Orientational Order in the Smectic F and G Phases: Deuterium Nuclear Magnetic Resonance (NMR) Study

A deuterium NMR investigation of molecular orientational order in the hexatic SF phase and the crystalline SG phases exhibited by three liquid crystalline mixtures is described. Two homologues of the terephthal-bis-n-alkylanilines series were used; TBOA and TBDA in mixtures with TBBA. The principal values and orientation of the principal axes of the averaged electric field gradient (EFG) tensors from deuterated sites in the core and chain regions of the molecule were determined using both aligned (monodomain) and powder samples. It was found in both phases that the averaged EFG tensors from different deuterated sites in the molecule show different asymmetries and do not share the same principal axes. While the averaged EFG tensor from the alpha deuterated positions shows a near zero asymmetry parameter implying no in-plane order, the averaged EFG tensors from the core are strongly asymmetric indicating the existence of in-plane order. The values of the averaged EFG tensors show no discontinuity at the SF−SG phase transition. This implies the existence of local herringbone order in the SF phase for the molecular core since this type of ordering is known to be present in the SG phase. From the measured values of the EFG tensors of the molecular core the order of this part of the molecule is estimated in the context of a simple model describing its motions.