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

TL;DR

This paper develops a theoretical framework to analyze how different molecular and phase attributes influence enantiotopic discrimination in NMR spectra of solutes in nematic phases formed by symmetric achiral bimesogens, highlighting the role of polar ordering.

Contribution

It introduces a mean torque model to explain enantiotopic discrimination mechanisms in the Nx phase, emphasizing the importance of polar ordering and identifying specific symmetry restrictions.

Findings

Polar ordering is crucial for enantiotopic discrimination in the Nx phase.

Discrimination occurs mainly for solutes with C_s and C_{2v} symmetry.

Three mechanisms—polarity, chirality, and spatial modulation—contribute to chiral asymmetry.

Abstract

The potential of mean torque governing the orientational ordering of prochiral solutes in the two nematic phases (N and Nx) formed by certain classes of symmetric achiral bimesogens is formulated and used for the analysis of existing NMR measurements on solutes of various symmetries dissolved in the two phases. Three distinct attributes of the solvent phase, namely polarity of the orientational ordering, chirality of the constituent molecules and spatial modulation of the local director, are identified as underlying three possible mechanisms for the generation of chiral asymmetry in the low temperature nematic phase (Nx). The role and quantitative contribution of each mechanism to enantiotopic discrimination in the NX phase are presented and compared with the case of the conventional chiral nematic phase (N*). It is found that polar ordering is essential for the appearance of enantiotopic discrimination in small rigid solutes dissolved in the Nx phase and that such discrimination is restricted to solutes belonging to the point group symmetries $C_s$ and $C_{2v}$.