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'Molecular motion' in keywords Facet   Publication Year 1993  [X]
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1Author    P. J. Sebastião, A. C. Ribeiro, H. T. Nguyen, F. NoackRequires cookie*
 Title    Proton NMR Relaxation Study of Molecular Motions in a Liquid Crystal with a Strong Polar Terminal Group  
 Abstract    Liquid crystalline compounds containing a cyano terminal group often exhibit peculiar molecular organizations of their mesophases. In this work we present proton NMR relaxation studies, per-formed by means of standard NMR and fast field-cycling NMR techniques, in the nematic (N) and bilayered smectic-,4 phase (S^ 2) of 4-pentyl-phenyl 4'-cyanobenzoyloxy-benzoate. The field-cycling measurements were used to clarify the relaxation behaviour in the low Larmor frequency range, where conventional techniques are not applicable. Self-diffusion and rotational reorientations are found to be the essential relaxation mechanisms at MHz frequencies in the smectic mesophase, while the contribution of collective modes appears only at lower frequencies in the kHz range. In the nematic mesophase the order director fluctuations mechanism dominates the relaxation dispersion up to 10 MHz, where the rotational reorientations become important, with minor corrections from the self-diffusion process. The agreement between the experimental findings and model fits could be improved by an additional relaxation mechanism in the kHz regime, ascribed to the interaction between protons and fast relaxing quadrupolar nitrogen 14 N nuclei. Though all four processes are present in the nematic and smectic-/l 2 phases, the overall 7\ frequency dependence is quite different in the two cases. This behaviour is discussed in terms of available theoretical calculations of the proton relaxation dispersion in liquid crystals, and it is also compared with data known from other cyano compounds. 
  Reference    Z. Naturforsch. 48a, 851—860 (1993); received April 22 1993 
  Published    1993 
  Keywords    NMR, Proton relaxation, Smectic-/! 2, Molecular motions, Bilayer smectic 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0851.pdf 
 Identifier    ZNA-1993-48a-0851 
 Volume    48 
2Author    Stefan Jurga, Kazimierz Jurga, EduardC. Reynhardt, Piotr KatowskiRequires cookie*
 Title    A Proton NMR Study of n-Decylammonium Chain Dynamics in the Perovskite-type Layered Compound (C 10 H 21 NH3) 2 CdCl4  
 Abstract    A detailed proton second moment and spin-lattice relaxation time investigation of the bilayered compound (C 10 H 21 NH 3) 2 CdCl 4 is reported. In the low temperature phase the methyl group exe-cutes a classical threefold reorientation, while the NH 3 group is involved in a similar reorientation in an asymmetric potential well. Evidence for defect chain motions in this phase has been found, and infomation regarding the potential wells associated with these motions has been extracted from the data. In the high temperature phase, slow chain defect motions and fast fourfold reorientations of chains about their long axes, parallel to the normal to the bilayer, have been observed. 
  Reference    Z. Naturforsch. 48a, 563—569 (1993); received October 9 1992 
  Published    1993 
  Keywords    Bilayered compounds, Phase transition, Spin-lattice relaxation, Molecular motions, Defect motions 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0563.pdf 
 Identifier    ZNA-1993-48a-0563 
 Volume    48