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'NMR' in keywords Facet   Publication Year 1993  [X]
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1Author    B.V S Murthy, K. P. Ramesh, J. RamakrishnaRequires cookie*
 Title    Molecular Dynamics in Tetramethyl Ammonium Hexabromo Selenate [N(CH 3 ) 4 ] 2 SeBr 6 . A Proton NMR Study  
 Abstract    spin-lattice relaxation time Ti and second moment M 2 have been studied as a function of tem-perature over the temperature range 445 to 77 K. A discontinuous change in Ti at 365 K, in-dicates the presence of a phase transition, while the slope change at 152 K, is attributed to a change in the TMA ion dynamics from tumbling to torsion. The 7i results could be explained in terms of inequivalent TMA ions and their small angle torsional oscillations. A second moment (M 2) transition occurs around 160 K and is ascribed to the freezing of TMA tumbling and CH 3 reorientation. 
  Reference    Z. Naturforsch. 48a, 868—870 (1993); received February 22 1993 
  Published    1993 
  Keywords    Molecular dynamics, Second moment, NMR, Spin-lattice relaxation time 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0868.pdf 
 Identifier    ZNA-1993-48a-0868 
 Volume    48 
2Author    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