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  • 1
    ISSN: 1520-5835
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3813-3820 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamic heterogeneities of glass-forming materials are elucidated for a simple model glass, for which we choose a generalized version of the well-known facilitated kinetic Ising model, originally proposed by Fredrickson and Andersen. The fluctuations within the heterogeneous distribution of reorientation rates are quantified in terms of a rate memory parameter by comparing appropriately chosen two-time and four-time correlation functions. Qualitatively, this parameter expresses after how many relaxation processes a typical relaxator has forgotten its dynamic history. In agreement with most experimental observations the rate memory parameter is close to its minimum value, indicating that the fluctuations within the heterogeneous distribution are on the same time scale as the relaxation processes itself. Upon increasing the "cooperativity parameter" in the glass model the rate memory parameter decreases. The same tendency is observed for decreasing temperature. The apparent dependence of the rate memory parameter on cooperativity is rationalized. Furthermore the results are compared with previous simulations of the facilitated kinetic Ising model. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8686-8696 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the standard tunneling model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass a-B2O3 shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP's are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP's, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in polymethylmethacrylate. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4201-4214 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The temperature dependence of the deuteron spin lattice relaxation time T1 in four carboxylic acids, which were deuterated in the carboxyl groups, is presented. These data allow determination of the rate Γ of the hydrogen transfer between the two minima of the double well potential in which the hydrogens move along the hydrogen bonds. The temperature dependence of Γ has clearly distinct low temperature quantum-mechanical and high temperature thermally activated regimes. Evidence is presented that the transfer of hydrogens in dimers of carboxylic acids is strongly influenced by a high-frequency mode which is strongly coupled to the hydrogens. This supports the conclusions recently put forward by the ETH group [J. Chem. Phys. 93, 1502 (1990)]. A model based on the spin–Boson–Hamiltonian with a bath consisting of acoustic phonons plus a unique high frequency mode explains fully the jump rate in the quantum mechanical regime of all four compounds studied. The way in which the unique bath mode influences the hydrogen transfer is clarified.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 88 (1992), S. 39-51 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We discuss the temperature dependence of deuterium NMR spectra and spin-lattice relaxation rates 1/T 1 of deuterated methyl groups. We restrict ourselves to the temperature range where it is sufficient to consider only the two lowest librational levels of the CD3 group. Specifically we derive explicit expressions for 1/T 1 and the widthГ NMR of the so-called tunneling resonances in the high-field NMR spectrum in the limit where the tunneling frequencyΔ 0 is large compared to the quadrupole coupling constant of a methyl group. We demonstrate that the disappearance of the tunneling resonances at elevated temperatures follows the scenario of the NMR of nuclei which exchange classically between two inequivalent sites. We further show that the NMR linewidths and relaxation rates are strongly related to the widths of the quasielastic and inelastic lines in incoherent neutron spectra. It turns out that for NMR spectra the classical limit is usually reached at lower temperatures than for neutron spectra. All calculations are based on the projection operator formalism.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Macromolecular Theory and Simulations 7 (1998), S. 665-674 
    ISSN: 1022-1344
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the rotational isomeric state (RIS) approach the conformational statistics of a polymer chain can be conveniently described in terms of the statistical matrix. In this work it is shown from a conceptual point of view to which degree the projection onto a few states, inherently present in any RIS approach, may lead to systematic deviations for estimation of the a priori probabilities. It is shown that these deviations scale with some power of the inverse temperature. Furthermore an algorithm is presented, deriving the statistical matrix from computer simulations. This algorithm is applied to polyethylene. Comparison with the conformational statistics as obtained from Monte Carlo simulations allows the quality of the statistical matrix elements to be estimated. It turns out that the systematic deviation does not hamper practical applications. Questions like the relevance of the anharmonicity of the energy landscape or consideration of more than nearest-neighbor correlations in the RIS approach are discussed.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Book
    Book
    Bonn :mitp-Verl.,
    Title: Datenbanken /
    Author: Saake, Gunter
    Contributer: Heuer, Andreas , Sattler, Kai-Uwe
    Edition: 2., aktualisierte und erw. Aufl
    Publisher: Bonn :mitp-Verl.,
    Year of publication: 2005
    Pages: XXI, 869 S. : , Ill., graph. Darst.
    ISBN: 3-8266-1438-0
    Type of Medium: Book
    Language: German
    URL: 04  (lizenzfrei)
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