Enantiotopic discrimination in the NMR spectrum of prochiral solutes in chiral liquid crystals.

The splitting of signals in the NMR spectra originating from enantiotopic sites in prochiral molecules when dissolved in chiral solvents is referred to as spectral enantiotopic discrimination. This phenomenon is particularly noticeable in chiral liquid crystals (CLCs) due to the combined effect of the anisotropic magnetic interactions and the ordering of the solute in the mesophase. The enantiorecognition mechanisms are different for rigid and flexible solutes.

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

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.

Conformation and dynamics of 18-membered hexathiametacyclophanes: a two step racemization as studied by deuterium NMR in chiral lyotropic liquid crystals.

The conformation and interconversion dynamics of two derivatives of the 18-membered hexathia metacyclophane 1 and 2 were studied by (1)H NMR spectroscopy in isotropic solvents and by (2)H NMR in chiral liquid crystalline (CLC) solutions, as well as by molecular structure computations. For the analysis of the dynamic effects, we made use of the concepts of "average symmetry" and "isodynamic groups", introduced by Altmann (Altmann, Proc. R.

Enantiotopic discrimination in the deuterium NMR spectrum of solutes with S4 symmetry in chiral liquid crystals.

Enantiotopic discrimination in the NMR spectra of prochiral rigid solutes in chiral liquid crystals (CLC), by the ordering mechanism, is limited to molecules possessing one of the four, so called, "allowed" symmetries, D(2d), C(2v), C(s), and S(4). So far, such spectral discrimination was demonstrated only for solutes possessing one of the first three symmetries. In this work, we present deuterium NMR measurements on a rigid S(4) compound dissolved in a chiral nematic solvent and demonstrate, for the first time, enantiotopic discrimination in such symmetry.

Mesomorphic properties of the neat enantiomers of a chiral pyramidic liquid crystal.

The crown form of nona-octanoyloxy tribenzocyclononatriene (C8) with C(3) symmetry was prepared and separated into its enantiomers (C8A and C8B) by HPLC. Like the racemate, the neat enantiomers are also mesogenic, exhibiting two mesophases, M and Col(h). Differential scanning calorimetry, X-ray diffraction, carbon-13 NMR and UV circular dichroism are used to study the structural and dynamic properties of these chiral mesophases.

Enantiodiscrimination in deuterium NMR spectra of flexible chiral molecules with average axial symmetry dissolved in chiral liquid crystals: the case of tridioxyethylenetriphenylene.

Flexible chiral molecules undergoing fast interconversion (on the NMR time scale) between different conformational enantiomers may yield "average" axial species with enantiotopically related sites. Contrary to the situation observed for rigid axial molecules, signals from these enantiotopic sites in NMR spectra recorded in chiral liquid-crystalline solvents can be resolved. In the present work, we studied the deuterium NMR spectra of tridioxyethylenetriphenylene (compound 4) statistically deuterated to 10% in the flexible side chains and dissolved in chiral and achiral lyotropic liquid crystals based on poly(gamma-benzylglutamate).

Isotopic difference spectra as an aide in determining absolute configuration using vibrational optical activity: vibrational circular dichroism of 13C- and 2H-labelled nonamethoxy cyclotriveratrylene.

The use of isotopic difference spectra in vibrational optical activity is demonstrated as a supplemental aide in determining the absolute configuration of chiral molecules. It is shown that IR and VCD difference spectra associated with isotopic substitution observed in experimental spectra can be accurately reproduced by density functional theory calculations when the IR and VCD spectra of the original isotopomer are calculated to reasonable accuracy. Results for isotopically substituted nonamethoxy cyclotriveratrylene are presented to illustrate the degree of agreement between measured and calculated IR and VCD difference spectra for several isotopomers of this molecule.

Structural and optical isomers of nonamethoxy cyclotriveratrylene: separation and physical characterization.

The paper concerns the structural and optical isomers of nonamethoxy-tribenzocyclononene (compound 1). In the first part of the paper it is shown that 1 exists in two structural isomers: a rigid crown (c-1) with C3 symmetry and a flexible saddle (s-1) with C1 symmetry. The latter, not previously known, can be prepared from the as-synthesized c-1 by quenching a hot solution (or the melt) followed by HPLC separation.

Chiral discrimination in the 13C and 2H NMR of the crown and saddle isomers of nonamethoxy-cyclotriveratrylene in chiral liquid-crystalline solutions.

We report 2H and 13C NMR spectra of the crown and saddle isomers of nonamethoxy-tribenzocyclononene (1), dissolved in lyotropic achiral and chiral liquid-crystalline solutions based on poly-gamma-benzyl-glutamate and poly-gamma-benzyl-L-glutamate (PBG and PBLG). The 2H-[1H] measurements include spectra of compound 1 deuterated in the ring methylene and in the aromatic sites as well as of the methyl groups in natural abundance. Carbon-13 spectra were recorded in natural abundance as well as in two isotopomers enriched in the ring methylene and one of the methoxy groups.

Mesomorphism, isomerization, and dynamics in a new series of pyramidic liquid crystals.

Nona-alkanoyloxy tribenzocyclononene (CTV-n, where n is the number of carbons in the side chains) were prepared for n = 2 to 14. The homologues of this series appear in two stable isomeric forms, rigid crown and flexible saddle. We report on their isomerization equilibria and dynamics in solution and on their mesomorphic properties in the neat state.

Enantiomeric and enantiotopic analysis of cone-shaped compounds with C(3) and C(3)(v) symmetry using NMR spectroscopy in chiral anisotropic solvents.

We describe the enantiomeric and enantiotopic analysis of the NMR spectra of compounds derived from the functionalized cone-shaped core, cyclotriveratrylenes (CTV), dissolved in weakly oriented lyotropic chiral liquid crystals (CLCs) based on organic solutions of poly-gamma-benzyl-L-glutamate. The CTV core lacks prostereogenic as well as stereogenic tetrahedral centers. However, depending on the pattern of substitution, chiral and achiral compounds with different symmetries can be obtained.