| 1. |
- B. Brant Carvalho, Paulo H., 1990-, et al.
(författare)
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Neutron scattering study of polyamorphic THF·17(H2O) : toward a generalized picture of amorphous states and structures derived from clathrate hydrates
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 25:21, s. 14981-14991
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Tidskriftsartikel (refereegranskat)abstract
- From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77-140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.
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| 2. |
- Barros Brant Carvalho, Paulo Henrique, et al.
(författare)
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Neutron scattering study of polyamorphic THF ∙ (H2O)17 – toward a generalized picture of amorphous states and structures derived from clathrate hydrates
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF ∙ 17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, very-HDA (VHDA), upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third, recovered amorphous (RA) form. Results from a compilation of neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF ∙ 17H2O solution (~2.5 M). The calculated density of (only in situ observable) HDA and VHDA at 2 GPa and 130 K is 1.287 and 1.328 g/cm3, respectively, whereas that of RA (at 1 atm) is 1.081 g/cm3. Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ~23 H2O. The local water structure in HDA is reminiscent of pure HDA-ice, featuring 5-coordinated H2O. In VHDA, this structure is maintained but the local water structure is densified to resemble pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ~18 H2O and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous, solid solutions of THF and water. The local water structure of water-rich (1:17) amorphous CHs resembles most that of the corresponding amorphous water ices when compared to guest-rich CHs, e.g., Ar ∙ ~6H2O. The proposed significance of different contributions of water local environments presents a simple view to justify neutron structure factor features.
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| 3. |
- Cladek, Bernadette R., et al.
(författare)
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In situ inelastic neutron scattering of mixed CH4–CO2 hydrates
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361 .- 1873-7153. ; 327
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Tidskriftsartikel (refereegranskat)abstract
- An abundant source of CH4 can be found in natural hydrate deposits. Recent demonstration of CH4 recovery from hydrates via CO2 exchange has revealed the potential as a fuel source that also provides a medium for carbon sequestration. It is vital to understand the structural and dynamic impacts of guest variation in CH4, CO2, and mixed hydrates and link the results to the stability of various deposits in nature, harvesting methane, and sequestering CO2. Molecular vibrations are examined in CH4, CO2, and mixed CH4-CO2 hydrates at 5 and 190 K and Xe hydrates for comparison. Inelastic neutron scattering (INS) is an ideal spectroscopy technique to observe the dynamic modes in the hydrate structure and enclathrated CH4, as it is extremely sensitive to 1H. The presence of CO2 in hydrates tightens the lattice. It introduces more active librational modes to the host lattice, while hindering the motion of CH4 in mixed CH4-CO2 hydrate at 5 K. At 190 K, a large broadening of the CH4 librational modes indicates disorder in the structure leading to dissociation.
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| 4. |
- Hartl, Monika, et al.
(författare)
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Hydrogen adsorption on two catalysts for the ortho- to parahydrogen conversion : Cr-doped silica and ferric oxide gel
- 2016
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 18:26, s. 17281-17293
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Tidskriftsartikel (refereegranskat)abstract
- Molecular hydrogen exists in two spin-rotation coupled states: parahydrogen and orthohydrogen. Due to the variation of energy with rotational level, the occupation of ortho- and parahydrogen states is temperature dependent, with parahydrogen being the dominant species at low temperatures. The equilibrium at 20 K (99.8% parahydrogen) can be reached by natural conversion only after a lengthy process. With the use of a suitable catalyst, this process can be shortened significantly. Two types of commercial catalysts currently being used for ortho- to parahydrogen conversion are: iron(iii) oxide (Fe2O3, IONEX®), and chromium(ii) oxide doped silica catalyst (CrO·SiO2, OXISORB®). We investigate the interaction of ortho- and parahydrogen with the surfaces of these ortho-para conversion catalysts using neutron vibrational spectroscopy. The catalytic surfaces have been characterized using X-ray absorption fine structure (XAFS) and X-ray/neutron pair distribution function measurements.
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| 5. |
- Kranak, Verina F., et al.
(författare)
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Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH
- 2009
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Ingår i: Solid State Sciences. - : Elsevier BV. - 1293-2558 .- 1873-3085. ; 11:11, s. 1847-1853
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Tidskriftsartikel (refereegranskat)abstract
- The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB2-type precursors SrAlGe and BaAlGe or reacting SrH2 with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.2435(2) and 4.3450(2) Å, c = 4.9710(3) and 5.2130(4) Å for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]2− which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]2− are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al–H stretching and bending mode frequencies at around 1250 and 870 cm−1, respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C. When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.
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| 6. |
- Martins, Murillo L, et al.
(författare)
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Restricted mobility of specific functional groups reduces anti-cancer drug activity in healthy cells.
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The most common cancer treatments currently available are radio- and chemo-therapy. These therapies have, however, drawbacks, such as, the reduction in quality of life and the low efficiency of radiotherapy in cases of multiple metastases. To lessen these effects, we have encapsulated an anti-cancer drug into a biocompatible matrix. In-vitro assays indicate that this bio-nanocomposite is able to interact and cause morphological changes in cancer cells. Meanwhile, no alterations were observed in monocytes and fibroblasts, indicating that this system might carry the drug in living organisms with reduced clearance rate and toxicity. X-rays and neutrons were used to investigate the carrier structure, as well as to assess the drug mobility within the bio-nanocomposite. From these unique data we show that partial mobility restriction of active groups of the drug molecule suggests why this carrier design is potentially safer to healthy cells.
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| 7. |
- Stishov, Sergei M., et al.
(författare)
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Lost Heat Capacity and Entropy in the Helical Magnet MnSi
- 2010
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 105:23, s. 236403-
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Tidskriftsartikel (refereegranskat)abstract
- The heat capacity of MnSi at B = 0 and B = 4 T was measured in the temperature range 2.5-100 K. To analyze the data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. The calculated phonon frequencies were confirmed by means of inelastic neutron scattering. The analysis of the data suggests the existence of negative contributions to the heat capacity and entropy of MnSi at T > T-c that may imply a specific ordering in the spin subsystem in the paramagnetic phase of MnSi.
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| 8. |
- Tsapatsaris, Nikolaos, et al.
(författare)
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Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics.
- 2014
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Ingår i: Molecular Pharmaceutics. - : American Chemical Society (ACS). - 1543-8392 .- 1543-8384. ; 11:3, s. 1032-1041
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Tidskriftsartikel (refereegranskat)abstract
- This study focuses on the interplay of molecular flexibility and hydrogen bonding manifested in the monoclinic (form I) and orthorhombic (form II) polymorphs of paracetamol. By means of incoherent inelastic neutron scattering and density functional theory calculations, the relaxation processes related to the methyl side-group reorientation were analyzed in detail. Our computational study demonstrates the importance of considering quantum effects to explain how methyl reorientations and subtle conformational changes of the molecule are intertwined. Indeed, by analyzing the quasi elastic signal of the neutron data, we were able to show a unique and complex motional flexibility in form II, reflected by a coupling between the methyl and the phenyl reorientation. This is associated with a higher energy barrier of the methyl rotation and a lower Gibbs free energy when compared to form I. We put forward the idea that correlating solubility and molecular flexibility, through the relation between pKa and methyl rotation activation energy, might bring new insights to understanding and predicting drug bioavailability.
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