Molecular dynamics in a liquid crystal

Abstract

Experiments are described for determining the auto-correlation spectral densities J ₁(ω₀) and J ₂(2ω₀) for deuterons at each position in the alkyl chain and in one aromatic ring of the nematogen 4-n-pentyl-d₁₁-4′-cyano-2,3,5,6-d₄-biphenyl (5CB-d₁₅). The experiments involve selective population inversions followed by non-selective monitoring of the spectrum. The efficiency of the method is compared with that of the Jeener-Broekaert sequence. The results are discussed in terms of models for the molecular motion which treat the molecules either as rigid, cylindrically symmetric rods, or which assume that the internal modes of molecular motion are fast enough to be decoupled from the overall molecular rotations. Both models fail to explain the dependence of the ratio J ₁(ω₀)/J ₂(2ω₀) on position in the molecule. It is suggested that the variation of the ratio of spectral densities with position reflects the importance of both internal modes of molecular motion, which affects both J ₁(ω₀) and J ₂(2ω₀) and director fluctuations which affect J ₁(ω₀) alone.

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Notes on contributors

P.A. Beckmann

On leave 1980–81 from the Department of Physics, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, U.S.A.