Aharon Loewenstein

Enantiomeric visualization using proton-decoupled natural abundance deuterium NMR in poly(γ-benzyl-l-glutamate) liquid crystalline solutions

Abstract We report the first visualization of chiral molecules oriented in a polypeptide liquid crystalline system (PBLG) using proton-decoupled natural abundance deuterium NMR. The chiral discrimination is observed through measurements of the quadrupolar splitting differences and we demonstrate that the sensitivity of natural abundance deuterium NMR is sufficient to measure the differential ordering effects (DOEs) without the need for isotopic enrichment. The feasibility and the potential of this novel method were investigated using a 5.87 T spectrometer (proton frequency 250 MHz). Several examples of chiral discrimination are presented and particular emphasis is given to demonstrate the potential of this approach.

Anisotropic Translational Diffusion of Methane and Chloroform in Thermotropic Nematic and Smectic Liquid Crystals

Abstract Nuclear magnetic resonance pulsed-gradient spin-echo measurements are reported for the anisotropic translational diffusion coefficients and their temperature dependence, of methane, monodeuteromethane and chloroform dissolved in several thermotropic liquid crystals exhibiting nematic, smectic A and smectic B phases. For methane, in those liquid crystals forming only nematic phases, the diffusion parallel to the director (D ⊥) is greater than the perpendicular diffusion (D); the magnitude of the diffusional anisotropy (D ⊥/D ⊥) depends on the nature of the nematic medium. In the nematic phases of smectic-forming materials, D ⊥/D ⊥ < 1. In the smectic A phases, D ⊥/D ⊥ ≪ 1 with a much larger activation energy for D ⊥. In the smectic B phase, D ⊥/D ⊥ ≪ 1 with similar activation energies. Chloroform exhibits similar behavior with deviations in the nematic phases of a polymorphic liquid crystal. A qualitative interpretation of these results is given in terms of several factors which govern the diffusi...

Deuterium NMR stereochemical analysis of threo–erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents

Abstract We report the proton-decoupled deuterium NMR study of labelled diastereomers with remote stereogenic carbons dissolved in various mixtures of poly-γ-benzyl- l -glutamate (PBLG) and poly-γ-benzyl- d -glutamate (PBDG) liquid crystalline solutions. The evolution of quadrupolar splitting as well as the diastereomeric and the enantiomeric discrimination versus the proportion of PBLG and PBDG in the liquid crystalline phase is studied. It is shown that racemic liquid crystalline solutions of PBLG and PBDG may be used to measure diastereomeric excess ( de ). Thereafter the spectrum in PBLG solution allows for measuring the enantiomeric excess ( ee ) of each diastereomer. These first results suggest substantial prospects in the field of the analysis of diastereomers with remote stereogenic carbons.

Visualization of enantiomers in a polypeptide liquid-crystal solvent through carbon-13 NMR spectroscopy

High-resolution, proton-coupled and decoupled, carbon-13 NMR in natural abundance is used to observe the spectra of enantiomers in chiral molecules dissolved in the poly(γ-benzyl-L-glutamate)(PBLG)/CD2Cl2 liquid-crystalline system. Discrimination between enantiomers can be obtained either through differences in their dipolar interactions or in their chemical shift anisotropies. This technique provides a new tool for the visualization of enantiomers and the measurement of the enantiomeric ratio. Several specific examples are presented and discussed.

An achiral deuterated derivatizing agent for enantiomeric analysis through NMR in a liquid crystalline solvent

Abstract The application of a ‘deuterium probe’ for the enantiomeric analysis of chiral amines, aminoacids and alcohols is proposed. The probe is the perdeuterated benzoyl fragment which can easily and reversibly be attached to the corresponding materials. The analysis is performed through measurements of the differential ordering effect (DOE) in the deuterium NMR of the chiral material dissolved in a cholesteric-nematic liquid crystalline system. Since all the chiral compounds which we studied were not racemic, the assignments of the observed DOEs to the enantiomers enabled us to draw tentative conclusions relating the results to the absolute configurations. This may open the way for the empirical derivation of absolute configurations of enantiomers from DOE measurements.

Synthesis and Stereochemical Assignment of Crypto-Optically Active (2) H6 -Neopentane.

The determination of the absolute configuration of chiral molecules is at the heart of asymmetric synthesis. Here we probe the spectroscopic limits for chiral discrimination with NMR spectroscopy in chiral aligned media and with vibrational circular dichroism spectroscopy of the sixfold-deuterated chiral neopentane. The study of this compound presents formidable challenges since its stereogenicity is only due to small mass differences. For this purpose, we selectively prepared both enantiomers of (2) H6 -1 through a concise synthesis utilizing multifunctional intermediates. While NMR spectroscopy in chiral aligned media could be used to characterize the precursors to (2) H6 -1, the final assignment could only be accomplished with VCD spectroscopy, despite the fleetingly small dichroic properties of 1. Both enantiomers were assigned by matching the VCD spectra with those computed with density functional theory.

Anisotropic Translational Diffusion of Methane in N-(4-n-pentyloxybenzyl dene)-4-n-hexylaniline (50.6)

Abstract The temperature dependence of the translational diffusion coefficients is reported for methane dissolved in the thermotropic liquid crystal N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6). The general features of methane diffusion behavior in the N, SA and crystal SB phases of 50.6 are quite similar, with respect to the magnitude of the rates and corresponding anisotropies, to those found in the homolog 40.6. The mutual characteristics are: that considerable smectic character (D || < D ||) exists in the nematic phase, that D || ≪ D || with E || ≫ E || in the SA phase because of increasing layer definition and of methane expulsion from the cores to the chains and that D ⊥/D|| ∼ 30 (with E || ∼ E ⊥) in the crystalline SB phase. Methane behavior in the SF phase of 50.6 is very unusual, showing a slight decrease in D⊥ and large increases in D || upon decreasing temperature with D ⊥/D|| values of from 10 to 5. This is possibly the result of the decrease in the close-packed core thickness coupled ...