Competitive Microstructures Versus Cooperative Dynamics of Hydrogen Bonding and pi-Type Stacking Interactions in Imidazolium Bis(oxalato)borate Ionic Liquids

摘要

The delicate trade-off between hydrogen bonding and pi-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 pi-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 pi-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 pi-type coordinations between imidazolium and oxalato ring planes, indicating distinct competitive structural characteristics between hydrogen bonding and pi-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 it-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.