Anisotropic reorientation and internal chain dynamics in liquid crystals: A deuterium nuclear magnetic resonance study

The spectral densities of motion J1(ω) and J2(2ω) were measured at 15.3 MHz in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline for the methine and butyl chain deuterons. The present study concentrates on measurements near the clearing temperature as well as extends measurements to temperatures lower than those of an earlier report. We propose to interpret these spectral density parameters using a new model of molecular reorientation suggested by Vold and Vold. The reorientation motion of the molecules in a Maier–Saupe potential is governed by three rotational diffusion rate constants Dα, Dβ, and Dγ. The subscripts in the rate constants are the Euler angles ΩL,M which are involved in time-dependent transformation between the molecule fixed frame and the laboratory (director) frame. Dα and Dβ are the principal components of a rotational diffusion tensor in the director frame. The motion about the molecular ZM axis (γ motion) is assumed to be independent of the α,β motions. The internal chain dynamics is treated using the superimposed-rotations model in which rotation about each C–C bond can either be free or jump among t (trans) and g± (gauche) sites. The diffusion constants Dα, Dβ, and Dγ are found to show the Arrhenius temperature dependence. Similar behaviors are observed in the derived rotational diffusion constants for internal rotation of the C–C bonds in the butyl chain of MBBA.