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

Chiral Discrimination in the 13 C and 2 H NMR of the Crown and Saddle Isomers of Nonamethoxy-Cyclotriveratrylene in Chiral Liquid-Crystalline Solutions

International audienceWe 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-ç-benzyl-glutamate and poly-ç-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. The crown isomer (c-1) is rigid with C3 symmetry and can be separated into its enantiomers using a chiral high-performance liquid chromatography column. The NMR spectra of racemic c-1 in PBLG solutions exhibit two sets of lines due to the enantiomers. The peaks were identified by comparing the spectra with those of the neat enantiomers. Analysis of the 2H quadrupolar splittings and the 13C residual chemical shift anisotropies shows that the dominant factor determining the chiral discrimination is the difference in the ordering of the two enantiomers in the chiral liquid crystals. The saddle isomer (s-1) is highly flexible, undergoing fast pseudorotation between six conformers. The “frozen” conformers have C1 symmetry and are therefore chiral. Three of these comprise one enantiomer, and the other three the second one. However, the rapidly interconverting species has, on the average, a C3h symmetry and is therefore achiral. The methylene groups in the latter are, however, prostereogenic, and their hydrogen/deuterium-carbon bonds constitute enantiotopic pairs. The 2H NMR spectra of the s-1 methylene-deuterated in PBLG solutions exhibit, in fact, enantio-discrimination with two quadrupolar doublets. This is in contrast to rigid prochiral molecules with a threefold symmetry axis, which normally do not show such discrimination. A detailed analysis of the effect is presented, and it is argued that the discrimination observed for s-1 reflects the different ordering of its enantiomers during the pseudorotation cycle

Molecular Reorientation and Self−Diffusion in Solid Cubane by Deuterium and Proton NMR

Deuterium and proton NMR line shape and relaxation measurements are reported for the sym−cubane−d2 isotopomer (D3d symmetry) in the temperature range from 200 K to the melting point of the solid. The results confirm the polymorphic phase sequence: solid II solid I liquid, reported earlier by White et al. The deuterium spectrum below 215 K exhibits a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a coupling constant of Qc = 178 kHz. Above this temperature the spectrum develops features characteristic of the onset of dynamic processes leading to a single line at about room temperature. Longitudinal relaxation time measurements show a T1 minimum at 344 K. On transition to solid I there is a discontinuous narrowing of the line and a concomitant increase in T1. These results are quantitatively interpreted in terms of reorientational jumps of the cubane molecules between their various equivalent orientations. The jump rate k in solid II follows an Arrhenius behavior over a range of almost seven decades with an activation energy E = 62 kJ mol−1. Transition to solid I results in a discontinuous increase of k. Proton line width measurements in solid II show two motional narrowing steps. The first, at around 240 K, is due to the cubic jumps while the second, at around 375 K, corresponds to molecular self− diffusion between the lattice sites. The activation energy for this process is Ed = 83 kJ mol−1. On transition to solid I there is also a discontinuous increase in the rate of this process. The proton T1 values are predominantly affected by the reorientation process and are consistent with the deuteron data. At temperatures above 340 K, where the deuterium NMR spectrum is expected to be a single Lorentzian with a width of less than 400 Hz, it actually exhibits, in both phases II and I, a Pake doublet corresponding to an axially symmetric quadrupole coupling tensor with a very small coupling constant Q'c = 0.48 kHz. This indicates that the reorientation process is not perfectly cubic. Possible reasons for this surprising effect are discusse

Bond-Shift Rearrangement in Solid Li3</sub<P7(Monoglyme)3: A 31P MAS NMR Study

The 31P MAS NMR spectrum of solid Li3P7 (monoglyme) 3 has been reinvestigated over a wide temperature range (-70 to +77&deg;C) and under conditions of better resolution (Larmor frequency of 162 MHz and spinning rate of &tilde;30 kHz) than previously measured (121 MHz and 13 kHz). At low temperatures three spinning sideband (ssb) manifolds are observed: a singlet (centered at -45 ppm relative to 85% H3PO4) due to the apical atom (A) of the P7-cage trianion; a 1 : 1 : 1 triplet (at -110, -117, and -124.5 ppm) due to the negatively charged equatorial (E) atoms, and a one to two doublet (at -161 and -168.5 ppm) due to the basal (B) atoms. These results are consistent with the P7 cage having nearly, but not perfect, C3v symmetry. The compound appears to be well ordered in the solid state with very little structural dispersity. On heating, the NMR lines broaden and eventually coalesce into a single ssb manifold. This behavior is ascribed to bond-shift rearrangement similar to the Cope rearrangement in bullvalene. A MAS 2D exchange experiment and a quantitative analysis of the 1D NMR lineshapes indicate that, unlike in solution where the rearrangement involves a single bond shift at a time, in the solid the process involves a succession of two bond shifts: The first leads to an intermediate species in which the rearranged P7 cage is inverted, while in the subsequent step a second bond shift takes place that also restores the original orientation of the cage in the lattice. The overall effect of the double bond shift is equivalent to cyclic permutation of the phosphorus atoms within the five member rings of the P7-cage. The quantitative analysis of the dynamic lineshapes shows that this cyclic permutation proceeds at a different rate in one ring (kd1) than in the other two (kd2,3). The kinetic parameters for these processes are E<SMALL>&alpha;</SMALL>1=18.7 kJ/mol, E<SMALL>&alpha;</SMALL>2,3=58.0 kJ/mol, kd1 (17&deg;C)=kd2,3 (17&deg;C)=104 s-1. No indications for independent threefold molecular jumps of the P7 cage were found

The Saddle Form of Cyclotriveratrylene

Cyclotriveratrylene (CTV), or hexamethoxy tribenzocyclononatriene (hexamethoxy−TBCN) and other peripherally hexa−(or tri−)substituted TBCN were so far only known to exist in their crown form. Attempts to detect the corresponding saddle isomers failed, even though when structurally chiral these compounds undergo racemization, a process that is believed to proceed via the saddle form. We show that the saddle form of CTV (and other peripherally hexa−substituted TBCN) can be obtained by rapidly quenching a hot solution or the high−temperature melt to below room temperature. The saddle form of CTV was quantitatively separated from the quenched material by column chromatography, and some of its thermodynamic and kinetic properties in solution and in the solid state were determined by proton and carbon−13 NMR. Detailed measurement were performed in chloroform solutions, for which it was found that at room temperature the equilibrium constant, K = [saddle]/[crown] = exp[−(H − TS)/RT, is ˜0.1 (with H = 9.96 ± 0.5 kJ mol−1, S = 13.8 ± 1.6 J mol−1 K−1). The isomerization half−life at room temperature is about 1 day with the rate constant for the crown to saddle transformation, (k(crownsaddle) = A exp(−Ea/RT), characterized by the kinetic parameters, Ea = 97.4 ± 4.8 kJ mol−1 and log[A (s−1)] = 11.0 ± 0.8). These parameters are consistent with those for the racemization rate of the isotopically chiral CTV−d9 measured by Collet and Gabard (J. Org. Chem. 1980, 45, 5400−5401). Attempts to freeze−out the fast pseudorotation of the saddle isomer by cooling a Freon solution to almost 100 K, failed, setting a lower limit of 106 to 107s−1 for the pseudorotation rate at 120 K. Carbon−13 MAS spectra indicate that the saddle isomer of CTV in the solid state is crystalline with two (nonequivalent and distorted) molecules per asymmetric unit