Morris Daniel C, Prescott Stuart W, Harper Jason B
School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia.
Phys Chem Chem Phys. 2021 Apr 28;23(16):9878-9888. doi: 10.1039/d0cp06066f.
A series of ionic liquids based on the 1-alkyl-3-methylimidazolium cations were examined as components of the solvent mixture for a bimolecular substitution process. The effects on both the rate coefficient of the process and the NMR spin-spin relaxation of the solvent components of changing either the alkyl chain length or the amount of ionic liquid in the reaction mixture were determined. At a constant mole fraction, a shorter alkyl chain length resulted in a greater rate coefficient enhancement and a longer relaxation time, with the opposite effects for a longer alkyl chain length. For a given ionic liquid, increasing the proportion of salt in the reaction mixture resulted in a greater rate coefficient and a shorter relaxation time. The microscopic origins of the rate coefficient enhancement were determined and a step change found in the activation parameters on increasing the alkyl chain length from hexyl to octyl, suggesting notable structuring in solution. Across a range of ionic liquids and solvent compositions, the relaxation time from NMR measurements was shown to relate to the reaction rate coefficient. The approach of using fast and simple NMR relaxation measurements to predict reaction outcomes was exemplified using a morpholinium-based ionic liquid.
研究了一系列基于1-烷基-3-甲基咪唑鎓阳离子的离子液体作为双分子取代过程溶剂混合物的成分。确定了改变反应混合物中烷基链长度或离子液体用量对该过程的速率系数以及溶剂成分的核磁共振自旋-自旋弛豫的影响。在恒定摩尔分数下，较短的烷基链长度导致更大的速率系数增强和更长的弛豫时间，而较长的烷基链长度则产生相反的效果。对于给定的离子液体，增加反应混合物中盐的比例会导致更大的速率系数和更短的弛豫时间。确定了速率系数增强的微观起源，并发现随着烷基链长度从己基增加到辛基，活化参数发生阶跃变化，这表明溶液中存在显著的结构。在一系列离子液体和溶剂组成中，核磁共振测量得到的弛豫时间与反应速率系数相关。使用基于吗啉鎓的离子液体举例说明了利用快速简单的核磁共振弛豫测量来预测反应结果的方法。
