the CO2 asymmetric stretch in the parallel-and perpendicular-pumped geometries yield significantly different FFCFs due to a phenomenon known as reorientation-induced spectral diffusion (RISD), revealing strong vector interactions with the liquid structures that evolve slowly on the (independently measured) rotation time scales. To separate the RISD contribution to the FFCF from the structural spectral diffusion contribution, the previously developed first order Stark effect RISD model is reformulated to describe the second order (quadratic) Stark effect-the first order Stark effect vanishes because CO2 does not have a permanent dipole moment. Through this analysis, we characterize the structural fluctuations of CO2 in the ionic liquid solvation environment, which separate into magnitude-only and combined magnitude and directional correlations of the liquid's time dependent electric field. This new methodology will enable highly incisive comparisons between CO2 dynamics in a variety of ionic liquid systems. (C) 2016 AIP Publishing LLC.