Carbon dioxide in an ionic liquid: Structural and rotational dynamics,The Journal of Chemical Physics

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Carbon dioxide in an ionic liquid: Structural and rotational dynamics
The Journal of Chemical Physics ( IF 3.1 ) Pub Date : 2016-03-11 15:44:42 , DOI: 10.1063/1.4943390
Chiara H. Giammanco ₁ , Patrick L. Kramer ₁ , Steven A. Yamada ₁ , Jun Nishida ₁ , Amr Tamimi ₁ , Michael D. Fayer ₁

Affiliation

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 (EmimNTf₂) 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 spectraldiffusion, 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 spectraldiffusion (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 spectraldiffusion 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.

中文翻译：

离子液体中的二氧化碳：结构和旋转动力学

离子液体（ILs）具有广泛的可调节结构基序和与溶质的分子间相互作用，已被提议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，了解存在的复杂溶剂化结构内溶解气体（尤其是二氧化碳（CO ₂））的基本时间尺度（飞秒到皮秒）的动力学和相互作用的细节可能很有用。在这些独特组织的材料中。旋转和局部结构波动CO的动力学₂在室温离子液体1-乙基-3-甲基咪唑鎓双（三氟甲基磺酰）亚胺（EmimNTf ₂）通过使用超快红外光谱来询问调查CO ₂不对称拉伸。极化选择性泵浦探头的测量结果得出了CO ₂振动跃迁偶极子的方向相关函数。发现二氧化碳的重新定向发生在三个时间尺度上：0.91±0.03、8.3±0.1、54±1ps。最初的两个归因于源于CO ₂门控的受限制的摆动运动IL阳离子和阴离子的运动。最终（最慢）衰减对应于完整的定向随机化。二维红外振动回波（2D IR）光谱通过2D线形的时间依赖性提供了有关引起光谱扩散的结构重排的信息。分析随时间变化的2D红外光谱，得出频率-频率相关函数（FFCF）。在CO ₂上进行的极化选择性2D IR实验平行和垂直泵浦几何形状中的不对称拉伸会产生明显不同的FFCF，这是由于一种称为重新定向诱导光谱扩散（RISD）的现象，揭示了与液体结构的强大矢量相互作用，这些液体结构在（独立测量的）旋转时间尺度上缓慢演化。为了将RISD对FFCF的贡献与结构光谱扩散的贡献分开，重新开发了先前开发的一阶Stark效应RISD模型以描述二阶（二次）Stark效应-因为CO ₂没有永久性，所以一阶Stark效应消失了。偶极矩。通过此分析，我们表征了CO ₂的结构波动在离子液体溶剂化环境中，它分为液体的随时间变化的电场的仅幅度和合并的幅度和方向相关性。这种新方法将使各种离子液体系统中的CO ₂动力学之间能够进行高度敏锐的比较。

更新日期：2016-03-12

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