Giammanco Chiara H, Kramer Patrick L, Yamada Steven A, Nishida Jun, Tamimi Amr, Fayer Michael D
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Chem Phys. 2016 Mar 14;144(10):104506. doi: 10.1063/1.4943390.
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 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.
离子液体（ILs）具有广泛可调的结构基序以及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。为了指导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）在这些独特组织材料中存在的复杂溶剂化结构内基本时间尺度（飞秒到皮秒）上的动力学和相互作用细节可能会有所帮助。通过使用超快红外光谱探测CO₂不对称伸缩，研究了室温离子液体1-乙基-3-甲基咪唑双（三氟甲基磺酰）亚胺（EmimNTf₂）中CO₂的旋转和局部结构波动动力学。偏振选择性泵浦探测测量得到了CO₂振动跃迁偶极子的取向相关函数。发现二氧化碳的重新取向发生在三个时间尺度上：0.91±0.03、8.3±0.1、54±1皮秒。前两个归因于由离子液体阳离子和阴离子对CO₂运动的门控引起的受限摆动运动。最后的（最慢的）衰减对应于完全取向随机化。二维红外振动回波（2D IR）光谱通过二维线形的时间依赖性提供了关于导致光谱扩散的结构重排的信息。对时间相关的二维红外光谱的分析产生了频率-频率相关函数（FFCF）。由于一种称为重新取向诱导光谱扩散（RISD）的现象，在平行泵浦和垂直泵浦几何结构中对CO₂不对称伸缩进行的偏振选择性二维红外实验产生了显著不同的FFCF，揭示了与在（独立测量的）旋转时间尺度上缓慢演化的液体结构的强矢量相互作用。为了将RISD对FFCF的贡献与结构光谱扩散贡献分开，对先前开发的一阶斯塔克效应RISD模型进行了重新表述，以描述二阶（二次）斯塔克效应——由于CO₂没有永久偶极矩，一阶斯塔克效应消失。通过这种分析，我们表征了离子液体溶剂化环境中CO₂的结构波动，其分为仅幅度相关以及液体随时间变化的电场的幅度和方向组合相关。这种新方法将能够对各种离子液体系统中CO₂的动力学进行高度精确的比较。
