FERROELECTRIC LIQUID-CRYSTALS DERIVED FROM ISOLEUCINE II - ORIENTATIONAL ORDERING BY C-13 SEPARATED LOCAL-FIELD SPECTROSCOPY

The orientational ordering of a series of ferroelectric liquid crystal s )-3-methyl-2-halopentanoyloxy]-4-hexyloxybiphenyls (3M2XPHOB, X = F for fluorine, C for chlorine, B for bromine) and their racemates have been studied by two-dimensional carbon-13 separated local field spectr oscopy combined with off-magic-angle spinning. All carbons are well-re solved in the carbon-13 NMR spectrum so that carbon-proton dipolar cou pling constants for all carbons in the liquid crystal molecule can be determined, giving detailed segmental orientational ordering informati on. (The order parameters of the biphenyl core segments and each carbo n-proton bond in the aliphatic chains were measured as a function of t emperature.) The results show that the substitution of different halog ens on the chiral chain affects the carbon-proton bond orientational o rdering of the entire chiral chain, while the orientational ordering o f the rest of the liquid crystal molecule (core and achiral chain) rem ains unchanged. For the fluoro-substituted liquid crystals, carbon-flu orine dipolar couplings are also observed. At the SmA-to-SmC transiti on, there is a discontinuous change in the magnitudes of all C-H bond order parameters. However, the aliphatic ordering relative to the core ordering is unchanged, suggesting that there is a negligible change i n the mesogen conformation and the environment of the aliphatic chain at this transition, i.e. there is no evidence for substantial 'chiral interactions' in the SmC phase.