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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1269-1278 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Second moments (M2) and spin-lattice relaxation times (T1) of proton nuclear magnetic resonance (NMR) measurements were analyzed for the hydrogensulfides of sodium, potassium, and rubidium. Three modifications are known at ambient pressure for each of MHS with M=Na, K,Rb. They are isotypic for the different cations, with an ordered monoclinic low temperature modification, a dynamically disordered rhombohedral middle temperature modification (MTM), and a cubic high temperature modification (HTM). The number of proton sites for one proton increases from two in the MTM to at least eight in the HTM. The title compounds were investigated by proton NMR in the temperature range 180 K≤T≤560 K with a resonance frequency of ν0=400 MHz. In addition, KHS was measured from T=90 K up to T=297 K, using proton resonance frequencies ν0=45 MHz and ν0=96 MHz. The M2 calculated from the absorption signal of the individual modification for the various cations do not differ from those calculated for the known crystal structures. The minima of T1 are in good agreement with results of calculations based on the crystal structure of the MTM of these compounds and two-site 180° reorientations of the anions. Activation enthalpies and attempt frequencies (NaHS:26.2(4) kJ mol−1, 6.1(2)×1014 s−1, KHS:19.0(4) kJ mol−1, 4.0(7)×1013 s−1, RbHS:16.3(4) kJ mol−1, 2.0(2)×1013 s−1) agree with those determined by quasielastic neutron scattering [Haarmann et al., J. Chem. Phys. 113, 8161 (2000)]. For the cubic HTM, translational motion of the cations is probable. Only in the case of KHS and RbHS, was the onset of this translational motion already observed in the MTM at temperatures close to the phase transition into the HTM. This seems to be a difference in the mechanism of the MTM(arrow-right-and-left)HTM phase transition for NaHS, on the one hand, and KHS, RbHS, on the other hand. © 2002 American Institute of 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 113 (2000), S. 8161-8167 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The isostructural hydrogensulfides of the alkali metals sodium, potassium, and rubidium were investigated by quasielastic neutron scattering in order to gain insight into the dynamics of the molecular anion HS− as a function of temperature. A phase transition between the rhombohedral middle temperature modification (MTM) and the cubic high temperature modification (HTM) of the title compounds was of special interest. A two site jump model is clearly supported by the data of the MTM and an eight site jump model is consistent with the data of the HTM. The jump distances are in reasonable agreement with distances of the hydrogen positions determined by neutron powder diffraction on the fully deuterated samples. At the MTM→HTM phase transition the correlation times of the reorientational motion decrease by about one order of magnitude. Within the investigated temperature range (T=170 K to T=450 K) they change from 10−8 s to 10−12 s. They show Arrhenius behavior. The activation enthalpies of the MTM decrease with increasing radius of the cation from 26(1) kJ mol−1, 22(1) kJ mol−1 to 18(2) kJ mol−1 for NaHS, KHS and RbHS respectively. In the HTM they are equal within the estimated error 6(1) kJ mol−1 and 7(1) kJ mol−1 for NaHS and KHS. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution neutron powder diffraction experiments on NaDS and KDS were carried out as a function of temperature between T=4 K and T=470 K at ROTAX (ISIS, UK). The probability density function (pdf) of the atoms was studied in detail, in particular that of deuterium. Several forms of temperature dependent reorientational disorder of the anions are the reason for structural polymorphism of these compounds. A monoclinic low (LTM), a rhombohedral middle (MTM), and a cubic high temperature (HTM) modification are the polymorphs. For the structures of the different modifications the parameters were refined by the use of split-atom models, Fourier synthesis, and/or cubic harmonics. As a main result of this study the anisotropy of the pdf of deuterium in NaDS and KDS was evaluated. It is due to large amplitudes of librations of the anions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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