Molecular dynamics in a nematogen studied by deuterium nuclear magnetic resonance spectroscopy

Abstract

The deuterium spin relaxation times of the nematic liquid crystal p-[²H₃]methoxy[²H₁]benzylidene-p-n-[²H₉]butylaniline ([²H₁₃]MBBA) have been determined using the phase-cycled Jeener–Broekaert pulse sequence. The spectral densities of motion of the methine and chain deuterons have been derived; they contain information on the overall and internal motions of the molecules. The overall motion is discussed in terms of the small-step rotational diffusion model, and the segmental motion of the butyl chain is considered using the superimposed free rotations model. In addition, the possible contribution of director fluctuations to the relaxation of deuteron spins is considered. The computed values of the various rotational diffusion coefficients are given. When the effects of director fluctuations are included the results are generally physically reasonable. One interesting point is that the contribution to spin relaxation due to director fluctuations appears to show an odd–even effect along the butyl chain of MBBA.