Avula Nikhil V S, Mondal Anirban, Balasubramanian Sundaram
Chemistry and Physics of Materials Unit , Jawaharlal Nehru Centre for Advanced Scientific Research , Bangalore 560064 , India.
J Phys Chem Lett. 2018 Jun 21;9(12):3511-3516. doi: 10.1021/acs.jpclett.8b01481. Epub 2018 Jun 13.
The extent of charge transfer between the cation and the anion in a room-temperature ionic liquid depends on the basicity of the anion. Ion charges determined in the condensed state via density functional theory calculations capture this effect rather well, and charges derived in such a manner have been employed in force field-based molecular dynamics simulations to quantitatively reproduce several physical properties of the liquids. However, the issue of transferability of cation charges in mixtures of ionic liquids, say with one type of cation and two different anion types needs to be addressed. Herein, we demonstrate that the cation charge in such a mixture varies linearly with anion composition, a result that ties in rather well with X-ray photoelectron spectroscopic experiments. The variation in cation charge with bulk anion composition is shown to be a result of changes in its coordination environment. Cations surrounded by a higher proportion of more basic anions possess lower charges than those surrounded by less basic anions. Time scales for the exchange of anion types for the occupation of hydrogen bonding sites around the cation have been determined and are seen to be constituted by three processes-breakage of existing hydrogen bond, diffusion to the hydrogen bonding site and displacement of the incumbent anion from its site in the cation coordination shell. These time scales explain the differences observed between infrared and NMR spectroscopic experiments in ionic liquid mixtures rather well.
室温离子液体中阳离子与阴离子之间电荷转移的程度取决于阴离子的碱性。通过密度泛函理论计算在凝聚态下确定的离子电荷能较好地捕捉到这种效应，并且以这种方式得出的电荷已被用于基于力场的分子动力学模拟中，以定量再现液体的几种物理性质。然而，离子液体混合物中阳离子电荷的可转移性问题，比如说含有一种阳离子和两种不同阴离子类型的混合物，需要加以解决。在此，我们证明了这种混合物中的阳离子电荷随阴离子组成呈线性变化，这一结果与X射线光电子能谱实验相当吻合。阳离子电荷随本体阴离子组成的变化被证明是其配位环境变化的结果。被比例更高的碱性更强的阴离子包围的阳离子所带电荷比被碱性较弱的阴离子包围的阳离子所带电荷更低。已经确定了阳离子周围氢键位点被不同阴离子类型占据时阴离子交换的时间尺度，并且发现其由三个过程组成——现有氢键的断裂、扩散到氢键位点以及取代阳离子配位壳层中在位阴离子。这些时间尺度很好地解释了在离子液体混合物的红外光谱和核磁共振光谱实验中观察到的差异。
