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Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling ConstantsâComparison 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 (câa), and the unusual hydrogen bonds between cation and cation (câc), 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 (câa) ion pairs and (câc) H-bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (câc) species successfully compete with the (câa) 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 (câc) 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 <sup>2</sup>H NMR SpinâLattice Relaxation Analysis
<sup>2</sup>H NMR spinâlattice relaxation was used to probe
small-amplitude torsional vibrations (librations) of the organic terephthalate
linkers in metalâorganic framework (MOF) UiO-66Â(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 Ï-flips around the <i>C</i><sub>2</sub> symmetry axis. We show that due to the intrinsic broad
distribution of the Ï-flips rate, the relaxation process for
the deuterium in the CâD group of phenylene fragment is multiexponential.
Two main modes of <i>T</i><sub>1</sub> relaxation are clearly
detected, corresponding to the fast <i>T</i><sub>1</sub><sup>fast</sup> and the slow <i>T</i><sub>1</sub><sup>slow</sup> relaxation. Based on the experimental observation of two-exponential
relaxation, a computational model for this <i>T</i><sub>1</sub> 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><sub>1</sub><sup>slow</sup>, while <i>T</i><sub>1</sub><sup>fast</sup> remains unaffected by this motion. Simulation
of the <i>T</i><sub>1</sub><sup>slow</sup> dependence on
the libration rate <i>k</i><sub>lib</sub> shows that in
the range of the librational frequencies of 10<sup>6</sup>â10<sup>9</sup> Hz the <i>T</i><sub>1</sub><sup>slow</sup> is not
sensitive to the <i>k</i><sub>lib</sub> variation, and therefore
a precise correspondence between <i>T</i><sub>1</sub><sup>slow</sup> and <i>k</i><sub>lib</sub> cannot be established. <i>T</i><sub>1</sub><sup>slow</sup> exhibits a specific âpeak-like-shapeâ
dependence of <i>k</i><sub>lib</sub> in the range of 10<sup>9</sup>â10<sup>12</sup> Hz. In this range of libration frequencies
an unambiguous relation between <i>T</i><sub>1</sub><sup>slow</sup> and <i>k</i><sub>lib</sub> exists only in a
very narrow frequency window of 0.1 Ă 10<sup>10</sup>â5
Ă 10<sup>10</sup> Hz. The best conditions to characterize the
librational motion by means of <i>T</i><sub>1</sub> relaxation
analysis are met when the flipping motion is almost frozen (<i>k</i><sub>flip</sub> < 10<sup>3</sup> Hz) because <i>T</i><sub>1</sub><sup>slow</sup> becomes extremely sensitive
to the variation of <i>k</i><sub>lib</sub>
Molecular Mobility of Tertâbutyl Alcohol Confined in a Breathing MILâ53 (Al) MetalâOrganic Framework
Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling ConstantsâComparison 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 (câa), and the unusual hydrogen bonds between cation and cation (câc), 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 (câa) ion pairs and (câc) H-bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (câc) species successfully compete with the (câa) 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 (câc) hydrogen bonds persist, resulting in supercooling and glass formation. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA