Abstract
We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.
Highlights
Salt bridges play an important role in proteins and supramolecular chemistry.[1,2,3,4,5,6,7,8,9,10,11,12,13,14] They are characterized by the sum of two types of intermolecular interaction: ionic bonding and hydrogen bonding
We present deuteron quadrupole coupling constants (DQCC) for hydroxyl-functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb-enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces
We measure these sensitive probes of hydrogen bonding by means of solid-state NMR spectroscopy
Summary
Salt bridges play an important role in proteins and supramolecular chemistry.[1,2,3,4,5,6,7,8,9,10,11,12,13,14] They are characterized by the sum of two types of intermolecular interaction: ionic bonding and hydrogen bonding. Salt bridges are crucial for the structure, dynamics, and function of proteins This type of Coulomb-enhanced hydrogen bonding is typical for ionic liquids, which consist solely of ions.[15,16,17,18,19] So-called “doubly ionic hydrogen bonds” (DIHB) usually result in the formation of ion pairs.[20,21,22,23,24,25] Hbonds in ionic liquids are manifold. For hydroxyl-functionalized ILs, both types of ionic interaction are present in equilibrium: hydrogen bonding (OÀH···O) between oppositely charged ions (c–a) and between like-charged ions, here cations (c–c). All samples were dried under vacuum (at 3 10À3 mbar) for several days and the final water concentration (< 15 ppm) was checked by Karl-Fischer titration
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