NMR discrimination in nonrigid prochiral solutes dissolved in chiral liquid crystals: symmetry considerations

Abstract

Enantiodiscrimination in the NMR spectra of flexible prochiral solutes dissolved in chiral liquid crystals (CLCs) is reviewed and compared with the analog phenomenon in such rigid solutes. In rigid prochiral solutes, the discrimination is brought about by the cancellation of improper symmetry elements upon dissolving in CLC within the frame of solute-solvent ordering mechanisms. If this reduction in symmetry renders the ordering of enantiotopic sites dissimilar, spectral discrimination may be observed. Symmetry considerations indicate that this is only possible for improper nonaxial groups lacking inversion symmetry. Nonrigid prochiral solutes consist of rapidly (on the NMR timescale) interconverting enantiomers, in which the racemization is accompanied by exchange of nonequivalent sites. These sites become, on the average, enantiotopically related, and in CLC, they exhibit spectral discrimination. The mechanism of the effect and the symmetry selection rules are different for the two cases. Specifically, the discrimination in flexible prochiral compounds results from the different ordering of the interchanging enantiomers in CLC. Using Altman's definition of average symmetry (Proc. R. Soc. A, 1967, 298, 184), selection rules for the phenomenon are derived. It follows that chiral discrimination in nonrigid prochiral solutes is much more abundant and can occur in all symmetry types except those possessing inversion. In particular, contrary to earlier thoughts, the effect can occur in compounds with axial symmetry. Illustrative examples of such studies with particular emphasis on compounds with average axial symmetry of the type D(3h), C(3v) and C(3h) are reviewed in this contribution.

Integrated Computational Protocol for the Analysis of Quadrupolar Splittings from Natural‐Abundance Deuterium NMR Spectra in (Chiral) Oriented Media

“…It is worth noting that the doubling of QDs is also observed for enantiotopic elements in prochiral molecules, . In NAD NMR spectroscopy, this doubling of signals corresponds to the detection of a pair of enantio‐isotopomers (RC*(DH)R′) …”

Investigation of Solute-Fiber Affinity and Orientational Ordering of Norbornadiene Interacting with Two-Polypeptide Chiral Liquid Crystalline Solvents by Natural Abundance Deuterium (NAD) NMR

“…In this new study, we examine the problem by investigating the interactional/orientational behavior of a weakly apolar, prochiral bicyclic compound dissolved in oriented systems made of two polypeptides (PBLG, PCBLL) and using DMF as cosolvent. Norbornadiene (bicyclo[2.2.1]­hept-2,5-diene) denoted NBD (Figure a) was selected as an ideal model molecule for three reasons: (i) it is a rigid molecule of C 2 v symmetry containing homo- and enantiotopic directions; (ii) it possesses single and double bonds; (iii) it intrinsically possesses a sufficient number of 2 H residual quadrupolar couplings for calculating the Saupe order matrix whether the enantiotopic sites are discriminated in the chiral mesophase. ,, …”

“…Norbornadiene (bicyclo[2.2.1]hept-2,5-diene) denoted NBD (Figure 1a) was selected as an ideal model molecule for three reasons: (i) it is a rigid molecule of C 2v symmetry containing homo-and enantiotopic directions; (ii) it possesses single and double bonds; (iii) it intrinsically possesses a sufficient number of 2 H residual quadrupolar couplings for calculating the Saupe order matrix whether the enantiotopic sites are discriminated in the chiral mesophase. 1,35,36 Experimentally, the alignment behavior of NBD is studied by following the variation of RQC( 2 H) measured on the anisotropic natural abundance deuterium (NAD) 2D-NMR spectra recorded in various chiral mixtures (PBLG/ PCBLL). 37,38 Then the variation of derived Saupe's order matrix parameters and related parameters (assuming a model geometry) versus the molar fraction of peptide unit of PBLG is discussed.…”