Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants—Comparison with Salt Bridges and Molecular Systems

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. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C8H17PD3][NTf2] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD3 group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD3 deuterons. A transition enthalpy of about 12 kJmol 1 from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD3 group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces

Characterization of Fast Restricted Librations of Terephthalate Linkers in MOF UiO-66(Zr) by ²H NMR Spin–Lattice Relaxation Analysis

²H NMR spin–lattice relaxation was used to probe small-amplitude torsional vibrations (librations) of the organic terephthalate linkers in metal–organic framework (MOF) UiO-66­(Zr) saturated with benzene molecules. In UiO-66 (Zr) the mobile phenylene fragments exhibit a complex rotational dynamics of the phenylene rings with fast librations and much slower π-flips around the <i>C</i>₂ symmetry axis. We show that due to the intrinsic broad distribution of the π-flips rate, the relaxation process for the deuterium in the C–D group of phenylene fragment is multiexponential. Two main modes of <i>T</i>₁ relaxation are clearly detected, corresponding to the fast <i>T</i>₁^fast and the slow <i>T</i>₁^slow relaxation. Based on the experimental observation of two-exponential relaxation, a computational model for this <i>T</i>₁ relaxation behavior capable to reproduce the peculiarities of the MOF linkers dynamics was built. Computational analysis allows to establish that the librational motion affects mostly the <i>T</i>₁^slow, while <i>T</i>₁^fast remains unaffected by this motion. Simulation of the <i>T</i>₁^slow dependence on the libration rate <i>k</i>_lib shows that in the range of the librational frequencies of 10⁶–10⁹ Hz the <i>T</i>₁^slow is not sensitive to the <i>k</i>_lib variation, and therefore a precise correspondence between <i>T</i>₁^slow and <i>k</i>_lib cannot be established. <i>T</i>₁^slow exhibits a specific “peak-like-shape” dependence of <i>k</i>_lib in the range of 10⁹–10¹² Hz. In this range of libration frequencies an unambiguous relation between <i>T</i>₁^slow and <i>k</i>_lib exists only in a very narrow frequency window of 0.1 × 10¹⁰–5 × 10¹⁰ Hz. The best conditions to characterize the librational motion by means of <i>T</i>₁ relaxation analysis are met when the flipping motion is almost frozen (<i>k</i>_flip < 10³ Hz) because <i>T</i>₁^slow becomes extremely sensitive to the variation of <i>k</i>_lib

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants—Comparison with Salt Bridges and Molecular Systems

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. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA