13C NMR study of orientational ordering in the chiral- C phase of partially deuterated smectic liquid crystals

Carbon-13 NMR spectroscopy is used to probe the orientational ordering in the tilted chiral smectic phase of two partially deuterated liquid crystals. Quantitative analyses of the chemical shifts of the aromatic carbons in the smectic- A and - C* phases enable us to obtain the location of the long molecular axis of the core and the off-diagonal order parameter S01 of the molecular core in the smectic- C* phase. The advantage of knowing the ordering information of these molecules in their mesophases from deuteron studies is demonstrated in this work, as they can be used to guide the C13 assignments. The temperature behaviors of the C13 chemical shifts from the aromatic sites in the helical structure of the smectic- C* phase is caused by both an increase of the tilt angle and a change in the orientation of the principal frame that describes the molecular ordering.