Biswas Aritri, Mallik Bhabani S
Department of Chemistry, Indian Institute of Technology Hyderabad, Sangareddy502285, Telangana, India.
J Phys Chem B. 2022 Nov 3;126(43):8838-8850. doi: 10.1021/acs.jpcb.2c04901. Epub 2022 Oct 20.
Classical molecular dynamics simulations were performed to assess an atomistic interpretation of the ion-probe structural interactions in two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm][NTf] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BDimIm][NTf] through computational ultrafast spectroscopy. The nitrile stretching vibrations of the thiocyanate anion, [SCN], serve as the local mode of the ultrafast system dynamics within the imidazolium-based ionic liquid environment. The wavelet transform of classical trajectories determines the time-varying fluctuating frequencies and the stretch spectral signatures of SCN in the normalized distribution. However, computational modeling of the two-dimensional (2D) spectra from the wavelet-derived vibrational frequencies yields time evolution of the local molecular structure along with the varied time-dependent dynamics of the spectral diffusion process. We calculated the frequency-frequency correlation functions (FFCFs), time correlations associated with the ion-pair and -cage dynamics, and mean square displacements as a function of time, depicting diffusive dynamics. The calculated results based on the pair correlation functions and the distribution of atomic density suggest that the hydrogen and methylated carbon at the two-position of the imidazolium ring of [BMIm] and [BDimIm] cations, respectively, strongly interact with the probe through the N of the thiocyanate anion rather than the S atom. The center-of-mass center-of-mass (COM-COM) cation-probe radial distribution functions (RDFs) in conjunction with the site-specific structural analysis further reveal well-structured interactions of the thiocyanate ion and [BMIm] cation rather than the [BDimIm] cation. In contrast, the anion-probe COM-COM RDFs depict weak interactive associations within the vibrational probe [SCN] and [NTf] ions. Methylation at the two-position of the imidazolium ring predicts slower structural reorganization and breaking and reformation dynamics of the ion pairs and cages within the ionic liquid framework.
通过计算超快光谱法进行经典分子动力学模拟，以评估两种典型离子液体（ILs），即1-丁基-3-甲基咪唑双（三氟甲基磺酰）亚胺[BMIm][NTf]和1-丁基-2,3-二甲基咪唑双（三氟甲基磺酰）亚胺[BDimIm][NTf]中离子-探针结构相互作用的原子解释。硫氰酸根阴离子[SCN]的腈伸缩振动，作为基于咪唑鎓的离子液体环境中超快系统动力学的局部模式。经典轨迹的小波变换确定了归一化分布中SCN随时间变化的波动频率和拉伸光谱特征。然而，从小波导出的振动频率计算二维（2D）光谱的计算模型，产生了局部分子结构的时间演化以及光谱扩散过程中随时间变化的动力学。我们计算了频率-频率相关函数（FFCFs）、与离子对和笼动力学相关的时间相关性以及作为时间函数的均方位移，描述了扩散动力学。基于对相关函数和原子密度分布的计算结果表明，[BMIm]和[BDimIm]阳离子咪唑环2位的氢和甲基化碳分别通过硫氰酸根阴离子的N而不是S原子与探针强烈相互作用。质心-质心（COM-COM）阳离子-探针径向分布函数（RDFs）结合位点特异性结构分析进一步揭示了硫氰酸根离子与[BMIm]阳离子而非[BDimIm]阳离子之间结构良好的相互作用。相比之下，阴离子-探针COM-COM RDFs描绘了振动探针[SCN]和[NTf]离子之间的弱相互作用关联。咪唑环2位的甲基化预示着离子液体框架内离子对和笼的结构重组以及断裂和再形成动力学较慢。
