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  • Articles  (6)
  • Articles: DFG German National Licenses  (6)
  • 1995-1999  (6)
  • Chemistry and Pharmacology  (6)
  • 1
    Electronic Resource
    Electronic Resource
    Thermal Broadening of an Optical Transition in a Chromoprotein between 50 mK and 15 K (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 14561-14565 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100039a051
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  • 2
    Electronic Resource
    Electronic Resource
    Optical spectroscopy and ground state dynamics of methyl groups (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 942-949 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigated dimethyl-s-tetrazine (-h6 and -d6) in a n-heptane (-h16 and -d16) lattice with hole burning techniques. Stark experiments definitely show that the two methyl groups in dimethyl-s-tetrazine (DMST) are inequivalent. Hence, the respective rotational tunneling dynamics can be different. This is clearly seen in the behavior of DMST-d6 in both lattices n-heptane-h16 and -d16: Hole burning produces three pairs of sideholes symmetrically shifted from the laser frequency. The tunneling dynamics of the central hole as well as of the sideholes is governed by two relaxation times which differ by about one order of magnitude. Due to the narrow temperature range accessible, an unambiguous assignment of the microscopic tunneling relaxation process is not possible. On the other hand, DMST-h6 relaxes via a Raman process above 3 K and via a direct process below. In the Raman-active regime the tunneling rates for both host lattices fall on top of each other in agreement with theory. There are, however, features in the tunneling dynamics which are not yet understood. For instance, in the temperature regime where the direct process prevails, host deuteration speeds up the tunneling relaxation by an order of magnitude. In addition, the sidehole pattern of DMST-h6 is quite different from DMST-d6: Only one pair is observed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470817
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  • 3
    Electronic Resource
    Electronic Resource
    Nonequilibrium phenomena in spectral diffusion physics of organic glasses (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1798-1806 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We performed two different types of spectral diffusion experiments on persistent spectral holes. In all cases, we measured the holewidth as a function of time. The two experiments differed in their initial conditions: In the type 1 experiment (the "aging experiment''), the sample was cooled from room temperature to the final temperatures which were 100 and 800 mK, respectively. Holes were burnt at various time intervals after the final temperature was reached. In the type 2 experiment (the "cycling experiment''), the sample was allowed to relax for a period of about 10 days. Then, a hole was burnt and subjected to a temperature cycle. In all cases, the time dependence of the holewidths was strongly nonlogarithmic. The temperature cycled hole showed a narrowing regime which prevailed for the whole observation period of roughly one week. We will show that the deviation from the logarithmic time dependences is a nonequilibrium phenomenon. All features observed could be modelled within the standard tunneling model. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472055
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  • 4
    Electronic Resource
    Electronic Resource
    Rotational tunneling of partially deuterated methyl groups: Optical studies of dimethyl-s-tetrazine in different crystalline matrices (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 9762-9770 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotational tunneling of monodeuterated methyl groups of dimethyl-s-tetrazine guest molecules in n-octane and tetramethylbenzene crystalline matrices is characterized by optical spectroscopy including hole burning methods. An analysis of the level structure shows that the asymmetric substitution leads to a partial localization of one of the methyl groups in n-octane, while in the tetramethylbenzene host both groups are equivalent and are strongly localized. The relaxation, by incoherent tunneling, between different orientations proceeds on a time scale of 〉107 s−1 in n-octane at temperatures below 2 K. The coupling of the orientation of the methyl group with the environment is estimated from information on inhomogeneous broadening attributed to random strain fields in the crystal. The strength of this coupling is related to the observed relaxation behavior. The origin of the asymmetry of the rotational potential is discussed in terms of static and dynamic contributions which are due to changes of bond length upon isotopic substitution and the dependence of vibrational frequencies on orientation. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472846
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  • 5
    Electronic Resource
    Electronic Resource
    Conformational dynamics of a low temperature protein: Free base cytochrome-c (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 3229-3234 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spectral diffusion waiting time experiments at 100 mK combined with aging time experiments are used to shed light on the features of the energy landscape of a cytochrome c-type protein and the respective conformational dynamics. The energy landscape shows features of a hierarchical organization. The time law which governs the dynamics in conformational phase space is a power law. The respective processes seem to be related to generalized diffusive-like motions. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477845
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  • 6
    Electronic Resource
    Electronic Resource
    Über die Produkte der Reaktion von Methylbromid und Ethylbromid mit Kaliumhydroxid in wäßrig-methanolischen Lösungen und den Verlauf dieser SN2-ReaktionHerrn Prof. Dr. Manfred Weidenbruch zur Vollendung des 60. Lebensjahres gewidmet (1998)
    New York, NY : Wiley-Blackwell
    Journal für Praktische Chemie/Chemiker-Zeitung 340 (1998), S. 73-80 
    Details
    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Concerning the Products of the Reaction of Methyl Bromide and Ethyl Bromide with Potassium Hydroxide in Aqueous Methanolic Solutions and the Progress of this SN2-ReactionInvestigations of the reaction of methyl bromide and ethyl bromide with potassium hydroxide in methanolic and aqueous methanolic solutions show that the main products of these reactions are dimethyl ether and ethylmethyl ether. The reaction rates measured in methanolic or aqueous methanolic solutions are the same whether potassium hydroxide or potassium methoxide are used. These results are caused by an equilibrium between hydroxide and methoxide ions with which we could establish the equilibrium constant near 0.6. This means that a solution of sodium hydroxide c = 0.1 moll-1 in methanol contains roughly 99.8% of methoxide ions. The reaction rates in methanolic as well as in aqueous methanolic solutions are strict second order. The reaction rate measured at several temperatures permitted the calculation of EA≠, ΔH≠, ΔS≠ and ΔG≠. Furthermore the kinetic investigations show that the nucleophilicity of methoxide ions is lower compared to hydroxide ions. The calculation of the Swain-Scott-parameter n results in a nucleophilicity scale in order to methoxide, hydroxide, ethoxide ions. The kinetic investigations of the reaction of ethyl bromide with methoxide and hydroxide ions in methanolic solutions demonstrate that at high temperatures the rate constant of methoxide ions is higher than that of hydroxide ions. The opposite case can be observed at lower temperatures. At the temperature of 20 °C the rate constants of both reactions are equal. This is to do with the isokinetic effect which one is rarely able to observe at room temperatures.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/prac.19983400111
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