Single-particle rotations in molecular crystals

Press, W.

doi:10.1007/bfb0048204

Cited by 144 publications

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“…If the methyl groups are terminating some of the missing-linker defects, then we would observe the ground-state tunneling of a quasi-free methyl rotation because the methyl groups pointing toward the pores in the structure should have rather small rotational barriers. 32,33 In Figure 4 we show the neutron inelastic spectrum of our deuterated UiO-66 sample (the one used in our neutron powder diffraction experiment), measured at 1.5 K using a Disk−Chopper Spectrometer 34 (see measurement details in Supporting Information). The linker deuteration of the sample effectively reduced the interference of lowenergy framework vibrational modes (note that only the methyl groups are not deuterated because normal hydrogenated acetic acid was used in the sample synthesis).…”

Section: Journal Of the American Chemical Societymentioning

confidence: 99%

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

et al. 2013

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UiO-66 is a highly important prototypical zirconium metal−organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missinglinker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm 3 / g and Brunauer−Emmett−Teller surface areas ranging from 1000 to 1600 m 2 /g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO 2 . Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO 2 ) suggesting the beneficial effect of the −OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the −OH group was further supported by our firstprinciples calculations. ■ INTRODUCTIONMetal−organic frameworks (MOFs), consisting of inorganic metal centers connected by organic linkers, are a relatively new family of porous materials that possess rich chemistry and offer great promise for gas adsorption related applications.

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Section: Journal Of the American Chemical Societymentioning

confidence: 99%

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

et al. 2013

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“…We note that the methyl group orientation and associated tunnel-splitting is a very sensitive probe for determination of the guest-host interactions. 6 For comparison, the neutron diffraction patterns from ZIF8 with the following D 2 concentrations: n D 2 ) 3, 16, 28 per 6 Zn are also shown in Figure 2 (also see Supporting Information). Using the model of the refined ZIF8 host structure, we performed Rietveld † NIST Center for Neutron Research.…”

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Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8

2007

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Zeolitic imidazolate frameworks (ZIFs) are a new class of nanoporous compounds which consist of tetrahedral clusters of MN 4 (M ) Co, Cu, Zn, etc.) linked by simple imidazolate ligands. 1,2 As a subfamily of metal-organic frameworks (MOFs), ZIFs exhibit the tunable pore size and chemical functionality of classical MOFs. At the same time, they possess the exceptional chemical stability and rich structural diversity of zeolites. 2 Because of these combined features, ZIFs show great promise for hydrogen storage applications. However, in contrast to a large number of extensive studies for other MOFs, [3][4][5] no experimental data concerning the nature of H 2 -ZIF interactions and the manner in which hydrogen molecules are adsorbed have been reported yet. Such fundamental studies hold the key to optimizing this new class of ZIF materials for practical hydrogen storage applications. In particular, the major adsorption sites and their binding energies are the key features of a system that determine its adsorption properties at a given temperature and pressure.ZIF8 is a prototypical ZIF compound (Zn(MeIM) 2 , MeIM ) 2-methylimidazolate) with a SOD (sodalite) zeolite-type structure, exhibiting an interesting nanopore topology formed by four-ring and six-ring ZnN 4 clusters as shown in Figure 1. Since the nanopores are only accessible through narrow six-ring funnel-like channels (Figure 1a), one wonders how H 2 molecules are adsorbed, where the major binding sites are, and what the binding energies are. Herein, using the difference Fourier analysis of neutron powder diffraction data along with first-principles calculations, we provide answers to these questions for the first time. Surprisingly, the strongest adsorption sites that we identified (see Figure 1b) are directly associated with the organic linkers, instead of the triangular faces of the ZnN 4 tetrahedra (i.e., metal sites), in strong contrast to other MOFs, where the faces of the metal-oxide tetrahedra are typically the primary adsorption sites. At high H 2 loading, the ZIF8 structure is capable of holding up to 28 H 2 molecules (i.e., 4.2 wt %) in the form of highly symmetric novel three-dimensional (3D) interlinked H 2 -nanoclusters.ZIF8 was synthesized using a solvothermal method as described in ref 2. Neutron powder diffraction data were collected on the high-resolution neutron powder diffractometer (BT-1) at NIST Center for Neutron Research. Because of the large incoherent cross section of H 2 , adsorption was studied as a function of D 2 concentration per ZIF8 molecular formula (Zn 6 N 24 C 48 H 60 ). Target amounts of D 2 , that is, 3, 16, and 28 D 2 per 6 Zn, were loaded into the ZIF8 sample at 70 K. One H 2 /6 Zn corresponds to ≈0.15 wt % hydrogen uptake. The sample was then cooled to 30 K at which point the D 2 was completely adsorbed. Once the system was equilibrated at 30 K, the sample was further cooled to 3.5 K before the diffraction measurement. No evidence of solid deuterium was observed on the structural refinement of the D 2 loaded samples, in...

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“…In the glassy state of polymeric materials, at temperatures well below the glass transition region, one can assume as a good approximation that the main-chain dynamics are completely frozenin and only small side units, as the methyl groups, can still remain mobile. In such conditions, the interaction between the methyl group and its environment is often well approximated by an effective mean-field or "single-particle" rotational potential [5][6][7]. The methyl group can be regarded as a rigid rotor because the strength of the C-H covalent bonds allows one to neglect the internal degrees of freedom in comparison with both translational and rotational motions of the group as a whole.…”

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confidence: 99%

“…Figure 1 shows the quantized energy levels of a threefold potential with barrier height V 3 = 500 K (in the following V 3 will be given as V 3 /k B , with k B the Boltzmann constant). Such levels are obtained [5][6][7] by solving the corresponding stationary Schrödinger equation, which in the case of a threefold potential takes the form of the well-known Mathieu equation (see, e.g., Ref. [8]).…”

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Neutron scattering investigations on methyl group dynamics in polymers

Colmenero

Moreno

Alegría

2005

Progress in Polymer Science

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Among the different dynamical processes that take place in polymers, methyl group rotation is perhaps the simplest one, since all the relevant interactions on the methyl group can be condensed in an effective mean-field one-dimensional potential. Recent experimental neutron scattering results have stimulated a new revival of the interest on methyl group dynamics in glasses and polymer systems. The existence of quantum rotational tunnelling of methyl groups in polymers was expected for a long time but only very recently (1998), these processes have been directly observed by high-resolution neutron scattering techniques. This paper revises and summarizes the work done on this topic over last ten years by means of neutron scattering. It is shown that the results obtained in many chemically and structurally different polymers can be consistently described in the whole temperature range -from the quantum tunnelling limit to the classical hopping regime -as well as in the librational spectrum, in terms of the Rotation Rate Distribution Model (RRDM), which was first proposed in 1994. This model introduces a distribution of potential barriers for methyl group rotation, which is associated to the disorder present in any structural glass. The molecular and structural origin of the barrier distribution in polymers is discussed on the basis of a huge collection of investigations reported in the literature, including recent fully atomistic molecular dynamics simulations.

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Single-particle rotations in molecular crystals

Cited by 144 publications

References 0 publications

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

Hydrogen Storage in a Prototypical Zeolitic Imidazolate Framework-8

Neutron scattering investigations on methyl group dynamics in polymers

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