The sets of residual dipolar couplings between carbon and hydrogen nuclei obtained from the proton-encoded 13C 2D NMR experiment are used to investigate the conformational changes which occur when the achiral symmetric liquid crystal dimer CB7CB changes from the achiral nematic to the chiral twist-bend nematic phase. It is found that these changes are a consequence of the chirality of the twist-bend nematic phase, rather than being the driving force for the stability of this phase.

One of the defining characteristics of the twist–bend nematic phase, formed by the methylene-linked liquid crystal dimer 1″,7″-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB), is its chirality. This new nematic phase, predicted by Dozov, is of particular interest because although the constituent molecules are achiral the phase itself is chiral. Here, we describe the use of NMR spectroscopy to determine experimentally whether in reality the phase is chiral or not. The basis of this novel procedure is that the equivalence of the protons or deuterons in a prochiral methylene group in a nematic phase with D∞h symmetry is lost in a chiral phase because its symmetry is reduced to D∞ on removal of the mirror plane. Recording proton-enhanced local field (PELF) NMR experiments shows that in the standard nematic phase all of the methylene groups in the heptane spacer have equivalent pairs of C–H groups but this equivalence is lost for the six prochiral methylene groups with their enantiotopic protons on passing to the twist–bend nematic. Strikingly, this equivalence is not lost for the central methylene group where the two protons are homotopic. We also show how the phase chirality can be demonstrated with probe molecules which contain deuteriated prochiral methylene groups, using 4-octyl-4′-cyanobiphenyl-d2, perdeuteroacenaphthene-d10, and acenaphthene-d4 as examples. For the standard nematic phase deuterium, NMR shows that the deuterons in these methylene groups are equivalent but, as expected, in the twist–bend nematic phase this equivalence is lost. The deuterium NMR spectra of these probe molecules dissolved in CB7CB have been recorded from the isotropic phase, through the nematic and deep into the supercooled twist–bend nematic.

The orientational order and conformational distributions of the two enantiomers of (±)-α-ethylhexanoic acid-d15, a flexible chiral molecule, dissolved in a chiral nematic liquid crystalline solvent made of PBLG in an organic co-solvent are obtained by analysis of NMR data. The anisotropic, NMR parameters are obtained from the separated analysis of the two enantiomers using carbon–deuterium, deuterium–deuterium 2D correlation experiments and proton-coupled carbon-13 1D experiments. The analysis of conformational distributions, and the conformationally dependent orientational order parameters are derived using the additive potential, AP, model.