48 research outputs found
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
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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
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 MetalâOrganic Framework MIL-53(Al): Effect of Different Xylene Loadings
MIL-53 is an interesting metalâorganic
framework (MOF) with a âbreathingâ 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 <sup>2</sup>H 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 2âMethylimidazolate Linkers in ZIFâ8 Probed by <sup>2</sup>H NMR: Saloon Doors for the Guests
ZIF-8
is one of the most interesting metalâorganic 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, âŒ3.4 Ă
,
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 <sup>2</sup>H 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 2Ï<sub>f</sub> = 34° with very low activation barrier (1.5 kJ mol<sup>â1</sup>) and high rates (âŒ10<sup>12</sup> s<sup>â1</sup>). Above 380 K the linkers begin to demonstrate additional fast librations
with gradually increasing amplitudes Îł<sub>lib</sub> comparable
with the two-site flips (γ<sub>lib</sub> = ±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<sup>â1</sup>
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>
Water dynamics in bulk and dispersed in silica CaCl2 hydrates studied by neutron scattering methods
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