Polar molecular ordering in the Nx phase of bimesogens and enantiotopic discrimination in the NMR spectra of rigid prochiral solutes

摘要

The potential of mean torque governing the orientational ordering ofprochiral solutes in the two nematic phases (N and Nx) formed by certainclasses of symmetric achiral bimesogens is formulated and used for the analysisof existing NMR measurements on solutes of various symmetries dissolved in thetwo phases. Three distinct attributes of the solvent phase, namely polarity ofthe orientational ordering, chirality of the constituent molecules and spatialmodulation of the local director, are identified as underlying three possiblemechanisms for the generation of chiral asymmetry in the low temperaturenematic phase (Nx). The role and quantitative contribution of each mechanism toenantiotopic discrimination in the NX phase are presented and compared with thecase of the conventional chiral nematic phase (N*). It is found that polarordering is essential for the appearance of enantiotopic discrimination insmall rigid solutes dissolved in the Nx phase and that such discrimination isrestricted to solutes belonging to the point group symmetries $C_s$ and$C_{2v}$.