Analysis of enantiotopic discrimination in the NMR spectra of prochiral solutes in chiral liquid crystals by symmetry factorization of the Saupe ordering matrix

International audienceThe enantiotopic discrimination in the NMR spectrum of prochiral molecules dissolved in chiral liquid crystals CLCs is governed by the ordering of the solute molecules. The lifting of the spectral degeneracy of enantiotopic sites in such solvents stems from the nonequivalence in their effective ordering. This, in turn, is brought about by a change in the directions or the equivalence of the principal axes of their ordering tensor, relative to those in achiral liquid crystals. This discrimination mechanism can only occur for solute molecules belonging to a limited number of “allowed” improper point groups, viz., D2d, C2v, Cs, and S4 (D. Merlet, J. W. Emsley, P. Lesot, and J. Courtieu, J. Chem. Phys. 111, 6890 1999). In this work it is shown that the ordering tensor of such prochiral solutes in CLC, SCLC, and likewise, the tensors, Tk, describing the anisotropic magnetic resonance interactions of enantiotopic pairs, can be partitioned into symmetric and antisymmetric and irrelevant parts. The NMR results in such solvents can be cast into separate sets of equations depending on either the symmetric or the antisymmetric parts of S(CLC) and Tk. The discrimination observed in such solvents depends only on the latter set of equation, while the former applies to the average splitting of enantiotopic pairs as well as to diastereotopic or homotopic sites in the prochiral molecules. The factorization procedure greatly facilitates the analysis of the ordering properties of prochiral solutes in CLC and provides new insight on the discrimination mechanism. In particular, it allows correlation between independent enantiotopic partners and the identification of NMR signals belonging to common prosterogenic faces in the molecule. Expressions relating NMR observables with the symmetric and antisymmetric parts of S(CLC) are derived for each of the four allowed groups. Model examples are presented and discussed

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

International audienceThe splitting of signals in the NMR spectra originating from enantiotopic sites in prochiral moleculeswhen dissolved in chiral solvents is referred to as spectral enantiotopic discrimination. This phenomenonis particularly noticeable in chiral liquid crystals (CLCs) due to the combined effect of the anisotropicmagnetic interactions and the ordering of the solute in the mesophase. The enantiorecognitionmechanisms are different for rigid and flexible solutes. For the former, discrimination results fromsymmetry breaking and is restricted to solutes whose point groups belong to one of the following four(‘‘allowed’’) symmetries, Cs, C2v, D2d and S4. The nature of the symmetry breaking for each one of thesegroups is discussed and experimental examples, using mainly 2H 1D/2D-NMR in chiral polypeptide lyotropicmesophases, are presented and analyzed. When flexible optically active solutes undergo fast racemization (onthe NMR timescale) their spectrum corresponds to that of an average prochiral molecule and may exhibitenantiotopic sites. In CLCs, such sites will become discriminated, irrespective of their average (improper)symmetry. This enantiodiscrimination results mainly from the different ordering of the interchangingenantiomers. Several examples of such flexible molecules, including solutes with average axial and planarsymmetries, are commented. Dynamic processes in solution that are not accompanied by the modulation ofmagnetic interactions remain ‘‘NMR blind’’. This is sometimes the case for interconversion of enantiomers(racemization) or exchange of enantiotopic sites in isotropic solvents. The limitation can be lifted by usingCLCs. In such solvents, non-equivalence between enantiomers or between enantiotopic sites is induced bythe chiral environment, thus providing the necessary interactions to be modulated by the dynamic processes.Illustrative examples involving exchange of both, enantiotopic sites and enantiomers are examined. Inthis comprehensive review, various important aspects of enantiodiscrimination by NMR are presented.Thus the possibility to reveal enantiotopic recognition using residual dipolar couplings or to determinethe absolute configuration of enantiotopic NMR signals is discussed. The various kinds of chiralmesophases able to reveal enantiotopic discrimination in guest prochiral molecules are also describedand compared with each other. Finally to illustrate the high analytical potentialities of NMR in CLCs,several and various applications involving the enantiodiscrimination phenomenon are described.A strategy for assigning the NMR signals of meso compound in a meso–threo mixture of cyclic moleculesis first discussed. This is followed by a description of advantages of the method for the determination of(D/H) natural isotopic fractionation in biocompounds

Enantiodiscrimination in Deuterium NMR Spectra of Flexible Chiral Molecules with Average Axial Symmetry Dissolved in Chiral Liquid Crystals: The Case of Tridioxyethylenetriphenylene

International audienceFlexible 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(γ-benzylglutamate). The fast chair-chair flipping of the side chains in 4 on average renders the molecule axially symmetric (D3h) with pairs of enantiotopic ethylene deuterons. These deuterons exhibit unusually large enantiodiscrimination. To explain this observation, we first describe how the average symmetry of flexible molecules can be derived from the symmetry of the “frozen” conformers and the nature of the averaging process. The procedure is then applied to 4 and used to analyze the NMR results. It is shown that the large enantiodiscrimination in the present case reflects a large difference in the orientational ordering of the conformational enantiomers participating in the interconversion processes as well as a large geometrical factor due to the special shape of the dioxyethylene side groups. 1H and 13C NMR spectra of 4 in the same lyotropic liquid crystalline solvent are analyzed to determine its ordering characteristics. Several related cases are also discusse

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

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Dynamic disorder in 2,3-dimethylnaphthalene: a single crystal deuteron nuclear magnetic resonance study

