Orientational ordering of a dimer liquid crystal by high-resolution solid-state C-13 NMR spectroscopy

The ordering of a dimer liquid crystal, alpha,omega-bis[(4,4'-cyanobiphenylyl)oxy]decane (CBA-IO), was studied by variable-temperature high-resolution solid-state C-13 NMR. A large transitional change of the C-13 chemical shift was observed on going from the isotropic to the nematic phase due to uniaxial alignment of the molecule. The principal values of the C-13 chemical shift tensor have been determined by the Herzfeld-Berger method for CBA-IO itself. The molecular axis is assumed to lie parallel to the line connecting the centers of both mesogenic cores, and the evaluation of the order parameters along the molecular axis, (S-zz), mesogene axis (S-zz(ring)) and C-gamma-C-epsilon, axis (S-zz(C nu-C epsilon)) was attempted here, using the obtained isotropic C-13 chemical shift and principal values of the C-13 chemical shift tensor, and taking into account the conformational behavior of the flexible spacer in the frame of rotational isomeric state (RIS) approach. The order parameter of the molecule S-zz increased from 0.79 to 0.89 with decrease of 17.5 degrees C in temperature from nearby the nematic-isotropic transition point. S-zz(ring) increased from 0.74 to 0.80 with decrease of temperature whereas S-zz(C gamma-C epsilon) and the conformation of the spacer was essentially unaltered in the same temperature range.