Institute of Chemical Physics, Faculty of Physics, Vilnius University, Vilnius LT-10257, Lithuania.
DTU Chemistry, Technical University of Denmark, Kongens Lyngby DK-2800, Denmark.
J Phys Chem B. 2021 Dec 9;125(48):13255-13266. doi: 10.1021/acs.jpcb.1c08215. Epub 2021 Nov 22.
The H NMR chemical shift of water exhibits non-monotonic dependence on the composition of an aqueous mixture of 1-butyl-3-methylimidazolium chloride, [C4mim][Cl], ionic liquid (IL). A clear minimum is observed for the H NMR chemical shift at a molar fraction of the IL of 0.34. To scrutinize the molecular mechanism behind this phenomenon, extensive classical molecular dynamics simulations of [C4mim][Cl] IL and its mixtures with water were carried out. A combined quantum mechanics/molecular mechanics approach based on the density functional theory was applied to predict the NMR chemical shifts. The proliferation of strongly hydrogen-bonded complexes between chloride anions and water molecules is found to be the reason behind the increasing H NMR chemical shift of water when its molar fraction in the mixture is low and decreasing. The model shows that the chemical shift of water molecules that are trapped in the IL matrix without direct hydrogen bonding to the anions is considerably smaller than the H NMR chemical shift predicted for the neat water. The structural features of neat IL and its mixtures with water have also been analyzed in relation to their NMR properties. The H NMR spectrum of neat [C4mim][Cl] was predicted and found to be in very reasonable agreement with the experimental data. Finally, the experimentally observed strong dependence of the chemical shift of the proton at position 2 in the imidazolium ring on the composition of the mixture was rationalized.
水的 H NMR 化学位移对 1-丁基-3-甲基咪唑氯([C4mim][Cl])离子液体 (IL) 的水混合物的组成表现出非单调依赖性。在 IL 的摩尔分数为 0.34 时，观察到 H NMR 化学位移的明显最小值。为了仔细研究这种现象背后的分子机制，对 [C4mim][Cl] IL 及其与水的混合物进行了广泛的经典分子动力学模拟。应用基于密度泛函理论的量子力学/分子力学方法来预测 NMR 化学位移。发现氯离子和水分子之间强烈氢键复合物的增殖是水的 H NMR 化学位移增加的原因，当混合物中水的摩尔分数较低时，H NMR 化学位移会降低。该模型表明，没有直接与阴离子形成氢键的 IL 基质中捕获的水分子的化学位移明显小于纯净水中预测的 H NMR 化学位移。还分析了纯净 IL 及其与水的混合物的结构特征与其 NMR 性质的关系。预测了纯净 [C4mim][Cl] 的 H NMR 光谱，发现与实验数据非常吻合。最后，对实验观察到的咪唑环上 2 位质子的化学位移对混合物组成的强烈依赖性进行了合理化解释。
