Wang Yong-Lei
Department of Chemistry , Stanford University , Stanford, California 94305 , United States.
J Phys Chem B. 2018 Jun 28;122(25):6570-6585. doi: 10.1021/acs.jpcb.8b02899. Epub 2018 Jun 13.
The delicate trade-off between hydrogen bonding and π-type coordinations plays a pivotal role in stabilizing molecular structures in ionic liquids bearing multiple hydrogen bonding sites and heteroaromatic ring planes. By performing extensive atomistic simulations, we have investigated the effect of aliphatic chain length in imidazolium cations on liquid morphologies, hydrogen bonding and π-type structures, and the corresponding dynamical quantities in imidazolium bis(oxalato)borate ionic liquids. The liquid morphologies are characterized by segregated apolar clusters (domains) within polar framework in liquid samples with short aliphatic chains in imidazolium cations and are transformed to spongelike polar and apolar arrangements in liquid matrices with lengthening aliphatic chains in imidazolium cations. Such a striking evolution of liquid morphologies of imidazolium bis(oxalato)borate ionic liquids is qualitatively characterized by total and partial X-ray scattering static structural factors. Preferential hydrogen bonds and distinctive π-type coordinations among imidazolium and oxalato ring planes coexist in ionic liquid matrices. A gradual addition of methylene units to imidazolium cations leads to a substantial increase in hydrogen bonding strength, which, however, results in decreased π-type coordinations between imidazolium and oxalato ring planes, indicating distinct competitive structural characteristics between hydrogen bonding and π-type associations between imidazolium and oxalato ring planes. A prevalent cooperative feature is observed in continuous and intermittent hydrogen bonding dynamics and in translational and reorientational dynamics of imidazolium and oxalato ring planes with lengthening aliphatic chains in imidazolium cations. The competitive structural trade-off and cooperative dynamical interplay of hydrogen bonding and π-type interactions between imidazolium cations and bis(oxalato)borate anions are intrinsically correlated with short-range collective interactions between alkyl units in imidazolium cations and long-range Coulombic interactions between imidazolium and oxalato ring planes in heterogeneous ionic environments.
在含有多个氢键位点和杂芳环平面的离子液体中，氢键与π型配位之间微妙的平衡在稳定分子结构方面起着关键作用。通过进行广泛的原子模拟，我们研究了咪唑阳离子中脂肪链长度对双（草酸根）硼酸咪唑离子液体的液体形态、氢键和π型结构以及相应动力学量的影响。液体形态的特征是，在咪唑阳离子中具有短脂肪链的液体样品的极性框架内存在分离的非极性簇（域），随着咪唑阳离子中脂肪链的延长，液体基质转变为海绵状的极性和非极性排列。双（草酸根）硼酸咪唑离子液体这种显著的液体形态演变通过全X射线散射和部分X射线散射静态结构因子进行定性表征。在离子液体基质中，咪唑和草酸根环平面之间存在优先氢键和独特的π型配位。向咪唑阳离子逐渐添加亚甲基单元会导致氢键强度大幅增加，然而，这会导致咪唑和草酸根环平面之间的π型配位减少，表明咪唑和草酸根环平面之间的氢键和π型缔合具有明显的竞争结构特征。在咪唑阳离子中脂肪链延长时，在连续和间歇的氢键动力学以及咪唑和草酸根环平面的平移和重取向动力学中观察到普遍的协同特征。咪唑阳离子与双（草酸根）硼酸根阴离子之间氢键和π型相互作用的竞争结构平衡以及协同动力学相互作用，与咪唑阳离子中烷基单元之间的短程集体相互作用以及异质离子环境中咪唑和草酸根环平面之间的长程库仑相互作用本质上相关。
