Ad. Chen et al., FERROELECTRIC LIQUID-CRYSTALS DERIVED FROM ISOLEUCINE II - ORIENTATIONAL ORDERING BY C-13 SEPARATED LOCAL-FIELD SPECTROSCOPY, Liquid crystals, 24(2), 1998, pp. 255-262
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.