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NMR Study of Molecular Dynamics in a D

ho

Columnar Mesophase

C. Cruz

a;b

, P. J. Sebasti˜ao

a;b

, J. Figueirinhas

a;b

, A. C. Ribeiro

a;b

, H. T. Nguyen

c

, C. Destrade

c

, and F. Noack

d

aCentro de F´ısica da Mat´eria Condensada, Av. Prof. Gama Pinto 2, 1699 Lisboa Codex, Portugal

bInstituto Superior T´ecnico, Departamento de F´ısica, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal

cCentre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence, France

dPhysikalisches Institut der Universit¨at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart Z. Naturforsch. 53 a, 823–827 (1998); received July 16, 1998

In this paper we report the first molecular dynamics study combining fast field-cycling and conven- tional NMR techniques in a thermotropic liquid crystal of discotic molecules exhibiting an ordered columnar hexagonal mesophase. Using the association of these techniques we obtained protonT1

data over a very large domain of Larmor frequencies (!=2from 500 Hz to 85 MHz). The proton spin-lattice relaxation results were analysed considering the structure of the mesophase and the types of movements which are expected to influence significantly the relaxation rate, namely local molecu- lar rotational reorientations, inter-columnar self-diffusion and collective movements corresponding to bending and compression of the columns. We verified that these mechanisms dominate the relaxation respectively for high, medium and low Larmor frequencies.

Key words: NMR; Proton Relaxation; Molecular Dynamics; Columnar Hexagonal Mesophases (Dho);

Discotic Molecules

Reprint requests to A. C. Ribeiro; Fax: 795 42 88, E-mail: ribeiro@alf1.cii.fc.ul.pt

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