| Περίληψη: | The occurrence of the splitting of NMR spectra of chiral or prochiral solute molecules dissolved in the chiral media is known as enantio-discrimination. For the chiral molecules, the splitting or doubling in the NMR spectra originates due to the different signals coming from different enantiomers. However, for the prochiral molecules, the spectral splitting emerges because different enantiotopic sites produce different signals. In the achiral or isotropic solvents, the two enantiomers or the two enantiotopic sites cannot be distinguishable as they give rise to the same signal. On the other hand, a well-resolved splitting in the NMR spectra is observed when the solute molecules are dissolved in chiral media. This discrimination depends on the symmetry of solutes and is primarily caused by the anisotropic interactions through the ordering of the solute. The discrimination due to different ordering occurs in the solutes exhibiting specific (reflection and improper: rotation-reflection) symmetries.
A new low-temperature nematic phase (referred as Nx) has recently been discovered which is formed by certain classes of symmetric liquid crystal dimers (two mesogenic units attached by a flexible spacer) having an odd number of methylene groups in the spacer. This phase occurs just below the conventional nematic phase and enantio-discrimination is possible in this phase, although the constituting liquid crystalline dimers are not chiral. This has been confirmed by different NMR spectroscopic studies of these dimers. At first, we have performed the molecular dynamics studies of CB7CB and CB9CB (both odd) and CB8CB (even) dimers. This demonstrates the existence of two preferable sets of conformations for each dimer. Based on these findings, one can establish a relationship between the occurrence of NMR spectral splitting and the slow and fast dynamics (conformational exchange) of odd liquid crystal dimers.
We have developed a molecular theory for the enantiotopic discrimination in prochiral solutes dissolved in chiral nematic solvents by means of NMR spectroscopy. The existence of enantiotopic discrimination for different prochiral molecular symmetries has been discussed systemically. The proposed potential of mean torque, established for rigid molecules dissolved in the chiral nematic phases, has been applied to fit the experimental data obtained by NMR spectroscopy for the norbornene, acenaphthene, and moderately flexible ethanol molecule. The proposed molecular theory has been extended to formulate the potential of mean torque governing the orientational ordering of prochiral solutes in the nematic and Nx phases formed by certain types of liquid crystal dimers. The formulated theory is then used for the analysis of NMR data of some solutes of different symmetries dissolved in the two phases. The possibility of three different mechanisms has been discussed for the generation of chiral asymmetry in the low temperature Nx phase. The relevance and importance of each mechanism for the enantiotopic discrimination in the Nx phase are described and compared with the case of the conventional chiral nematic phase.
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