Preparation of protic ionic liquids with minimal water content and 15N NMR study of proton transfer
作者:Geoffrey L. Burrell、Iko M. Burgar、Frances Separovic、Noel F. Dunlop
DOI:10.1039/b921432a
日期:——
Low-molecular-weight Brønsted acids and amine bases were used to reproducibly prepare very dry, high-purity room-temperature protic ionic liquids (PILs). A series of eight amine bases and six Brønsted acids were combined to produce 48 mixtures, of which 18 were liquid at room temperature. The phase transitions and thermal decomposition temperatures were determined for each mixture; whereas viscosity, density and conductivity were determined for the room-temperature liquids. By utilising 15N NMR it was possible to distinguish between neutral and ionised amine bases (ammonia vs. ammonium-type ion), which indicated that the protic ionic liquids were completely ionised when made as a stoichiometric mixture. However, a Walden plot comparison of fluidity and molar conductivity indicated the majority of PILs had much lower conductivity than predicted by viscosity unless the base contained excess proton-donating groups. This disparity is indicative of protic ionic molecules forming neutral aggregates or non-Newtonian fluid hydrogen-bonded networks with a secondary Grotthuss proton-hopping mechanism arising from polyprotic bases.
低分子量的布朗斯特酸和胺碱被用来重复制备非常干燥、高纯度的室温质子型离子液体(PILs)。将一系列八种胺碱和六种布朗斯特酸组合,产生了48种混合物,其中18种在室温下为液态。每种混合物的相变和热分解温度被测定;而室温液体的粘度、密度和电导率也被测定。通过利用15N NMR,能够区分中性和离子化的胺碱(氨与铵型离子),这表明在作为化学计量混合物制备时,质子型离子液体是完全离子化的。然而,流动性和摩尔电导率的沃尔登图比较表明,除了当碱包含过量的质子给体基团时,大多数PIL的电导率远低于粘度所预测的值。这种差异表明,质子型离子分子形成了中性聚集体或非牛顿流体氢键网络,并出现了由多质子碱引起的次级格罗图斯质子跳跃机制。