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

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

Tailoring porosity and rotational dynamics in a series of octacarboxylate metal-organic frameworks

Modulation and precise control of porosity of metal-organic frameworks (MOFs) are of critical importance to their materials function. Here we report the first modulation of porosity for a series of isoreticular octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs show absence of network interpenetration, robust structures and permanent porosity. Interestingly, activated MFM-185a shows a record high BET surface area of 4734 m2 g-1 for an octacarboxylate MOF. These MOFs show remarkable CH4 and CO2 adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH4 capacities of 0.24 g g-1 and 163 v/v (298 K, 5-65 bar) recorded for MFM-185a due to selective elongation of tubular cages. Dynamics of molecular rotors in deuterated MFM-180a-d16 and MFM-181a-d16 were investigated by variable-temperature 2H solid state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups on the linkers, their rotational rates and transition temperatures, paves the way to controlling and understanding the role of molecular rotors through organic linker design within porous MOF materials

Guest Controlled Rotational Dynamics of Terephthalate Phenylenes in Metal–Organic Framework MIL-53(Al): Effect of Different Xylene Loadings

MIL-53 is an interesting metal–organic framework (MOF) with a “breathing” framework which is envisioned for a number of potential applications. It is suggested that the processes of hydrocarbon adsorption, diffusion, and separation by this material are strongly influenced by flexibility of the framework and fastly moving terephthalate phenylene fragments, representing the array of molecular rotors. To govern the mentioned processes we need to learn how to monitor the flexibility of the framework and identify the specific effects of particular hydrocarbons on the rotational motion of phenylene fragments. Here we demonstrate that flexibility, i.e., large pore (LP) and narrow pore (NP) crystalline state interconversions of the framework, can be monitored by following the evolution of phenylene fragments dynamics with temperature by ²H solid-state nuclear magnetic resonance. We have established that the dynamics of phenylene fragments is very sensitive to the loading of xylene guests and the MOF structural state. The rotation rate is higher and the activation barrier lower for the LP state of the guest-free or loosely loaded material, whereas the NP and LP states with high loadings and dense guest packing show a decrease of the rotation rates and increase of the rotation energetic barrier

Mobility of the 2‑Methylimidazolate Linkers in ZIF‑8 Probed by ²H NMR: Saloon Doors for the Guests

ZIF-8 is one of the most interesting metal–organic frameworks due to its high stability and unique capabilities for hydrocarbons separation. Its porous network is formed by large almost spherical cavities interconnected by very narrow windows, ∼3.4 Å, which should be too small even for methane. At the same time the direct experimental observations show that ZIF-8 cavities are able to host even such large molecules as benzene. This effect is associated with the flexibility of the cavity widows, arising from dynamical freedom on the 2-methylimidazole (2-mIM) linkers that form the framework. In this work, by means of ²H NMR we show that the 2-mIM linkers of the ZIF-8 are very mobile and their mobility is sensitive to the presence of benzene guest. In contrast with other known MOFs based on linearly bonded carboxylates, in guest-free ZIF-8 the plane of 2-mIM linker exhibits two-site flips within a sector of 2φ_f = 34° with very low activation barrier (1.5 kJ mol^–1) and high rates (∼10¹² s^–1). Above 380 K the linkers begin to demonstrate additional fast librations with gradually increasing amplitudes γ_lib comparable with the two-site flips (γ_lib = ±17° above 560 K). This is direct evidence that the ZIF-8 linkers twist notably, thus increasing the aperture of the windows sufficiently to fit very large molecules. Upon benzene loading, the geometry of the observed motions remains similar but the potential barrier of the linkers flipping rises up to 9 kJ mol^–1

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

Water dynamics in bulk and dispersed in silica CaCl2 hydrates studied by neutron scattering methods

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