Deuteron NMR experiments on single crystals of 2,3-dimethylnaphthalene (DMN) deuterated in the aromatic sites are reported. The deuteron resonances are inhomogeneously broadened and their widths are strongly anisotropic. This is shown to be due to orientational disorder of the DMN molecules. Two types of disorder are distinguished: polar disorder, also seen by X-ray and neutron diffraction, in which the direction of the long molecular axis alternates between up and down, and alignment disorder, in which the direction of this axis varies statistically about an average direction with a mean deviation of about 1.8o. It is argued that the two types of disorder are linked. Their coexistence suggests that the polar disorder in DMN is statistical rather than correlated. One- and two- dimensional magnetization transfer experiments show effects due to spin diffusion, molecular reorientations (pi-flips) and molecular self-diffusion. The self-diffusion process involves two types of molecular jumps called flips and slips. These jumps cause both types of disorder to become dynamic. The mean time between such jumps is about 3s at T 338K

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

International audienceThe splitting of signals in the NMR spectra originating from enantiotopic sites in prochiral moleculeswhen dissolved in chiral solvents is referred to as spectral enantiotopic discrimination. This phenomenonis particularly noticeable in chiral liquid crystals (CLCs) due to the combined effect of the anisotropicmagnetic interactions and the ordering of the solute in the mesophase. The enantiorecognitionmechanisms are different for rigid and flexible solutes. For the former, discrimination results fromsymmetry breaking and is restricted to solutes whose point groups belong to one of the following four(‘‘allowed’’) symmetries, Cs, C2v, D2d and S4. The nature of the symmetry breaking for each one of thesegroups is discussed and experimental examples, using mainly 2H 1D/2D-NMR in chiral polypeptide lyotropicmesophases, are presented and analyzed. When flexible optically active solutes undergo fast racemization (onthe NMR timescale) their spectrum corresponds to that of an average prochiral molecule and may exhibitenantiotopic sites. In CLCs, such sites will become discriminated, irrespective of their average (improper)symmetry. This enantiodiscrimination results mainly from the different ordering of the interchangingenantiomers. Several examples of such flexible molecules, including solutes with average axial and planarsymmetries, are commented. Dynamic processes in solution that are not accompanied by the modulation ofmagnetic interactions remain ‘‘NMR blind’’. This is sometimes the case for interconversion of enantiomers(racemization) or exchange of enantiotopic sites in isotropic solvents. The limitation can be lifted by usingCLCs. In such solvents, non-equivalence between enantiomers or between enantiotopic sites is induced bythe chiral environment, thus providing the necessary interactions to be modulated by the dynamic processes.Illustrative examples involving exchange of both, enantiotopic sites and enantiomers are examined. Inthis comprehensive review, various important aspects of enantiodiscrimination by NMR are presented.Thus the possibility to reveal enantiotopic recognition using residual dipolar couplings or to determinethe absolute configuration of enantiotopic NMR signals is discussed. The various kinds of chiralmesophases able to reveal enantiotopic discrimination in guest prochiral molecules are also describedand compared with each other. Finally to illustrate the high analytical potentialities of NMR in CLCs,several and various applications involving the enantiodiscrimination phenomenon are described.A strategy for assigning the NMR signals of meso compound in a meso–threo mixture of cyclic moleculesis first discussed. This is followed by a description of advantages of the method for the determination of(D/H) natural isotopic fractionation in biocompounds

Temperature Dependent Sign Reversal of the Optical Anisotropy in Pyramidic Mesophases

Hexasubstituted tribenzocyclononene derivatives with R = -OC(O)C6H4CnH2n+i and R = -OC(O)C6H4OCnH2n+1 possess enantiotropic pyramidic mesophases. These mesophases exhibit an uncommon feature in that their optical anisotropy changes sign as function of temperature within the mesophase region. It is suggested that this effect reflects conformational changes involving the side chain benzene ring

Ellagic Acid Derivatives: A New Mesogenic Series Exhibiting a Thermotropic Cubic Mesophase

Nine members of the tetra-n-alkanoyloxy ellagic acid series, ranging from the octanoyl to hexadecanoyl were synthesized and studied by calorimetry and optical microscopy. The octanoyloxy and nonanoyloxy derivatives are not mesogenic, while the other homologues exhibit a highly organized birefringent enantiotropic mesophase. Of these the five members with the longest side chains exhibit in addition a monotropic optically isotropic, and consequently, a cubic mesophase. From miscibility studies it appears that this cubic phase, which we name CD, is different from other cubic mesophases previously identified in non-chiral thermotropic mesogens. This monotropic CD mesophase can be maintained at room temperature in a metastable state for several weeks.Neuf termes de la série des dérivés tétra-n-alcanoyloxy de l'acide ellagique (de l'octanoyle à l'hexadécanoyle) sont synthétisés et étudiés par calorimétrie et microscopie optique. Les dérivés octanoyloxy et nonanoyloxy ne sont pas mésogènes, les autres termes présentent une mésophase thermodynamiquement stable, biréfringente et fortement organisée. Les cinq substances qui ont les plus longues chaînes latérales présentent, en plus, une mésophase métastable optiquement isotrope, donc cubique. D'après les observations de miscibilités cette phase cubique, que nous appelons CD, est différente des autres mésophases cubiques antérieurement identifiées dans les mésogènes thermotropes achiraux. Cette mésophase CD peut être conservée pendant plusieurs semaines à la température ambiante