On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB

O. MENSIO; R.C. ZAMAR; E. ANOARDO; R. H. ACOSTA; R.Y. DONG

JOURNAL OF CHEMICAL PHYSICS

Año: 2005 vol. 123 p. 204911 - 204911

0021-9606

We investigate the role that local motions and slow cooperative fluctuations have on the relaxationof the intrapair dipolar order in the nematic 5CB. With this purpose we present a theoretical andexperimental systematic study which allow us to quantify the contribution from each type ofmolecular fluctuation to the intrapair dipolar order relaxation time, T1D. The experimental workincludes measurements of Zeeman and intrapair dipolar order relaxation times
T1Z and T1D as afunction of temperature at conventional NMR frequencies, in three complementary samples: normaland chain deuterated 4-n-pentyl-4
-cyanobiphenyl
5CB and 5CBd11 and a mixture of normal 5CBand fully deuterated 4-n-pentyl-4
-cyanobiphenyl
5CBd19, 50% in weight. Additionally weperform T1Z field-cycling Larmor frequency-dependent measurements to obtain the spectral densityof the cooperative fluctuations. The obtained results are as follows.
a The cooperative molecularfluctuations have a strong relative weight in the relaxation of the intrapair dipolar order state, evenat Larmor frequencies in the range of conventional NMR.
b Alkyl chain rotations are an importantrelaxation mechanism of the intrapair dipolar order at megahertz frequencies.
c Intermolecularfluctuations mediated by translational self-diffusion of the molecules is not an efficient mechanismof relaxation of the intrapair dipolar order. © 2005 American Institute of Physics.